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Subject: Re: [PATCH 1/2] media: docs-rst: Document memory-to-memory video
 decoder interface
To:     Tomasz Figa <tfiga@chromium.org>
Cc:     Linux Media Mailing List <linux-media@vger.kernel.org>,
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From:   Hans Verkuil <hverkuil@xs4all.nl>
Message-ID: <f0aa7c84-08e3-9b04-8d1b-95f741d6817b@xs4all.nl>
Date:   Tue, 7 Aug 2018 09:37:31 +0200
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On 08/07/2018 09:05 AM, Tomasz Figa wrote:
> On Thu, Jul 26, 2018 at 7:57 PM Hans Verkuil <hverkuil@xs4all.nl> wrote:
>>>> What if you set the format to 0x0 but the stream does not have meta data with
>>>> the resolution? How does userspace know if 0x0 is allowed or not? If this is
>>>> specific to the chosen coded pixel format, should be add a new flag for those
>>>> formats indicating that the coded data contains resolution information?
>>>
>>> Yes, this would definitely be on a per-format basis. Not sure what you
>>> mean by a flag, though? E.g. if the format is set to H264, then it's
>>> bound to include resolution information. If the format doesn't include
>>> it, then userspace is already aware of this fact, because it needs to
>>> get this from some other source (e.g. container).
>>>
>>>>
>>>> That way userspace knows if 0x0 can be used, and the driver can reject 0x0
>>>> for formats that do not support it.
>>>
>>> As above, but I might be misunderstanding your suggestion.
>>
>> So my question is: is this tied to the pixel format, or should we make it
>> explicit with a flag like V4L2_FMT_FLAG_CAN_DECODE_WXH.
>>
>> The advantage of a flag is that you don't need a switch on the format to
>> know whether or not 0x0 is allowed. And the flag can just be set in
>> v4l2-ioctls.c.
> 
> As far as my understanding goes, what data is included in the stream
> is definitely specified by format. For example, a H264 elementary
> stream will always include those data as a part of SPS.
> 
> However, having such flag internally, not exposed to userspace, could
> indeed be useful to avoid all drivers have such switch. That wouldn't
> belong to this documentation, though, since it would be just kernel
> API.

Why would you keep this internally only?

>>>> I wonder if we should make these min buffer controls required. It might be easier
>>>> that way.
>>>
>>> Agreed. Although userspace is still free to ignore it, because REQBUFS
>>> would do the right thing anyway.
>>
>> It's never been entirely clear to me what the purpose of those min buffers controls
>> is. REQBUFS ensures that the number of buffers is at least the minimum needed to
>> make the HW work. So why would you need these controls? It only makes sense if they
>> return something different from REQBUFS.
>>
> 
> The purpose of those controls is to let the client allocate a number
> of buffers bigger than minimum, without the need to allocate the
> minimum number of buffers first (to just learn the number), free them
> and then allocate a bigger number again.

I don't feel this is particularly useful. One problem with the minimum number
of buffers as used in the kernel is that it is often the minimum number of
buffers required to make the hardware work, but it may not be optimal. E.g.
quite a few capture drivers set the minimum to 2, which is enough for the
hardware, but it will likely lead to dropped frames. You really need 3
(one is being DMAed, one is queued and linked into the DMA engine and one is
being processed by userspace).

I would actually prefer this to be the recommended minimum number of buffers,
which is >= the minimum REQBUFS uses.

I.e., if you use this number and you have no special requirements, then you'll
get good performance.

> 
>>>
>>>>
>>>>> +7.  If all the following conditions are met, the client may resume the
>>>>> +    decoding instantly, by using :c:func:`VIDIOC_DECODER_CMD` with
>>>>> +    ``V4L2_DEC_CMD_START`` command, as in case of resuming after the drain
>>>>> +    sequence:
>>>>> +
>>>>> +    * ``sizeimage`` of new format is less than or equal to the size of
>>>>> +      currently allocated buffers,
>>>>> +
>>>>> +    * the number of buffers currently allocated is greater than or equal to
>>>>> +      the minimum number of buffers acquired in step 6.
>>>>
>>>> You might want to mention that if there are insufficient buffers, then
>>>> VIDIOC_CREATE_BUFS can be used to add more buffers.
>>>>
>>>
>>> This might be a bit tricky, since at least s5p-mfc and coda can only
>>> work on a fixed buffer set and one would need to fully reinitialize
>>> the decoding to add one more buffer, which would effectively be the
>>> full resolution change sequence, as below, just with REQBUFS(0),
>>> REQBUFS(N) replaced with CREATE_BUFS.
>>
>> What happens today in those drivers if you try to call CREATE_BUFS?
> 
> s5p-mfc doesn't set the .vidioc_create_bufs pointer in its
> v4l2_ioctl_ops, so I suppose that would be -ENOTTY?

Correct for s5p-mfc.

Regards,

	Hans