Re: [PATCH v3] media: venus: amend buffer size for bitstream plane

[Hans Verkuil]

On 11/26/2018 04:44 PM, Tomasz Figa wrote:
> Hi Hans,
> 
> On Tue, Nov 27, 2018 at 12:24 AM Hans Verkuil <hverkuil@xxxxxxxxx> wrote:
>>
>> On 11/26/2018 03:57 PM, Stanimir Varbanov wrote:
>>> Hi Hans,
>>>
>>> On 11/26/18 3:37 PM, Hans Verkuil wrote:
>>>> On 11/26/2018 11:12 AM, Malathi Gottam wrote:
>>>>> Accept the buffer size requested by client and compare it
>>>>> against driver calculated size and set the maximum to
>>>>> bitstream plane.
>>>>>
>>>>> Signed-off-by: Malathi Gottam <mgottam@xxxxxxxxxxxxxx>
>>>>
>>>> Sorry, this isn't allowed. It is the driver that sets the sizeimage value,
>>>> never the other way around.
>>>
>>> I think for decoders (OUTPUT queue) and encoders (CAPTURE queue) we
>>> allowed userspace to set sizeimage for buffers. See [1] Initialization
>>> paragraph point 2:
>>>
>>>     ``sizeimage``
>>>        desired size of ``CAPTURE`` buffers; the encoder may adjust it to
>>>        match hardware requirements
>>>
>>> Similar patch we be needed for decoder as well.
>>
>> I may have missed that change since it wasn't present in v1 of the stateful
>> encoder spec.
> 
> It's been there from the very beginning, even before I started working
> on it. Actually, even the early slides from Kamil mention the
> application setting the buffer size for compressed streams:
> https://events.static.linuxfound.org/images/stories/pdf/lceu2012_debski.pdf
> 
>>
>> Tomasz, what was the reason for this change? I vaguely remember some thread
>> about this, but I forgot the details. Since this would be a departure of
>> the current API this should be explained in more detail.
> 
> The kernel is not the place to encode assumptions about optimal
> bitstream chunk sizes. It depends on the use case and the application
> should be able decide. It may for example want to use smaller buffers,
> optimizing for the well compressible video streams and just reallocate
> if bigger chunks are needed.
> 
>>
>> I don't really see the point of requiring userspace to fill this in. For
>> stateful codecs it can just return some reasonable size. Possibly calculated
>> using the provided width/height values or (if those are 0) some default value.
> 
> How do we decide what's "reasonable"? Would it be reasonable for all
> applications?

In theory it should be the minimum size that the hardware supports. But it is
silly to i.e. provide the size of one PAGE as the minimum. In practice you
probably want to set sizeimage to something larger than that. Depending on
the typical compression ratio perhaps 5 or 10% of what a raw YUV 4:2:0 frame
would be.

> 
>>
>> Ditto for decoders.
>>
>> Stanimir, I certainly cannot merge this until this has been fully nailed down
>> as it would be a departure from the current API.
> 
> It would not be a departure, because I can see existing stateful
> drivers behaving like that:
> https://elixir.bootlin.com/linux/v4.20-rc4/source/drivers/media/platform/s5p-mfc/s5p_mfc_enc.c#L1444
> https://elixir.bootlin.com/linux/v4.20-rc4/source/drivers/media/platform/mtk-vcodec/mtk_vcodec_enc.c#L469

Yes, and that's out of spec. Clearly v4l2-compliance doesn't test for this.
It should have been caught at least for the mtk driver.

> 
> Also, Chromium has been setting the size on its own for long time
> using its own heuristics.
> 
>>
>> And looking at the venus patch I do not see how it helps userspace.
>>
>> Regards,
>>
>>         Hans
>>
>>>
>>>>
>>>> If you need to allocate larger buffers, then use VIDIOC_CREATE_BUFS instead
>>>> of VIDIOC_REQBUFS.
> 
> CREATE_BUFS wouldn't work, because one needs to use TRY_FMT to obtain
> a format for it and the format returned by it would have the sizeimage
> as hardcoded in the driver...

???

Userspace can change the sizeimage to whatever it wants before calling
CREATE_BUFS as long as it is >= the sizeimage of the current format.

If we want to allow smaller sizes, then I think that would not be
unreasonable for stateful codecs. I actually think that drivers can
already do this in queue_setup(), but the spec would have to be updated
a bit.

Regards,

	Hans

> 
> Best regards,
> Tomasz
>