Re: [FFmpeg-devel] [PATCH v4 1/1] avutils/hwcontext: When deriving a hwdevice, search for existing device in both directions

[Mark Thompson <sw@jkqxz.net>]

On 30/12/2021 00:29, Soft Works wrote:
>> -----Original Message-----
>> From: ffmpeg-devel <ffmpeg-devel-bounces@ffmpeg.org> On Behalf Of Mark
>> Thompson
>> Sent: Thursday, December 30, 2021 12:04 AM
>> To: ffmpeg-devel@ffmpeg.org
>> Subject: Re: [FFmpeg-devel] [PATCH v4 1/1] avutils/hwcontext: When deriving a
>> hwdevice, search for existing device in both directions
>>
>> On 25/11/2021 02:41, Soft Works wrote:
>>> The test /libavutil/tests/hwdevice checks that when deriving a device
>>> from a source device and then deriving back to the type of the source
>>> device, the result is matching the original source device, i.e. the
>>> derivation mechanism doesn't create a new device in this case.
>>>
>>> Previously, this test was usually passed, but only due to two different
>>> kind of flaws:
>>>
>>> 1. The test covers only a single level of derivation (and back)
>>>
>>> It derives device Y from device X and then Y back to the type of X and
>>> checks whether the result matches X.
>>>
>>> What it doesn't check for, are longer chains of derivation like:
>>>
>>> CUDA1 > OpenCL2 > CUDA3 and then back to OpenCL4
>>>
>>> In that case, the second derivation returns the first device (CUDA3 ==
>>> CUDA1), but when deriving OpenCL4, hwcontext.c was creating a new
>>> OpenCL4 context instead of returning OpenCL2, because there was no link
>>> from CUDA1 to OpenCL2 (only backwards from OpenCL2 to CUDA1)
>>
>> Yes, this is exactly what I expect.
>>
>> Because of how these APIs work, device derivation is always one-way - you can
>> make an OpenCL device from a D3D11 one, but not the other direction.  I don't
>> think there is any case which allows both directions
> 
> hwmap=reverse=1

Indeed, hwmap reverse exists because mapping is one-way and sometimes a filter graph wants to use it in the other direction.

>> Saying that derivation from A should always return the same B is not
>> intended, nor do I think it should be.
> 
> Why not?
> 
> Looking at the reality of API users:
> 
> - I'm covering a wide range of different processing pipelines and
>    found that this behavior is crucial to make important and relevant
>    processing pipelines work
> 
> - Intel have three different workaround-patches in their backlog/queue
>    of ffmpeg patches to get certain processing setups working
> 
> - The developers working on Vulkan have confirmed that this change
>    is necessary and crucial for certain setups to work
> 
> - Nobody has named any case or scenario that would be negatively
>    affected by this change
> 
> Given that situation, I don't think it's useful to talk about
> theoretical implications.

You are not talking about API users at all.  When does an API user ever want this patch?  From their point of view it is surprising and unwanted - if they want the same device again, they just use the same device again.

You are talking about users of the ffmpeg utility.  The change is a library hack to work around the inability to select devices per-filter in the ffmpeg utility.

Please, just implement device selection for filters in ffmpeg rather than adding unexpected behaviour elsewhere.  libavfilter has supported it for API users for a long time, no library changes should be needed.

- Mark
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