On 28/04/2022 15:09, Nicolas Dufresne wrote:
> Le jeudi 28 avril 2022 à 08:12 +0200, Hans Verkuil a écrit :
>> On 27/04/2022 17:08, Nicolas Dufresne wrote:
>>> Le mercredi 27 avril 2022 à 13:31 +0900, Tomasz Figa a écrit :
>>>> Hi Nicolas, Sebastian,
>>>>
>>>> On Tue, Apr 26, 2022 at 9:58 PM Nicolas Dufresne
>>>> <nicolas.dufresne@xxxxxxxxxxxxx> wrote:
>>>>>
>>>>> From: Sebastian Fricke <sebastian.fricke@xxxxxxxxxxxxx>
>>>>>
>>>>> Using V4L2_BUF_FLAG_M2M_HOLD_CAPTURE_BUF flag without specifying the
>>>>> subsystem flag VB2_V4L2_FL_SUPPORTS_M2M_HOLD_CAPTURE_BUF, results in
>>>>> silently ignoring it.
>>>>> Warn the user via a debug print when the flag is requested but ignored
>>>>> by the videobuf2 framework.
>>>>>
>>>>> Signed-off-by: Sebastian Fricke <sebastian.fricke@xxxxxxxxxxxxx>
>>>>> Signed-off-by: Nicolas Dufresne <nicolas.dufresne@xxxxxxxxxxxxx>
>>>>> Reviewed-by: Ezequiel Garcia <ezequiel@xxxxxxxxxxxxxxxxxxxx>
>>>>> ---
>>>>> drivers/media/common/videobuf2/videobuf2-v4l2.c | 7 ++++++-
>>>>> 1 file changed, 6 insertions(+), 1 deletion(-)
>>>>>
>>>>
>>>> Thanks for the patch. Please see my comments inline.
>>>>
>>>>> diff --git a/drivers/media/common/videobuf2/videobuf2-v4l2.c b/drivers/media/common/videobuf2/videobuf2-v4l2.c
>>>>> index 6edf4508c636..812c8d1962e0 100644
>>>>> --- a/drivers/media/common/videobuf2/videobuf2-v4l2.c
>>>>> +++ b/drivers/media/common/videobuf2/videobuf2-v4l2.c
>>>>> @@ -329,8 +329,13 @@ static int vb2_fill_vb2_v4l2_buffer(struct vb2_buffer *vb, struct v4l2_buffer *b
>>>>> */
>>>>> vbuf->flags &= ~V4L2_BUF_FLAG_TIMECODE;
>>>>> vbuf->field = b->field;
>>>>> - if (!(q->subsystem_flags & VB2_V4L2_FL_SUPPORTS_M2M_HOLD_CAPTURE_BUF))
>>>>> + if (!(q->subsystem_flags & VB2_V4L2_FL_SUPPORTS_M2M_HOLD_CAPTURE_BUF)) {
>>>>> + if (vbuf->flags & V4L2_BUF_FLAG_M2M_HOLD_CAPTURE_BUF)
>>>>> + dprintk(q, 1,
>>>>> + "Request holding buffer (%d), unsupported on output queue\n",
>>>>> + b->index);
>>>>
>>>> I wonder if we shouldn't just fail such a QBUF operation. Otherwise
>>>> the application would get unexpected behavior from the kernel.
>>>> Although it might be too late to do it now if there are applications
>>>> that rely on this implicit ignore...
>>>
>>> In the context of this patchset, the statu quo seems to be the logical thing to
>>> do. We can raise this up in a separate thread. The side effect is of course
>>> confusing for developers, but it is hard for me to tell if a hard failure may
>>> break an existing software.
>>
>> I am leaning towards returning an error as well. It makes no sense to try
>> to hold on to a buffer when this is not supported.
>>
>> I also thought that it should be enough to rely on the core to clear the
>> flag upon return if it isn't supported, but looking through the vb2 core code
>> it looks like we're not clearing unknown flags at all, so running this for
>> older kernels that do not support holding at all will not clear the flag
>> either.
>>
>> The handling for flags in vb2 can be improved, I think I'll take a look at
>> that myself.
>>
>> I plan to merge this series soon, but will skip this patch for now.
>
> Ok, no problem. For me, as long as we do something about it, since it was not
> obvious and time consuming to debug.
After thinking some more (it only took me 7 months :-) ) I believe that an error
should be returned and the documentation for V4L2_BUF_FLAG_M2M_HOLD_CAPTURE_BUF
should be updated to mention that it will return EINVAL if set for a queue that
doesn't support this. vb2_queue_or_prepare_buf() is probably the best place for
this check.
Regards,
Hans
>
> regards,
> Nicolas
>
>>
>> Regards,
>>
>> Hans
>>
>>>
>>> regards,
>>> Nicolas
>>>
>>>>
>>>> Best regards,
>>>> Tomasz
>>>
>>
>
