linux-kernel - Re: [PATCH] media: venus: fix reported size of 0-length buffers

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Date:   Wed, 14 Nov 2018 13:12:46 +0900
From:   Alexandre Courbot <acourbot@...omium.org>
To:     Nicolas Dufresne <nicolas@...fresne.ca>
Cc:     Stanimir Varbanov <stanimir.varbanov@...aro.org>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        linux-arm-msm@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] media: venus: fix reported size of 0-length buffers

On Wed, Nov 14, 2018 at 3:54 AM Nicolas Dufresne <nicolas@...fresne.ca> wrote:
>
>
>
> Le mar. 13 nov. 2018 04 h 30, Alexandre Courbot <acourbot@...omium.org> a écrit :
>>
>> The last buffer is often signaled by an empty buffer with the
>> V4L2_BUF_FLAG_LAST flag set. Such buffers were returned with the
>> bytesused field set to the full size of the OPB, which leads
>> user-space to believe that the buffer actually contains useful data. Fix
>> this by passing the number of bytes reported used by the firmware.
>
>
> That means the driver does not know on time which one is last. Why not just returned EPIPE to userspace on DQBUF and ovoid this useless roundtrip ?

Sorry, I don't understand what you mean. EPIPE is supposed to be
returned after a buffer with V4L2_BUF_FLAG_LAST is made available for
dequeue. This patch amends the code that prepares this LAST-flagged
buffer. How could we avoid a roundtrip in this case?

>
>>
>> Signed-off-by: Alexandre Courbot <acourbot@...omium.org>
>> ---
>>  drivers/media/platform/qcom/venus/vdec.c | 4 +---
>>  1 file changed, 1 insertion(+), 3 deletions(-)
>>
>> diff --git a/drivers/media/platform/qcom/venus/vdec.c b/drivers/media/platform/qcom/venus/vdec.c
>> index 189ec975c6bb..282de21cf2e1 100644
>> --- a/drivers/media/platform/qcom/venus/vdec.c
>> +++ b/drivers/media/platform/qcom/venus/vdec.c
>> @@ -885,10 +885,8 @@ static void vdec_buf_done(struct venus_inst *inst, unsigned int buf_type,
>>         vbuf->field = V4L2_FIELD_NONE;
>>
>>         if (type == V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE) {
>> -               unsigned int opb_sz = venus_helper_get_opb_size(inst);
>> -
>>                 vb = &vbuf->vb2_buf;
>> -               vb2_set_plane_payload(vb, 0, bytesused ? : opb_sz);
>> +               vb2_set_plane_payload(vb, 0, bytesused);
>>                 vb->planes[0].data_offset = data_offset;
>>                 vb->timestamp = timestamp_us * NSEC_PER_USEC;
>>                 vbuf->sequence = inst->sequence_cap++;
>> --
>> 2.19.1.930.g4563a0d9d0-goog
>>