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[media] DocBook: improve the error_idx field documentation

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The documentation of the error_idx field was incomplete and confusing.
This patch improves it.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
This commit is contained in:
Hans Verkuil 2013-01-11 10:33:30 -03:00 committed by Mauro Carvalho Chehab
parent 07b64b837d
commit 0568aeeeef
1 changed files with 40 additions and 7 deletions
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47
Documentation/DocBook/media/v4l/vidioc-g-ext-ctrls.xml
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@ -199,13 +199,46 @@ also be zero.</entry>
<row>
<entry>__u32</entry>
<entry><structfield>error_idx</structfield></entry>
<entry>Set by the driver in case of an error. If it is equal
to <structfield>count</structfield>, then no actual changes were made to
controls. In other words, the error was not associated with setting a particular
control. If it is another value, then only the controls up to <structfield>error_idx-1</structfield>
were modified and control <structfield>error_idx</structfield> is the one that
caused the error. The <structfield>error_idx</structfield> value is undefined
if the ioctl returned 0 (success).</entry>
<entry><para>Set by the driver in case of an error. If the error is
associated with a particular control, then <structfield>error_idx</structfield>
is set to the index of that control. If the error is not related to a specific
control, or the validation step failed (see below), then
<structfield>error_idx</structfield> is set to <structfield>count</structfield>.
The value is undefined if the ioctl returned 0 (success).</para>
<para>Before controls are read from/written to hardware a validation step
takes place: this checks if all controls in the list are valid controls,
if no attempt is made to write to a read-only control or read from a write-only
control, and any other up-front checks that can be done without accessing the
hardware. The exact validations done during this step are driver dependent
since some checks might require hardware access for some devices, thus making
it impossible to do those checks up-front. However, drivers should make a
best-effort to do as many up-front checks as possible.</para>
<para>This check is done to avoid leaving the hardware in an inconsistent state due
to easy-to-avoid problems. But it leads to another problem: the application needs to
know whether an error came from the validation step (meaning that the hardware
was not touched) or from an error during the actual reading from/writing to hardware.</para>
<para>The, in hindsight quite poor, solution for that is to set <structfield>error_idx</structfield>
to <structfield>count</structfield> if the validation failed. This has the
unfortunate side-effect that it is not possible to see which control failed the
validation. If the validation was successful and the error happened while
accessing the hardware, then <structfield>error_idx</structfield> is less than
<structfield>count</structfield> and only the controls up to
<structfield>error_idx-1</structfield> were read or written correctly, and the
state of the remaining controls is undefined.</para>
<para>Since <constant>VIDIOC_TRY_EXT_CTRLS</constant> does not access hardware
there is also no need to handle the validation step in this special way,
so <structfield>error_idx</structfield> will just be set to the control that
failed the validation step instead of to <structfield>count</structfield>.
This means that if <constant>VIDIOC_S_EXT_CTRLS</constant> fails with
<structfield>error_idx</structfield> set to <structfield>count</structfield>,
then you can call <constant>VIDIOC_TRY_EXT_CTRLS</constant> to try to discover
the actual control that failed the validation step. Unfortunately, there
is no <constant>TRY</constant> equivalent for <constant>VIDIOC_G_EXT_CTRLS</constant>.
</para></entry>
</row>
<row>
<entry>__u32</entry>