linux/drivers/rtc/rtc-mc146818-lib.c

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// SPDX-License-Identifier: GPL-2.0-only
#include <linux/bcd.h>
#include <linux/delay.h>
#include <linux/export.h>
#include <linux/mc146818rtc.h>
#ifdef CONFIG_ACPI
#include <linux/acpi.h>
#endif
unsigned int mc146818_get_time(struct rtc_time *time)
{
unsigned char ctrl;
unsigned long flags;
unsigned char century = 0;
rtc: mc146818: Prevent reading garbage The MC146818 driver is prone to read garbage from the RTC. There are several issues all related to the update cycle of the MC146818. The chip increments seconds obviously once per second and indicates that by a bit in a register. The bit goes high 244us before the actual update starts. During the update the readout of the time values is undefined. The code just checks whether the update in progress bit (UIP) is set before reading the clock. If it's set it waits arbitrary 20ms before retrying, which is ample because the maximum update time is ~2ms. But this check does not guarantee that the UIP bit goes high and the actual update happens during the readout. So the following can happen 0.997 UIP = False -> Interrupt/NMI/preemption 0.998 UIP -> True 0.999 Readout <- Undefined To prevent this rework the code so it checks UIP before and after the readout and if set after the readout try again. But that's not enough to cover the following: 0.997 UIP = False Readout seconds -> NMI (or vCPU scheduled out) 0.998 UIP -> True update completes UIP -> False 1.000 Readout minutes,.... UIP check succeeds That can make the readout wrong up to 59 seconds. To prevent this, read the seconds value before the first UIP check, validate it after checking UIP and after reading out the rest. It's amazing that the original i386 code had this actually correct and the generic implementation of the MC146818 driver got it wrong in 2002 and it stayed that way until today. Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com> Link: https://lore.kernel.org/r/20201206220541.594826678@linutronix.de
2020-12-07 00:46:14 +03:00
bool retry;
#ifdef CONFIG_MACH_DECSTATION
unsigned int real_year;
#endif
rtc: mc146818: Prevent reading garbage The MC146818 driver is prone to read garbage from the RTC. There are several issues all related to the update cycle of the MC146818. The chip increments seconds obviously once per second and indicates that by a bit in a register. The bit goes high 244us before the actual update starts. During the update the readout of the time values is undefined. The code just checks whether the update in progress bit (UIP) is set before reading the clock. If it's set it waits arbitrary 20ms before retrying, which is ample because the maximum update time is ~2ms. But this check does not guarantee that the UIP bit goes high and the actual update happens during the readout. So the following can happen 0.997 UIP = False -> Interrupt/NMI/preemption 0.998 UIP -> True 0.999 Readout <- Undefined To prevent this rework the code so it checks UIP before and after the readout and if set after the readout try again. But that's not enough to cover the following: 0.997 UIP = False Readout seconds -> NMI (or vCPU scheduled out) 0.998 UIP -> True update completes UIP -> False 1.000 Readout minutes,.... UIP check succeeds That can make the readout wrong up to 59 seconds. To prevent this, read the seconds value before the first UIP check, validate it after checking UIP and after reading out the rest. It's amazing that the original i386 code had this actually correct and the generic implementation of the MC146818 driver got it wrong in 2002 and it stayed that way until today. Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com> Link: https://lore.kernel.org/r/20201206220541.594826678@linutronix.de
2020-12-07 00:46:14 +03:00
again:
spin_lock_irqsave(&rtc_lock, flags);
/* Ensure that the RTC is accessible. Bit 6 must be 0! */
if (WARN_ON_ONCE((CMOS_READ(RTC_VALID) & 0x40) != 0)) {
spin_unlock_irqrestore(&rtc_lock, flags);
memset(time, 0xff, sizeof(*time));
return 0;
}
/*
rtc: mc146818: Prevent reading garbage The MC146818 driver is prone to read garbage from the RTC. There are several issues all related to the update cycle of the MC146818. The chip increments seconds obviously once per second and indicates that by a bit in a register. The bit goes high 244us before the actual update starts. During the update the readout of the time values is undefined. The code just checks whether the update in progress bit (UIP) is set before reading the clock. If it's set it waits arbitrary 20ms before retrying, which is ample because the maximum update time is ~2ms. But this check does not guarantee that the UIP bit goes high and the actual update happens during the readout. So the following can happen 0.997 UIP = False -> Interrupt/NMI/preemption 0.998 UIP -> True 0.999 Readout <- Undefined To prevent this rework the code so it checks UIP before and after the readout and if set after the readout try again. But that's not enough to cover the following: 0.997 UIP = False Readout seconds -> NMI (or vCPU scheduled out) 0.998 UIP -> True update completes UIP -> False 1.000 Readout minutes,.... UIP check succeeds That can make the readout wrong up to 59 seconds. To prevent this, read the seconds value before the first UIP check, validate it after checking UIP and after reading out the rest. It's amazing that the original i386 code had this actually correct and the generic implementation of the MC146818 driver got it wrong in 2002 and it stayed that way until today. Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com> Link: https://lore.kernel.org/r/20201206220541.594826678@linutronix.de
2020-12-07 00:46:14 +03:00
* Check whether there is an update in progress during which the
* readout is unspecified. The maximum update time is ~2ms. Poll
* every msec for completion.
*
* Store the second value before checking UIP so a long lasting NMI
* which happens to hit after the UIP check cannot make an update
* cycle invisible.
*/
rtc: mc146818: Prevent reading garbage The MC146818 driver is prone to read garbage from the RTC. There are several issues all related to the update cycle of the MC146818. The chip increments seconds obviously once per second and indicates that by a bit in a register. The bit goes high 244us before the actual update starts. During the update the readout of the time values is undefined. The code just checks whether the update in progress bit (UIP) is set before reading the clock. If it's set it waits arbitrary 20ms before retrying, which is ample because the maximum update time is ~2ms. But this check does not guarantee that the UIP bit goes high and the actual update happens during the readout. So the following can happen 0.997 UIP = False -> Interrupt/NMI/preemption 0.998 UIP -> True 0.999 Readout <- Undefined To prevent this rework the code so it checks UIP before and after the readout and if set after the readout try again. But that's not enough to cover the following: 0.997 UIP = False Readout seconds -> NMI (or vCPU scheduled out) 0.998 UIP -> True update completes UIP -> False 1.000 Readout minutes,.... UIP check succeeds That can make the readout wrong up to 59 seconds. To prevent this, read the seconds value before the first UIP check, validate it after checking UIP and after reading out the rest. It's amazing that the original i386 code had this actually correct and the generic implementation of the MC146818 driver got it wrong in 2002 and it stayed that way until today. Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com> Link: https://lore.kernel.org/r/20201206220541.594826678@linutronix.de
2020-12-07 00:46:14 +03:00
time->tm_sec = CMOS_READ(RTC_SECONDS);
if (CMOS_READ(RTC_FREQ_SELECT) & RTC_UIP) {
spin_unlock_irqrestore(&rtc_lock, flags);
mdelay(1);
goto again;
}
/* Revalidate the above readout */
if (time->tm_sec != CMOS_READ(RTC_SECONDS)) {
spin_unlock_irqrestore(&rtc_lock, flags);
goto again;
}
/*
* Only the values that we read from the RTC are set. We leave
* tm_wday, tm_yday and tm_isdst untouched. Even though the
* RTC has RTC_DAY_OF_WEEK, we ignore it, as it is only updated
* by the RTC when initially set to a non-zero value.
*/
time->tm_min = CMOS_READ(RTC_MINUTES);
time->tm_hour = CMOS_READ(RTC_HOURS);
time->tm_mday = CMOS_READ(RTC_DAY_OF_MONTH);
time->tm_mon = CMOS_READ(RTC_MONTH);
time->tm_year = CMOS_READ(RTC_YEAR);
#ifdef CONFIG_MACH_DECSTATION
real_year = CMOS_READ(RTC_DEC_YEAR);
#endif
#ifdef CONFIG_ACPI
if (acpi_gbl_FADT.header.revision >= FADT2_REVISION_ID &&
acpi_gbl_FADT.century)
century = CMOS_READ(acpi_gbl_FADT.century);
#endif
ctrl = CMOS_READ(RTC_CONTROL);
rtc: mc146818: Prevent reading garbage The MC146818 driver is prone to read garbage from the RTC. There are several issues all related to the update cycle of the MC146818. The chip increments seconds obviously once per second and indicates that by a bit in a register. The bit goes high 244us before the actual update starts. During the update the readout of the time values is undefined. The code just checks whether the update in progress bit (UIP) is set before reading the clock. If it's set it waits arbitrary 20ms before retrying, which is ample because the maximum update time is ~2ms. But this check does not guarantee that the UIP bit goes high and the actual update happens during the readout. So the following can happen 0.997 UIP = False -> Interrupt/NMI/preemption 0.998 UIP -> True 0.999 Readout <- Undefined To prevent this rework the code so it checks UIP before and after the readout and if set after the readout try again. But that's not enough to cover the following: 0.997 UIP = False Readout seconds -> NMI (or vCPU scheduled out) 0.998 UIP -> True update completes UIP -> False 1.000 Readout minutes,.... UIP check succeeds That can make the readout wrong up to 59 seconds. To prevent this, read the seconds value before the first UIP check, validate it after checking UIP and after reading out the rest. It's amazing that the original i386 code had this actually correct and the generic implementation of the MC146818 driver got it wrong in 2002 and it stayed that way until today. Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com> Link: https://lore.kernel.org/r/20201206220541.594826678@linutronix.de
2020-12-07 00:46:14 +03:00
/*
* Check for the UIP bit again. If it is set now then
* the above values may contain garbage.
*/
retry = CMOS_READ(RTC_FREQ_SELECT) & RTC_UIP;
/*
* A NMI might have interrupted the above sequence so check whether
* the seconds value has changed which indicates that the NMI took
* longer than the UIP bit was set. Unlikely, but possible and
* there is also virt...
*/
retry |= time->tm_sec != CMOS_READ(RTC_SECONDS);
spin_unlock_irqrestore(&rtc_lock, flags);
rtc: mc146818: Prevent reading garbage The MC146818 driver is prone to read garbage from the RTC. There are several issues all related to the update cycle of the MC146818. The chip increments seconds obviously once per second and indicates that by a bit in a register. The bit goes high 244us before the actual update starts. During the update the readout of the time values is undefined. The code just checks whether the update in progress bit (UIP) is set before reading the clock. If it's set it waits arbitrary 20ms before retrying, which is ample because the maximum update time is ~2ms. But this check does not guarantee that the UIP bit goes high and the actual update happens during the readout. So the following can happen 0.997 UIP = False -> Interrupt/NMI/preemption 0.998 UIP -> True 0.999 Readout <- Undefined To prevent this rework the code so it checks UIP before and after the readout and if set after the readout try again. But that's not enough to cover the following: 0.997 UIP = False Readout seconds -> NMI (or vCPU scheduled out) 0.998 UIP -> True update completes UIP -> False 1.000 Readout minutes,.... UIP check succeeds That can make the readout wrong up to 59 seconds. To prevent this, read the seconds value before the first UIP check, validate it after checking UIP and after reading out the rest. It's amazing that the original i386 code had this actually correct and the generic implementation of the MC146818 driver got it wrong in 2002 and it stayed that way until today. Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com> Link: https://lore.kernel.org/r/20201206220541.594826678@linutronix.de
2020-12-07 00:46:14 +03:00
if (retry)
goto again;
if (!(ctrl & RTC_DM_BINARY) || RTC_ALWAYS_BCD)
{
time->tm_sec = bcd2bin(time->tm_sec);
time->tm_min = bcd2bin(time->tm_min);
time->tm_hour = bcd2bin(time->tm_hour);
time->tm_mday = bcd2bin(time->tm_mday);
time->tm_mon = bcd2bin(time->tm_mon);
time->tm_year = bcd2bin(time->tm_year);
century = bcd2bin(century);
}
#ifdef CONFIG_MACH_DECSTATION
time->tm_year += real_year - 72;
#endif
if (century > 20)
time->tm_year += (century - 19) * 100;
/*
* Account for differences between how the RTC uses the values
* and how they are defined in a struct rtc_time;
*/
if (time->tm_year <= 69)
time->tm_year += 100;
time->tm_mon--;
return RTC_24H;
}
EXPORT_SYMBOL_GPL(mc146818_get_time);
/* Set the current date and time in the real time clock. */
int mc146818_set_time(struct rtc_time *time)
{
unsigned long flags;
unsigned char mon, day, hrs, min, sec;
unsigned char save_control, save_freq_select;
unsigned int yrs;
#ifdef CONFIG_MACH_DECSTATION
unsigned int real_yrs, leap_yr;
#endif
unsigned char century = 0;
yrs = time->tm_year;
mon = time->tm_mon + 1; /* tm_mon starts at zero */
day = time->tm_mday;
hrs = time->tm_hour;
min = time->tm_min;
sec = time->tm_sec;
if (yrs > 255) /* They are unsigned */
return -EINVAL;
#ifdef CONFIG_MACH_DECSTATION
real_yrs = yrs;
leap_yr = ((!((yrs + 1900) % 4) && ((yrs + 1900) % 100)) ||
!((yrs + 1900) % 400));
yrs = 72;
/*
* We want to keep the year set to 73 until March
* for non-leap years, so that Feb, 29th is handled
* correctly.
*/
if (!leap_yr && mon < 3) {
real_yrs--;
yrs = 73;
}
#endif
#ifdef CONFIG_ACPI
if (acpi_gbl_FADT.header.revision >= FADT2_REVISION_ID &&
acpi_gbl_FADT.century) {
century = (yrs + 1900) / 100;
yrs %= 100;
}
#endif
/* These limits and adjustments are independent of
* whether the chip is in binary mode or not.
*/
if (yrs > 169)
return -EINVAL;
if (yrs >= 100)
yrs -= 100;
if (!(CMOS_READ(RTC_CONTROL) & RTC_DM_BINARY)
|| RTC_ALWAYS_BCD) {
sec = bin2bcd(sec);
min = bin2bcd(min);
hrs = bin2bcd(hrs);
day = bin2bcd(day);
mon = bin2bcd(mon);
yrs = bin2bcd(yrs);
century = bin2bcd(century);
}
spin_lock_irqsave(&rtc_lock, flags);
save_control = CMOS_READ(RTC_CONTROL);
CMOS_WRITE((save_control|RTC_SET), RTC_CONTROL);
save_freq_select = CMOS_READ(RTC_FREQ_SELECT);
CMOS_WRITE((save_freq_select|RTC_DIV_RESET2), RTC_FREQ_SELECT);
#ifdef CONFIG_MACH_DECSTATION
CMOS_WRITE(real_yrs, RTC_DEC_YEAR);
#endif
CMOS_WRITE(yrs, RTC_YEAR);
CMOS_WRITE(mon, RTC_MONTH);
CMOS_WRITE(day, RTC_DAY_OF_MONTH);
CMOS_WRITE(hrs, RTC_HOURS);
CMOS_WRITE(min, RTC_MINUTES);
CMOS_WRITE(sec, RTC_SECONDS);
#ifdef CONFIG_ACPI
if (acpi_gbl_FADT.header.revision >= FADT2_REVISION_ID &&
acpi_gbl_FADT.century)
CMOS_WRITE(century, acpi_gbl_FADT.century);
#endif
CMOS_WRITE(save_control, RTC_CONTROL);
CMOS_WRITE(save_freq_select, RTC_FREQ_SELECT);
spin_unlock_irqrestore(&rtc_lock, flags);
return 0;
}
EXPORT_SYMBOL_GPL(mc146818_set_time);