Re: [PATCH 1/2] media: docs-rst: Document memory-to-memory video decoder interface

[Hans Verkuil]

On 10/09/2018 06:23 AM, Tomasz Figa wrote:
> On Mon, Oct 8, 2018 at 9:22 PM Hans Verkuil <hverkuil@xxxxxxxxx> wrote:
>>
>> On 09/19/2018 12:17 PM, Tomasz Figa wrote:
>>> Hi Hans,
>>>
>>> On Thu, Jul 26, 2018 at 7:20 PM Tomasz Figa <tfiga@xxxxxxxxxxxx> wrote:
>>>>
>>>> Hi Hans,
>>>>
>>>> On Wed, Jul 25, 2018 at 8:59 PM Hans Verkuil <hverkuil@xxxxxxxxx> wrote:
>>>>>
>>>>> Hi Tomasz,
>>>>>
>>>>> Many, many thanks for working on this! It's a great document and when done
>>>>> it will be very useful indeed.
>>>>>
>>>>> Review comments follow...
>>>>
>>>> Thanks for review!
>>>>
>>>>>
>>>>> On 24/07/18 16:06, Tomasz Figa wrote:
>>> [snip]
>>>>>> +13. Allocate destination (raw format) buffers via :c:func:`VIDIOC_REQBUFS`
>>>>>> +    on the ``CAPTURE`` queue.
>>>>>> +
>>>>>> +    * **Required fields:**
>>>>>> +
>>>>>> +      ``count``
>>>>>> +          requested number of buffers to allocate; greater than zero
>>>>>> +
>>>>>> +      ``type``
>>>>>> +          a ``V4L2_BUF_TYPE_*`` enum appropriate for ``CAPTURE``
>>>>>> +
>>>>>> +      ``memory``
>>>>>> +          follows standard semantics
>>>>>> +
>>>>>> +    * **Return fields:**
>>>>>> +
>>>>>> +      ``count``
>>>>>> +          adjusted to allocated number of buffers
>>>>>> +
>>>>>> +    * The driver must adjust count to minimum of required number of
>>>>>> +      destination buffers for given format and stream configuration and the
>>>>>> +      count passed. The client must check this value after the ioctl
>>>>>> +      returns to get the number of buffers allocated.
>>>>>> +
>>>>>> +    .. note::
>>>>>> +
>>>>>> +       To allocate more than minimum number of buffers (for pipeline
>>>>>> +       depth), use G_CTRL(``V4L2_CID_MIN_BUFFERS_FOR_CAPTURE``) to
>>>>>> +       get minimum number of buffers required, and pass the obtained value
>>>>>> +       plus the number of additional buffers needed in count to
>>>>>> +       :c:func:`VIDIOC_REQBUFS`.
>>>>>
>>>>>
>>>>> I think we should mention here the option of using VIDIOC_CREATE_BUFS in order
>>>>> to allocate buffers larger than the current CAPTURE format in order to accommodate
>>>>> future resolution changes.
>>>>
>>>> Ack.
>>>>
>>>
>>> I'm about to add a paragraph to describe this, but there is one detail
>>> to iron out.
>>>
>>> The VIDIOC_CREATE_BUFS ioctl accepts a v4l2_format struct. Userspace
>>> needs to fill in this struct and the specs says that
>>>
>>>   "Usually this will be done using the VIDIOC_TRY_FMT or VIDIOC_G_FMT
>>> ioctls to ensure that the requested format is supported by the
>>> driver."
>>>
>>> However, in case of a decoder, those calls would fixup the format to
>>> match the currently parsed stream, which would likely resolve to the
>>> current coded resolution (~hardware alignment). How do we get a format
>>> for the desired maximum resolution?
>>
>> You would call G_FMT to get the current format/resolution, then update
>> width and height and call TRY_FMT.
>>
>> Although to be honest you can also just set pixelformat and width/height
>> and zero everything else and call TRY_FMT directly, skipping the G_FMT
>> ioctl.
>>
> 
> Wouldn't TRY_FMT adjust the width and height back to match current stream?

Huh. Hmm. Grrr.

Good point and I didn't read your original comment carefully enough.

Suggestions on a postcard...

Regards,

	Hans