serial: 8250: Add proper clock handling for OxSemi PCIe devices
Oxford Semiconductor PCIe (Tornado) 950 serial port devices are driven
by a fixed 62.5MHz clock input derived from the 100MHz PCI Express clock.
We currently drive the device using its default oversampling rate of 16
and the clock prescaler disabled, consequently yielding the baud base of
3906250. This base is inadequate for some of the high-speed baud rates
such as 460800bps, for which the closest rate possible can be obtained
by dividing the baud base by 8, yielding the baud rate of 488281.25bps,
which is off by 5.9638%. This is enough for data communication to break
with the remote end talking actual 460800bps, where missed stop bits
have been observed.
We can do better however, by taking advantage of a reduced oversampling
rate, which can be set to any integer value from 4 to 16 inclusive by
programming the TCR register, and by using the clock prescaler, which
can be set to any value from 1 to 63.875 in increments of 0.125 in the
CPR/CPR2 register pair. The prescaler has to be explicitly enabled
though by setting bit 7 in the MCR or otherwise it is bypassed (in the
enhanced mode that we enable) as if the value of 1 was used.
Make use of these features then as follows:
- Set the baud base to 15625000, reflecting the minimum oversampling
rate of 4 with the clock prescaler and divisor both set to 1.
- Override the `set_mctrl' and set the MCR shadow there so as to have
MCR[7] always set and have the 8250 core propagate these settings.
- Override the `get_divisor' handler and determine a good combination of
parameters by using a lookup table with predetermined value pairs of
the oversampling rate and the clock prescaler and finding a pair that
divides the input clock such that the quotient, when rounded to the
nearest integer, deviates the least from the exact result. Calculate
the clock divisor accordingly.
Scale the resulting oversampling rate (only by powers of two) if
possible so as to maximise it, reducing the divisor accordingly, and
avoid a divisor overflow for very low baud rates by scaling the
oversampling rate and/or the prescaler even if that causes some
accuracy loss.
Also handle the historic spd_cust feature so as to allow one to set
all the three parameters manually to arbitrary values, by keeping the
low 16 bits for the divisor and then putting TCR in bits 19:16 and
CPR/CPR2 in bits 28:20, sanitising the bit pattern supplied such as
to clamp CPR/CPR2 values between 0.000 and 0.875 inclusive to 33.875.
This preserves compatibility with any existing setups, that is where
requesting a custom divisor that only has any bits set among the low
16 the oversampling rate of 16 and the clock prescaler of 33.875 will
be used as with the original 8250.
Finally abuse the `frac' argument to store the determined bit patterns
for the TCR, CPR and CPR2 registers.
- Override the `set_divisor' handler so as to set the TCR, CPR and CPR2
registers from the `frac' value supplied. Set the divisor as usual.
With the baud base set to 15625000 and the unsigned 16-bit UART_DIV_MAX
limitation imposed by `serial8250_get_baud_rate' standard baud rates
below 300bps become unavailable in the regular way, e.g. the rate of
200bps requires the baud base to be divided by 78125 and that is beyond
the unsigned 16-bit range. The historic spd_cust feature can still be
used to obtain such rates if so required.
See Documentation/tty/device_drivers/oxsemi-tornado.rst for more details.
Signed-off-by: Maciej W. Rozycki <macro@orcam.me.uk>
Link: https://lore.kernel.org/r/alpine.DEB.2.21.2204181519450.9383@angie.orcam.me.uk
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
2022-04-18 18:27:33 +03:00
.. SPDX-License-Identifier: GPL-2.0
====================================================================
Notes on Oxford Semiconductor PCIe (Tornado) 950 serial port devices
====================================================================
Oxford Semiconductor PCIe (Tornado) 950 serial port devices are driven
by a fixed 62.5MHz clock input derived from the 100MHz PCI Express clock.
The baud rate produced by the baud generator is obtained from this input
frequency by dividing it by the clock prescaler, which can be set to any
value from 1 to 63.875 in increments of 0.125, and then the usual 16-bit
divisor is used as with the original 8250, to divide the frequency by a
value from 1 to 65535. Finally a programmable oversampling rate is used
that can take any value from 4 to 16 to divide the frequency further and
determine the actual baud rate used. Baud rates from 15625000bps down
to 0.933bps can be obtained this way.
By default the oversampling rate is set to 16 and the clock prescaler is
set to 33.875, meaning that the frequency to be used as the reference
for the usual 16-bit divisor is 115313.653, which is close enough to the
frequency of 115200 used by the original 8250 for the same values to be
used for the divisor to obtain the requested baud rates by software that
is unaware of the extra clock controls available.
The oversampling rate is programmed with the TCR register and the clock
2022-05-14 01:42:08 +03:00
prescaler is programmed with the CPR/CPR2 register pair [OX200]_ [OX952]_
[OX954]_ [OX958]_ . To switch away from the default value of 33.875 for
the prescaler the enhanced mode has to be explicitly enabled though, by
setting bit 4 of the EFR. In that mode setting bit 7 in the MCR enables
the prescaler or otherwise it is bypassed as if the value of 1 was used.
Additionally writing any value to CPR clears CPR2 for compatibility with
old software written for older conventional PCI Oxford Semiconductor
devices that do not have the extra prescaler's 9th bit in CPR2, so the
CPR/CPR2 register pair has to be programmed in the right order.
serial: 8250: Add proper clock handling for OxSemi PCIe devices
Oxford Semiconductor PCIe (Tornado) 950 serial port devices are driven
by a fixed 62.5MHz clock input derived from the 100MHz PCI Express clock.
We currently drive the device using its default oversampling rate of 16
and the clock prescaler disabled, consequently yielding the baud base of
3906250. This base is inadequate for some of the high-speed baud rates
such as 460800bps, for which the closest rate possible can be obtained
by dividing the baud base by 8, yielding the baud rate of 488281.25bps,
which is off by 5.9638%. This is enough for data communication to break
with the remote end talking actual 460800bps, where missed stop bits
have been observed.
We can do better however, by taking advantage of a reduced oversampling
rate, which can be set to any integer value from 4 to 16 inclusive by
programming the TCR register, and by using the clock prescaler, which
can be set to any value from 1 to 63.875 in increments of 0.125 in the
CPR/CPR2 register pair. The prescaler has to be explicitly enabled
though by setting bit 7 in the MCR or otherwise it is bypassed (in the
enhanced mode that we enable) as if the value of 1 was used.
Make use of these features then as follows:
- Set the baud base to 15625000, reflecting the minimum oversampling
rate of 4 with the clock prescaler and divisor both set to 1.
- Override the `set_mctrl' and set the MCR shadow there so as to have
MCR[7] always set and have the 8250 core propagate these settings.
- Override the `get_divisor' handler and determine a good combination of
parameters by using a lookup table with predetermined value pairs of
the oversampling rate and the clock prescaler and finding a pair that
divides the input clock such that the quotient, when rounded to the
nearest integer, deviates the least from the exact result. Calculate
the clock divisor accordingly.
Scale the resulting oversampling rate (only by powers of two) if
possible so as to maximise it, reducing the divisor accordingly, and
avoid a divisor overflow for very low baud rates by scaling the
oversampling rate and/or the prescaler even if that causes some
accuracy loss.
Also handle the historic spd_cust feature so as to allow one to set
all the three parameters manually to arbitrary values, by keeping the
low 16 bits for the divisor and then putting TCR in bits 19:16 and
CPR/CPR2 in bits 28:20, sanitising the bit pattern supplied such as
to clamp CPR/CPR2 values between 0.000 and 0.875 inclusive to 33.875.
This preserves compatibility with any existing setups, that is where
requesting a custom divisor that only has any bits set among the low
16 the oversampling rate of 16 and the clock prescaler of 33.875 will
be used as with the original 8250.
Finally abuse the `frac' argument to store the determined bit patterns
for the TCR, CPR and CPR2 registers.
- Override the `set_divisor' handler so as to set the TCR, CPR and CPR2
registers from the `frac' value supplied. Set the divisor as usual.
With the baud base set to 15625000 and the unsigned 16-bit UART_DIV_MAX
limitation imposed by `serial8250_get_baud_rate' standard baud rates
below 300bps become unavailable in the regular way, e.g. the rate of
200bps requires the baud base to be divided by 78125 and that is beyond
the unsigned 16-bit range. The historic spd_cust feature can still be
used to obtain such rates if so required.
See Documentation/tty/device_drivers/oxsemi-tornado.rst for more details.
Signed-off-by: Maciej W. Rozycki <macro@orcam.me.uk>
Link: https://lore.kernel.org/r/alpine.DEB.2.21.2204181519450.9383@angie.orcam.me.uk
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
2022-04-18 18:27:33 +03:00
By using these parameters rates from 15625000bps down to 1bps can be
obtained, with either exact or highly-accurate actual bit rates for
standard and many non-standard rates.
Here are the figures for the standard and some non-standard baud rates
(including those quoted in Oxford Semiconductor documentation), giving
the requested rate (r), the actual rate yielded (a) and its deviation
from the requested rate (d), and the values of the oversampling rate
(tcr), the clock prescaler (cpr) and the divisor (div) produced by the
2022-05-14 01:42:08 +03:00
new `` get_divisor `` handler:
::
r: 15625000, a: 15625000.00, d: 0.0000%, tcr: 4, cpr: 1.000, div: 1
r: 12500000, a: 12500000.00, d: 0.0000%, tcr: 5, cpr: 1.000, div: 1
r: 10416666, a: 10416666.67, d: 0.0000%, tcr: 6, cpr: 1.000, div: 1
r: 8928571, a: 8928571.43, d: 0.0000%, tcr: 7, cpr: 1.000, div: 1
r: 7812500, a: 7812500.00, d: 0.0000%, tcr: 8, cpr: 1.000, div: 1
r: 4000000, a: 4000000.00, d: 0.0000%, tcr: 5, cpr: 3.125, div: 1
r: 3686400, a: 3676470.59, d: -0.2694%, tcr: 8, cpr: 2.125, div: 1
r: 3500000, a: 3496503.50, d: -0.0999%, tcr: 13, cpr: 1.375, div: 1
r: 3000000, a: 2976190.48, d: -0.7937%, tcr: 14, cpr: 1.500, div: 1
r: 2500000, a: 2500000.00, d: 0.0000%, tcr: 10, cpr: 2.500, div: 1
r: 2000000, a: 2000000.00, d: 0.0000%, tcr: 10, cpr: 3.125, div: 1
r: 1843200, a: 1838235.29, d: -0.2694%, tcr: 16, cpr: 2.125, div: 1
r: 1500000, a: 1492537.31, d: -0.4975%, tcr: 5, cpr: 8.375, div: 1
r: 1152000, a: 1152073.73, d: 0.0064%, tcr: 14, cpr: 3.875, div: 1
r: 921600, a: 919117.65, d: -0.2694%, tcr: 16, cpr: 2.125, div: 2
r: 576000, a: 576036.87, d: 0.0064%, tcr: 14, cpr: 3.875, div: 2
r: 460800, a: 460829.49, d: 0.0064%, tcr: 7, cpr: 3.875, div: 5
r: 230400, a: 230414.75, d: 0.0064%, tcr: 14, cpr: 3.875, div: 5
r: 115200, a: 115207.37, d: 0.0064%, tcr: 14, cpr: 1.250, div: 31
r: 57600, a: 57603.69, d: 0.0064%, tcr: 8, cpr: 3.875, div: 35
r: 38400, a: 38402.46, d: 0.0064%, tcr: 14, cpr: 3.875, div: 30
r: 19200, a: 19201.23, d: 0.0064%, tcr: 8, cpr: 3.875, div: 105
r: 9600, a: 9600.06, d: 0.0006%, tcr: 9, cpr: 1.125, div: 643
r: 4800, a: 4799.98, d: -0.0004%, tcr: 7, cpr: 2.875, div: 647
r: 2400, a: 2400.02, d: 0.0008%, tcr: 9, cpr: 2.250, div: 1286
r: 1200, a: 1200.00, d: 0.0000%, tcr: 14, cpr: 2.875, div: 1294
r: 300, a: 300.00, d: 0.0000%, tcr: 11, cpr: 2.625, div: 7215
r: 200, a: 200.00, d: 0.0000%, tcr: 16, cpr: 1.250, div: 15625
r: 150, a: 150.00, d: 0.0000%, tcr: 13, cpr: 2.250, div: 14245
r: 134, a: 134.00, d: 0.0000%, tcr: 11, cpr: 2.625, div: 16153
r: 110, a: 110.00, d: 0.0000%, tcr: 12, cpr: 1.000, div: 47348
r: 75, a: 75.00, d: 0.0000%, tcr: 4, cpr: 5.875, div: 35461
r: 50, a: 50.00, d: 0.0000%, tcr: 16, cpr: 1.250, div: 62500
r: 25, a: 25.00, d: 0.0000%, tcr: 16, cpr: 2.500, div: 62500
r: 4, a: 4.00, d: 0.0000%, tcr: 16, cpr: 20.000, div: 48828
r: 2, a: 2.00, d: 0.0000%, tcr: 16, cpr: 40.000, div: 48828
r: 1, a: 1.00, d: 0.0000%, tcr: 16, cpr: 63.875, div: 61154
serial: 8250: Add proper clock handling for OxSemi PCIe devices
Oxford Semiconductor PCIe (Tornado) 950 serial port devices are driven
by a fixed 62.5MHz clock input derived from the 100MHz PCI Express clock.
We currently drive the device using its default oversampling rate of 16
and the clock prescaler disabled, consequently yielding the baud base of
3906250. This base is inadequate for some of the high-speed baud rates
such as 460800bps, for which the closest rate possible can be obtained
by dividing the baud base by 8, yielding the baud rate of 488281.25bps,
which is off by 5.9638%. This is enough for data communication to break
with the remote end talking actual 460800bps, where missed stop bits
have been observed.
We can do better however, by taking advantage of a reduced oversampling
rate, which can be set to any integer value from 4 to 16 inclusive by
programming the TCR register, and by using the clock prescaler, which
can be set to any value from 1 to 63.875 in increments of 0.125 in the
CPR/CPR2 register pair. The prescaler has to be explicitly enabled
though by setting bit 7 in the MCR or otherwise it is bypassed (in the
enhanced mode that we enable) as if the value of 1 was used.
Make use of these features then as follows:
- Set the baud base to 15625000, reflecting the minimum oversampling
rate of 4 with the clock prescaler and divisor both set to 1.
- Override the `set_mctrl' and set the MCR shadow there so as to have
MCR[7] always set and have the 8250 core propagate these settings.
- Override the `get_divisor' handler and determine a good combination of
parameters by using a lookup table with predetermined value pairs of
the oversampling rate and the clock prescaler and finding a pair that
divides the input clock such that the quotient, when rounded to the
nearest integer, deviates the least from the exact result. Calculate
the clock divisor accordingly.
Scale the resulting oversampling rate (only by powers of two) if
possible so as to maximise it, reducing the divisor accordingly, and
avoid a divisor overflow for very low baud rates by scaling the
oversampling rate and/or the prescaler even if that causes some
accuracy loss.
Also handle the historic spd_cust feature so as to allow one to set
all the three parameters manually to arbitrary values, by keeping the
low 16 bits for the divisor and then putting TCR in bits 19:16 and
CPR/CPR2 in bits 28:20, sanitising the bit pattern supplied such as
to clamp CPR/CPR2 values between 0.000 and 0.875 inclusive to 33.875.
This preserves compatibility with any existing setups, that is where
requesting a custom divisor that only has any bits set among the low
16 the oversampling rate of 16 and the clock prescaler of 33.875 will
be used as with the original 8250.
Finally abuse the `frac' argument to store the determined bit patterns
for the TCR, CPR and CPR2 registers.
- Override the `set_divisor' handler so as to set the TCR, CPR and CPR2
registers from the `frac' value supplied. Set the divisor as usual.
With the baud base set to 15625000 and the unsigned 16-bit UART_DIV_MAX
limitation imposed by `serial8250_get_baud_rate' standard baud rates
below 300bps become unavailable in the regular way, e.g. the rate of
200bps requires the baud base to be divided by 78125 and that is beyond
the unsigned 16-bit range. The historic spd_cust feature can still be
used to obtain such rates if so required.
See Documentation/tty/device_drivers/oxsemi-tornado.rst for more details.
Signed-off-by: Maciej W. Rozycki <macro@orcam.me.uk>
Link: https://lore.kernel.org/r/alpine.DEB.2.21.2204181519450.9383@angie.orcam.me.uk
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
2022-04-18 18:27:33 +03:00
With the baud base set to 15625000 and the unsigned 16-bit UART_DIV_MAX
2022-05-14 01:42:08 +03:00
limitation imposed by `` serial8250_get_baud_rate `` standard baud rates
serial: 8250: Add proper clock handling for OxSemi PCIe devices
Oxford Semiconductor PCIe (Tornado) 950 serial port devices are driven
by a fixed 62.5MHz clock input derived from the 100MHz PCI Express clock.
We currently drive the device using its default oversampling rate of 16
and the clock prescaler disabled, consequently yielding the baud base of
3906250. This base is inadequate for some of the high-speed baud rates
such as 460800bps, for which the closest rate possible can be obtained
by dividing the baud base by 8, yielding the baud rate of 488281.25bps,
which is off by 5.9638%. This is enough for data communication to break
with the remote end talking actual 460800bps, where missed stop bits
have been observed.
We can do better however, by taking advantage of a reduced oversampling
rate, which can be set to any integer value from 4 to 16 inclusive by
programming the TCR register, and by using the clock prescaler, which
can be set to any value from 1 to 63.875 in increments of 0.125 in the
CPR/CPR2 register pair. The prescaler has to be explicitly enabled
though by setting bit 7 in the MCR or otherwise it is bypassed (in the
enhanced mode that we enable) as if the value of 1 was used.
Make use of these features then as follows:
- Set the baud base to 15625000, reflecting the minimum oversampling
rate of 4 with the clock prescaler and divisor both set to 1.
- Override the `set_mctrl' and set the MCR shadow there so as to have
MCR[7] always set and have the 8250 core propagate these settings.
- Override the `get_divisor' handler and determine a good combination of
parameters by using a lookup table with predetermined value pairs of
the oversampling rate and the clock prescaler and finding a pair that
divides the input clock such that the quotient, when rounded to the
nearest integer, deviates the least from the exact result. Calculate
the clock divisor accordingly.
Scale the resulting oversampling rate (only by powers of two) if
possible so as to maximise it, reducing the divisor accordingly, and
avoid a divisor overflow for very low baud rates by scaling the
oversampling rate and/or the prescaler even if that causes some
accuracy loss.
Also handle the historic spd_cust feature so as to allow one to set
all the three parameters manually to arbitrary values, by keeping the
low 16 bits for the divisor and then putting TCR in bits 19:16 and
CPR/CPR2 in bits 28:20, sanitising the bit pattern supplied such as
to clamp CPR/CPR2 values between 0.000 and 0.875 inclusive to 33.875.
This preserves compatibility with any existing setups, that is where
requesting a custom divisor that only has any bits set among the low
16 the oversampling rate of 16 and the clock prescaler of 33.875 will
be used as with the original 8250.
Finally abuse the `frac' argument to store the determined bit patterns
for the TCR, CPR and CPR2 registers.
- Override the `set_divisor' handler so as to set the TCR, CPR and CPR2
registers from the `frac' value supplied. Set the divisor as usual.
With the baud base set to 15625000 and the unsigned 16-bit UART_DIV_MAX
limitation imposed by `serial8250_get_baud_rate' standard baud rates
below 300bps become unavailable in the regular way, e.g. the rate of
200bps requires the baud base to be divided by 78125 and that is beyond
the unsigned 16-bit range. The historic spd_cust feature can still be
used to obtain such rates if so required.
See Documentation/tty/device_drivers/oxsemi-tornado.rst for more details.
Signed-off-by: Maciej W. Rozycki <macro@orcam.me.uk>
Link: https://lore.kernel.org/r/alpine.DEB.2.21.2204181519450.9383@angie.orcam.me.uk
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
2022-04-18 18:27:33 +03:00
below 300bps become unavailable in the regular way, e.g. the rate of
200bps requires the baud base to be divided by 78125 and that is beyond
the unsigned 16-bit range. The historic spd_cust feature can still be
used by encoding the values for, the prescaler, the oversampling rate
and the clock divisor (DLM/DLL) as follows to obtain such rates if so
required:
2022-05-14 01:42:08 +03:00
::
serial: 8250: Add proper clock handling for OxSemi PCIe devices
Oxford Semiconductor PCIe (Tornado) 950 serial port devices are driven
by a fixed 62.5MHz clock input derived from the 100MHz PCI Express clock.
We currently drive the device using its default oversampling rate of 16
and the clock prescaler disabled, consequently yielding the baud base of
3906250. This base is inadequate for some of the high-speed baud rates
such as 460800bps, for which the closest rate possible can be obtained
by dividing the baud base by 8, yielding the baud rate of 488281.25bps,
which is off by 5.9638%. This is enough for data communication to break
with the remote end talking actual 460800bps, where missed stop bits
have been observed.
We can do better however, by taking advantage of a reduced oversampling
rate, which can be set to any integer value from 4 to 16 inclusive by
programming the TCR register, and by using the clock prescaler, which
can be set to any value from 1 to 63.875 in increments of 0.125 in the
CPR/CPR2 register pair. The prescaler has to be explicitly enabled
though by setting bit 7 in the MCR or otherwise it is bypassed (in the
enhanced mode that we enable) as if the value of 1 was used.
Make use of these features then as follows:
- Set the baud base to 15625000, reflecting the minimum oversampling
rate of 4 with the clock prescaler and divisor both set to 1.
- Override the `set_mctrl' and set the MCR shadow there so as to have
MCR[7] always set and have the 8250 core propagate these settings.
- Override the `get_divisor' handler and determine a good combination of
parameters by using a lookup table with predetermined value pairs of
the oversampling rate and the clock prescaler and finding a pair that
divides the input clock such that the quotient, when rounded to the
nearest integer, deviates the least from the exact result. Calculate
the clock divisor accordingly.
Scale the resulting oversampling rate (only by powers of two) if
possible so as to maximise it, reducing the divisor accordingly, and
avoid a divisor overflow for very low baud rates by scaling the
oversampling rate and/or the prescaler even if that causes some
accuracy loss.
Also handle the historic spd_cust feature so as to allow one to set
all the three parameters manually to arbitrary values, by keeping the
low 16 bits for the divisor and then putting TCR in bits 19:16 and
CPR/CPR2 in bits 28:20, sanitising the bit pattern supplied such as
to clamp CPR/CPR2 values between 0.000 and 0.875 inclusive to 33.875.
This preserves compatibility with any existing setups, that is where
requesting a custom divisor that only has any bits set among the low
16 the oversampling rate of 16 and the clock prescaler of 33.875 will
be used as with the original 8250.
Finally abuse the `frac' argument to store the determined bit patterns
for the TCR, CPR and CPR2 registers.
- Override the `set_divisor' handler so as to set the TCR, CPR and CPR2
registers from the `frac' value supplied. Set the divisor as usual.
With the baud base set to 15625000 and the unsigned 16-bit UART_DIV_MAX
limitation imposed by `serial8250_get_baud_rate' standard baud rates
below 300bps become unavailable in the regular way, e.g. the rate of
200bps requires the baud base to be divided by 78125 and that is beyond
the unsigned 16-bit range. The historic spd_cust feature can still be
used to obtain such rates if so required.
See Documentation/tty/device_drivers/oxsemi-tornado.rst for more details.
Signed-off-by: Maciej W. Rozycki <macro@orcam.me.uk>
Link: https://lore.kernel.org/r/alpine.DEB.2.21.2204181519450.9383@angie.orcam.me.uk
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
2022-04-18 18:27:33 +03:00
2022-05-14 01:42:08 +03:00
31 29 28 20 19 16 15 0
+-----+-----------------+-------+-------------------------------+
|0 0 0| CPR2:CPR | TCR | DLM:DLL |
+-----+-----------------+-------+-------------------------------+
Use a value such encoded for the `` custom_divisor `` field along with the
ASYNC_SPD_CUST flag set in the `` flags `` field in `` struct serial_struct ``
serial: 8250: Add proper clock handling for OxSemi PCIe devices
Oxford Semiconductor PCIe (Tornado) 950 serial port devices are driven
by a fixed 62.5MHz clock input derived from the 100MHz PCI Express clock.
We currently drive the device using its default oversampling rate of 16
and the clock prescaler disabled, consequently yielding the baud base of
3906250. This base is inadequate for some of the high-speed baud rates
such as 460800bps, for which the closest rate possible can be obtained
by dividing the baud base by 8, yielding the baud rate of 488281.25bps,
which is off by 5.9638%. This is enough for data communication to break
with the remote end talking actual 460800bps, where missed stop bits
have been observed.
We can do better however, by taking advantage of a reduced oversampling
rate, which can be set to any integer value from 4 to 16 inclusive by
programming the TCR register, and by using the clock prescaler, which
can be set to any value from 1 to 63.875 in increments of 0.125 in the
CPR/CPR2 register pair. The prescaler has to be explicitly enabled
though by setting bit 7 in the MCR or otherwise it is bypassed (in the
enhanced mode that we enable) as if the value of 1 was used.
Make use of these features then as follows:
- Set the baud base to 15625000, reflecting the minimum oversampling
rate of 4 with the clock prescaler and divisor both set to 1.
- Override the `set_mctrl' and set the MCR shadow there so as to have
MCR[7] always set and have the 8250 core propagate these settings.
- Override the `get_divisor' handler and determine a good combination of
parameters by using a lookup table with predetermined value pairs of
the oversampling rate and the clock prescaler and finding a pair that
divides the input clock such that the quotient, when rounded to the
nearest integer, deviates the least from the exact result. Calculate
the clock divisor accordingly.
Scale the resulting oversampling rate (only by powers of two) if
possible so as to maximise it, reducing the divisor accordingly, and
avoid a divisor overflow for very low baud rates by scaling the
oversampling rate and/or the prescaler even if that causes some
accuracy loss.
Also handle the historic spd_cust feature so as to allow one to set
all the three parameters manually to arbitrary values, by keeping the
low 16 bits for the divisor and then putting TCR in bits 19:16 and
CPR/CPR2 in bits 28:20, sanitising the bit pattern supplied such as
to clamp CPR/CPR2 values between 0.000 and 0.875 inclusive to 33.875.
This preserves compatibility with any existing setups, that is where
requesting a custom divisor that only has any bits set among the low
16 the oversampling rate of 16 and the clock prescaler of 33.875 will
be used as with the original 8250.
Finally abuse the `frac' argument to store the determined bit patterns
for the TCR, CPR and CPR2 registers.
- Override the `set_divisor' handler so as to set the TCR, CPR and CPR2
registers from the `frac' value supplied. Set the divisor as usual.
With the baud base set to 15625000 and the unsigned 16-bit UART_DIV_MAX
limitation imposed by `serial8250_get_baud_rate' standard baud rates
below 300bps become unavailable in the regular way, e.g. the rate of
200bps requires the baud base to be divided by 78125 and that is beyond
the unsigned 16-bit range. The historic spd_cust feature can still be
used to obtain such rates if so required.
See Documentation/tty/device_drivers/oxsemi-tornado.rst for more details.
Signed-off-by: Maciej W. Rozycki <macro@orcam.me.uk>
Link: https://lore.kernel.org/r/alpine.DEB.2.21.2204181519450.9383@angie.orcam.me.uk
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
2022-04-18 18:27:33 +03:00
passed with the TIOCSSERIAL ioctl(2), such as with the setserial(8)
2022-05-14 01:42:08 +03:00
utility and its `` divisor `` and `` spd_cust `` parameters, and then select
serial: 8250: Add proper clock handling for OxSemi PCIe devices
Oxford Semiconductor PCIe (Tornado) 950 serial port devices are driven
by a fixed 62.5MHz clock input derived from the 100MHz PCI Express clock.
We currently drive the device using its default oversampling rate of 16
and the clock prescaler disabled, consequently yielding the baud base of
3906250. This base is inadequate for some of the high-speed baud rates
such as 460800bps, for which the closest rate possible can be obtained
by dividing the baud base by 8, yielding the baud rate of 488281.25bps,
which is off by 5.9638%. This is enough for data communication to break
with the remote end talking actual 460800bps, where missed stop bits
have been observed.
We can do better however, by taking advantage of a reduced oversampling
rate, which can be set to any integer value from 4 to 16 inclusive by
programming the TCR register, and by using the clock prescaler, which
can be set to any value from 1 to 63.875 in increments of 0.125 in the
CPR/CPR2 register pair. The prescaler has to be explicitly enabled
though by setting bit 7 in the MCR or otherwise it is bypassed (in the
enhanced mode that we enable) as if the value of 1 was used.
Make use of these features then as follows:
- Set the baud base to 15625000, reflecting the minimum oversampling
rate of 4 with the clock prescaler and divisor both set to 1.
- Override the `set_mctrl' and set the MCR shadow there so as to have
MCR[7] always set and have the 8250 core propagate these settings.
- Override the `get_divisor' handler and determine a good combination of
parameters by using a lookup table with predetermined value pairs of
the oversampling rate and the clock prescaler and finding a pair that
divides the input clock such that the quotient, when rounded to the
nearest integer, deviates the least from the exact result. Calculate
the clock divisor accordingly.
Scale the resulting oversampling rate (only by powers of two) if
possible so as to maximise it, reducing the divisor accordingly, and
avoid a divisor overflow for very low baud rates by scaling the
oversampling rate and/or the prescaler even if that causes some
accuracy loss.
Also handle the historic spd_cust feature so as to allow one to set
all the three parameters manually to arbitrary values, by keeping the
low 16 bits for the divisor and then putting TCR in bits 19:16 and
CPR/CPR2 in bits 28:20, sanitising the bit pattern supplied such as
to clamp CPR/CPR2 values between 0.000 and 0.875 inclusive to 33.875.
This preserves compatibility with any existing setups, that is where
requesting a custom divisor that only has any bits set among the low
16 the oversampling rate of 16 and the clock prescaler of 33.875 will
be used as with the original 8250.
Finally abuse the `frac' argument to store the determined bit patterns
for the TCR, CPR and CPR2 registers.
- Override the `set_divisor' handler so as to set the TCR, CPR and CPR2
registers from the `frac' value supplied. Set the divisor as usual.
With the baud base set to 15625000 and the unsigned 16-bit UART_DIV_MAX
limitation imposed by `serial8250_get_baud_rate' standard baud rates
below 300bps become unavailable in the regular way, e.g. the rate of
200bps requires the baud base to be divided by 78125 and that is beyond
the unsigned 16-bit range. The historic spd_cust feature can still be
used to obtain such rates if so required.
See Documentation/tty/device_drivers/oxsemi-tornado.rst for more details.
Signed-off-by: Maciej W. Rozycki <macro@orcam.me.uk>
Link: https://lore.kernel.org/r/alpine.DEB.2.21.2204181519450.9383@angie.orcam.me.uk
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
2022-04-18 18:27:33 +03:00
the baud rate of 38400bps. Note that the value of 0 in TCR sets the
oversampling rate to 16 and prescaler values below 1 in CPR2/CPR are
clamped by the driver to 1.
For example the value of 0x1f4004e2 will set CPR2/CPR, TCR and DLM/DLL
respectively to 0x1f4, 0x0 and 0x04e2, choosing the prescaler value,
the oversampling rate and the clock divisor of 62.500, 16 and 1250
respectively. These parameters will set the baud rate for the serial
port to 62500000 / 62.500 / 1250 / 16 = 50bps.
2022-05-14 01:42:08 +03:00
Maciej W. Rozycki <macro@orcam.me.uk>
serial: 8250: Add proper clock handling for OxSemi PCIe devices
Oxford Semiconductor PCIe (Tornado) 950 serial port devices are driven
by a fixed 62.5MHz clock input derived from the 100MHz PCI Express clock.
We currently drive the device using its default oversampling rate of 16
and the clock prescaler disabled, consequently yielding the baud base of
3906250. This base is inadequate for some of the high-speed baud rates
such as 460800bps, for which the closest rate possible can be obtained
by dividing the baud base by 8, yielding the baud rate of 488281.25bps,
which is off by 5.9638%. This is enough for data communication to break
with the remote end talking actual 460800bps, where missed stop bits
have been observed.
We can do better however, by taking advantage of a reduced oversampling
rate, which can be set to any integer value from 4 to 16 inclusive by
programming the TCR register, and by using the clock prescaler, which
can be set to any value from 1 to 63.875 in increments of 0.125 in the
CPR/CPR2 register pair. The prescaler has to be explicitly enabled
though by setting bit 7 in the MCR or otherwise it is bypassed (in the
enhanced mode that we enable) as if the value of 1 was used.
Make use of these features then as follows:
- Set the baud base to 15625000, reflecting the minimum oversampling
rate of 4 with the clock prescaler and divisor both set to 1.
- Override the `set_mctrl' and set the MCR shadow there so as to have
MCR[7] always set and have the 8250 core propagate these settings.
- Override the `get_divisor' handler and determine a good combination of
parameters by using a lookup table with predetermined value pairs of
the oversampling rate and the clock prescaler and finding a pair that
divides the input clock such that the quotient, when rounded to the
nearest integer, deviates the least from the exact result. Calculate
the clock divisor accordingly.
Scale the resulting oversampling rate (only by powers of two) if
possible so as to maximise it, reducing the divisor accordingly, and
avoid a divisor overflow for very low baud rates by scaling the
oversampling rate and/or the prescaler even if that causes some
accuracy loss.
Also handle the historic spd_cust feature so as to allow one to set
all the three parameters manually to arbitrary values, by keeping the
low 16 bits for the divisor and then putting TCR in bits 19:16 and
CPR/CPR2 in bits 28:20, sanitising the bit pattern supplied such as
to clamp CPR/CPR2 values between 0.000 and 0.875 inclusive to 33.875.
This preserves compatibility with any existing setups, that is where
requesting a custom divisor that only has any bits set among the low
16 the oversampling rate of 16 and the clock prescaler of 33.875 will
be used as with the original 8250.
Finally abuse the `frac' argument to store the determined bit patterns
for the TCR, CPR and CPR2 registers.
- Override the `set_divisor' handler so as to set the TCR, CPR and CPR2
registers from the `frac' value supplied. Set the divisor as usual.
With the baud base set to 15625000 and the unsigned 16-bit UART_DIV_MAX
limitation imposed by `serial8250_get_baud_rate' standard baud rates
below 300bps become unavailable in the regular way, e.g. the rate of
200bps requires the baud base to be divided by 78125 and that is beyond
the unsigned 16-bit range. The historic spd_cust feature can still be
used to obtain such rates if so required.
See Documentation/tty/device_drivers/oxsemi-tornado.rst for more details.
Signed-off-by: Maciej W. Rozycki <macro@orcam.me.uk>
Link: https://lore.kernel.org/r/alpine.DEB.2.21.2204181519450.9383@angie.orcam.me.uk
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
2022-04-18 18:27:33 +03:00
2022-05-14 01:42:08 +03:00
.. [OX200] "OXPCIe200 PCI Express Multi-Port Bridge", Oxford Semiconductor,
Inc., DS-0045, 10 Nov 2008, Section "950 Mode", pp. 64-65
serial: 8250: Add proper clock handling for OxSemi PCIe devices
Oxford Semiconductor PCIe (Tornado) 950 serial port devices are driven
by a fixed 62.5MHz clock input derived from the 100MHz PCI Express clock.
We currently drive the device using its default oversampling rate of 16
and the clock prescaler disabled, consequently yielding the baud base of
3906250. This base is inadequate for some of the high-speed baud rates
such as 460800bps, for which the closest rate possible can be obtained
by dividing the baud base by 8, yielding the baud rate of 488281.25bps,
which is off by 5.9638%. This is enough for data communication to break
with the remote end talking actual 460800bps, where missed stop bits
have been observed.
We can do better however, by taking advantage of a reduced oversampling
rate, which can be set to any integer value from 4 to 16 inclusive by
programming the TCR register, and by using the clock prescaler, which
can be set to any value from 1 to 63.875 in increments of 0.125 in the
CPR/CPR2 register pair. The prescaler has to be explicitly enabled
though by setting bit 7 in the MCR or otherwise it is bypassed (in the
enhanced mode that we enable) as if the value of 1 was used.
Make use of these features then as follows:
- Set the baud base to 15625000, reflecting the minimum oversampling
rate of 4 with the clock prescaler and divisor both set to 1.
- Override the `set_mctrl' and set the MCR shadow there so as to have
MCR[7] always set and have the 8250 core propagate these settings.
- Override the `get_divisor' handler and determine a good combination of
parameters by using a lookup table with predetermined value pairs of
the oversampling rate and the clock prescaler and finding a pair that
divides the input clock such that the quotient, when rounded to the
nearest integer, deviates the least from the exact result. Calculate
the clock divisor accordingly.
Scale the resulting oversampling rate (only by powers of two) if
possible so as to maximise it, reducing the divisor accordingly, and
avoid a divisor overflow for very low baud rates by scaling the
oversampling rate and/or the prescaler even if that causes some
accuracy loss.
Also handle the historic spd_cust feature so as to allow one to set
all the three parameters manually to arbitrary values, by keeping the
low 16 bits for the divisor and then putting TCR in bits 19:16 and
CPR/CPR2 in bits 28:20, sanitising the bit pattern supplied such as
to clamp CPR/CPR2 values between 0.000 and 0.875 inclusive to 33.875.
This preserves compatibility with any existing setups, that is where
requesting a custom divisor that only has any bits set among the low
16 the oversampling rate of 16 and the clock prescaler of 33.875 will
be used as with the original 8250.
Finally abuse the `frac' argument to store the determined bit patterns
for the TCR, CPR and CPR2 registers.
- Override the `set_divisor' handler so as to set the TCR, CPR and CPR2
registers from the `frac' value supplied. Set the divisor as usual.
With the baud base set to 15625000 and the unsigned 16-bit UART_DIV_MAX
limitation imposed by `serial8250_get_baud_rate' standard baud rates
below 300bps become unavailable in the regular way, e.g. the rate of
200bps requires the baud base to be divided by 78125 and that is beyond
the unsigned 16-bit range. The historic spd_cust feature can still be
used to obtain such rates if so required.
See Documentation/tty/device_drivers/oxsemi-tornado.rst for more details.
Signed-off-by: Maciej W. Rozycki <macro@orcam.me.uk>
Link: https://lore.kernel.org/r/alpine.DEB.2.21.2204181519450.9383@angie.orcam.me.uk
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
2022-04-18 18:27:33 +03:00
2022-05-14 01:42:08 +03:00
.. [OX952] "OXPCIe952 PCI Express Bridge to Dual Serial & Parallel Port",
Oxford Semiconductor, Inc., DS-0046, Mar 06 08, Section "950 Mode",
p. 20
serial: 8250: Add proper clock handling for OxSemi PCIe devices
Oxford Semiconductor PCIe (Tornado) 950 serial port devices are driven
by a fixed 62.5MHz clock input derived from the 100MHz PCI Express clock.
We currently drive the device using its default oversampling rate of 16
and the clock prescaler disabled, consequently yielding the baud base of
3906250. This base is inadequate for some of the high-speed baud rates
such as 460800bps, for which the closest rate possible can be obtained
by dividing the baud base by 8, yielding the baud rate of 488281.25bps,
which is off by 5.9638%. This is enough for data communication to break
with the remote end talking actual 460800bps, where missed stop bits
have been observed.
We can do better however, by taking advantage of a reduced oversampling
rate, which can be set to any integer value from 4 to 16 inclusive by
programming the TCR register, and by using the clock prescaler, which
can be set to any value from 1 to 63.875 in increments of 0.125 in the
CPR/CPR2 register pair. The prescaler has to be explicitly enabled
though by setting bit 7 in the MCR or otherwise it is bypassed (in the
enhanced mode that we enable) as if the value of 1 was used.
Make use of these features then as follows:
- Set the baud base to 15625000, reflecting the minimum oversampling
rate of 4 with the clock prescaler and divisor both set to 1.
- Override the `set_mctrl' and set the MCR shadow there so as to have
MCR[7] always set and have the 8250 core propagate these settings.
- Override the `get_divisor' handler and determine a good combination of
parameters by using a lookup table with predetermined value pairs of
the oversampling rate and the clock prescaler and finding a pair that
divides the input clock such that the quotient, when rounded to the
nearest integer, deviates the least from the exact result. Calculate
the clock divisor accordingly.
Scale the resulting oversampling rate (only by powers of two) if
possible so as to maximise it, reducing the divisor accordingly, and
avoid a divisor overflow for very low baud rates by scaling the
oversampling rate and/or the prescaler even if that causes some
accuracy loss.
Also handle the historic spd_cust feature so as to allow one to set
all the three parameters manually to arbitrary values, by keeping the
low 16 bits for the divisor and then putting TCR in bits 19:16 and
CPR/CPR2 in bits 28:20, sanitising the bit pattern supplied such as
to clamp CPR/CPR2 values between 0.000 and 0.875 inclusive to 33.875.
This preserves compatibility with any existing setups, that is where
requesting a custom divisor that only has any bits set among the low
16 the oversampling rate of 16 and the clock prescaler of 33.875 will
be used as with the original 8250.
Finally abuse the `frac' argument to store the determined bit patterns
for the TCR, CPR and CPR2 registers.
- Override the `set_divisor' handler so as to set the TCR, CPR and CPR2
registers from the `frac' value supplied. Set the divisor as usual.
With the baud base set to 15625000 and the unsigned 16-bit UART_DIV_MAX
limitation imposed by `serial8250_get_baud_rate' standard baud rates
below 300bps become unavailable in the regular way, e.g. the rate of
200bps requires the baud base to be divided by 78125 and that is beyond
the unsigned 16-bit range. The historic spd_cust feature can still be
used to obtain such rates if so required.
See Documentation/tty/device_drivers/oxsemi-tornado.rst for more details.
Signed-off-by: Maciej W. Rozycki <macro@orcam.me.uk>
Link: https://lore.kernel.org/r/alpine.DEB.2.21.2204181519450.9383@angie.orcam.me.uk
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
2022-04-18 18:27:33 +03:00
2022-05-14 01:42:08 +03:00
.. [OX954] "OXPCIe954 PCI Express Bridge to Quad Serial Port", Oxford
Semiconductor, Inc., DS-0047, Feb 08, Section "950 Mode", p. 20
serial: 8250: Add proper clock handling for OxSemi PCIe devices
Oxford Semiconductor PCIe (Tornado) 950 serial port devices are driven
by a fixed 62.5MHz clock input derived from the 100MHz PCI Express clock.
We currently drive the device using its default oversampling rate of 16
and the clock prescaler disabled, consequently yielding the baud base of
3906250. This base is inadequate for some of the high-speed baud rates
such as 460800bps, for which the closest rate possible can be obtained
by dividing the baud base by 8, yielding the baud rate of 488281.25bps,
which is off by 5.9638%. This is enough for data communication to break
with the remote end talking actual 460800bps, where missed stop bits
have been observed.
We can do better however, by taking advantage of a reduced oversampling
rate, which can be set to any integer value from 4 to 16 inclusive by
programming the TCR register, and by using the clock prescaler, which
can be set to any value from 1 to 63.875 in increments of 0.125 in the
CPR/CPR2 register pair. The prescaler has to be explicitly enabled
though by setting bit 7 in the MCR or otherwise it is bypassed (in the
enhanced mode that we enable) as if the value of 1 was used.
Make use of these features then as follows:
- Set the baud base to 15625000, reflecting the minimum oversampling
rate of 4 with the clock prescaler and divisor both set to 1.
- Override the `set_mctrl' and set the MCR shadow there so as to have
MCR[7] always set and have the 8250 core propagate these settings.
- Override the `get_divisor' handler and determine a good combination of
parameters by using a lookup table with predetermined value pairs of
the oversampling rate and the clock prescaler and finding a pair that
divides the input clock such that the quotient, when rounded to the
nearest integer, deviates the least from the exact result. Calculate
the clock divisor accordingly.
Scale the resulting oversampling rate (only by powers of two) if
possible so as to maximise it, reducing the divisor accordingly, and
avoid a divisor overflow for very low baud rates by scaling the
oversampling rate and/or the prescaler even if that causes some
accuracy loss.
Also handle the historic spd_cust feature so as to allow one to set
all the three parameters manually to arbitrary values, by keeping the
low 16 bits for the divisor and then putting TCR in bits 19:16 and
CPR/CPR2 in bits 28:20, sanitising the bit pattern supplied such as
to clamp CPR/CPR2 values between 0.000 and 0.875 inclusive to 33.875.
This preserves compatibility with any existing setups, that is where
requesting a custom divisor that only has any bits set among the low
16 the oversampling rate of 16 and the clock prescaler of 33.875 will
be used as with the original 8250.
Finally abuse the `frac' argument to store the determined bit patterns
for the TCR, CPR and CPR2 registers.
- Override the `set_divisor' handler so as to set the TCR, CPR and CPR2
registers from the `frac' value supplied. Set the divisor as usual.
With the baud base set to 15625000 and the unsigned 16-bit UART_DIV_MAX
limitation imposed by `serial8250_get_baud_rate' standard baud rates
below 300bps become unavailable in the regular way, e.g. the rate of
200bps requires the baud base to be divided by 78125 and that is beyond
the unsigned 16-bit range. The historic spd_cust feature can still be
used to obtain such rates if so required.
See Documentation/tty/device_drivers/oxsemi-tornado.rst for more details.
Signed-off-by: Maciej W. Rozycki <macro@orcam.me.uk>
Link: https://lore.kernel.org/r/alpine.DEB.2.21.2204181519450.9383@angie.orcam.me.uk
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
2022-04-18 18:27:33 +03:00
2022-05-14 01:42:08 +03:00
.. [OX958] "OXPCIe958 PCI Express Bridge to Octal Serial Port", Oxford
Semiconductor, Inc., DS-0048, Feb 08, Section "950 Mode", p. 20
