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[media] cec: document the special unconfigured case

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Even when the CEC device is unconfigured due to an invalid physical
address it is still allowed to send a message from 0xf (Unregistered)
to 0 (TV). This is a corner case explicitly allowed by the CEC specification.

Document this corner case.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
This commit is contained in:
Hans Verkuil 2017-02-27 10:53:39 -03:00 committed by Mauro Carvalho Chehab
parent 491314b8d4
commit fabd5bd68f
1 changed files with 14 additions and 0 deletions
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14
Documentation/media/uapi/cec/cec-ioc-receive.rst
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@ -69,6 +69,18 @@ The ``sequence`` field is filled in for every transmit and this can be
checked against the received messages to find the corresponding transmit
result.
Normally calling :ref:`ioctl CEC_TRANSMIT <CEC_TRANSMIT>` when the physical
address is invalid (due to e.g. a disconnect) will return ``ENONET``.
However, the CEC specification allows sending messages from 'Unregistered' to
'TV' when the physical address is invalid since some TVs pull the hotplug detect
pin of the HDMI connector low when they go into standby, or when switching to
another input.
When the hotplug detect pin goes low the EDID disappears, and thus the
physical address, but the cable is still connected and CEC still works.
In order to detect/wake up the device it is allowed to send poll and 'Image/Text
View On' messages from initiator 0xf ('Unregistered') to destination 0 ('TV').
.. tabularcolumns:: |p{1.0cm}|p{3.5cm}|p{13.0cm}|
@ -315,6 +327,8 @@ EPERM
ENONET
The CEC adapter is not configured, i.e. :ref:`ioctl CEC_ADAP_S_LOG_ADDRS <CEC_ADAP_S_LOG_ADDRS>`
was called, but the physical address is invalid so no logical address was claimed.
An exception is made in this case for transmits from initiator 0xf ('Unregistered')
to destination 0 ('TV'). In that case the transmit will proceed as usual.
EBUSY
Another filehandle is in exclusive follower or initiator mode, or the filehandle