[FFmpeg-devel] [PATCH] ffmpeg_qsv: use -hwaccel_device to specify a device for VAAPI backend

[Rogozhkin, Dmitry V]

On Wed, 2020-09-02 at 07:31 +0000, Soft Works wrote:
> > -----Original Message-----
> > From: ffmpeg-devel <ffmpeg-devel-bounces at ffmpeg.org> On Behalf Of
> > Rogozhkin, Dmitry V
> > Sent: Wednesday, September 2, 2020 9:00 AM
> > To: ffmpeg-devel at ffmpeg.org
> > Subject: Re: [FFmpeg-devel] [PATCH] ffmpeg_qsv: use -hwaccel_device
> > to
> > specify a device for VAAPI backend
> > 
> > On Wed, 2020-09-02 at 04:28 +0000, Xiang, Haihao wrote:
> > > On Wed, 2020-09-02 at 04:20 +0000, Soft Works wrote:
> > > > > -----Original Message-----
> > > > > From: ffmpeg-devel <ffmpeg-devel-bounces at ffmpeg.org> On
> > > > > Behalf
> > 
> > Of
> > > > > Haihao Xiang
> > > > > Sent: Wednesday, September 2, 2020 6:01 AM
> > > > > To: ffmpeg-devel at ffmpeg.org
> > > > > Cc: Haihao Xiang <haihao.xiang at intel.com>
> > > > > Subject: [FFmpeg-devel] [PATCH] ffmpeg_qsv: use
> > > > > -hwaccel_device to
> > > > > specify a device for VAAPI backend
> > > > > 
> > > > > According to the FFmpeg documentation, -hwaccel_device can be
> > > > > used
> > > > > to select a device for hardware acceleration
> > > > > 
> > > > > Example:
> > > > > ffmpeg -hwaccel qsv -hwaccel_device /dev/dri/renderD129 -c:v
> > > > > h264_qsv \ -i
> > > > > in.h264 -c:v hevc_qsv out.h265
> > > > 
> > > > No. The docs say about hwaccel_device
> > > > 
> > > > It can either refer to an existing device created with
> > > > -init_hw_device by name,
> > > > 
> > > > => /dev/dri/renderD129 is not an existing device name
> > > > 
> > > > or it can create a new device as if ‘-init_hw_device’
> > > > type:hwaccel_device were called immediately before.
> > > > 
> > > > => /dev/dri/renderD129 is not a valid string for init_hw_device
> > 
> > Well, it seems that's where documentation does not reflect the
> > reality since
> > (as pointed by Haihao below) 
> > https://trac.ffmpeg.org/wiki/Hardware/VAAPI
> > is full of contradictory examples.
> > 
> > > > 
> > > > 
> > > > Hence, the patch doesn't make sense.
> > > 
> > > in https://trac.ffmpeg.org/wiki/Hardware/VAAPI,
> > > 
> > > If only one stream is being used, -hwaccel_device can also accept
> > > a
> > > device path directly
> > > 
> > > ffmpeg -hwaccel vaapi -hwaccel_device /dev/dri/renderD128 -i ...
> > > 
> > > I think it can be applied to qsv too.
> > > 
> > 
> > I agree. Part of ffmpeg help:
> >     -hwaccel hwaccel name  use HW accelerated decoding
> >     -hwaccel_device devicename  select a device for HW acceleration
> > 
> > To me specifying device via -hwaccel_device in the way it is being
> > suggest in
> > commit is pretty intuitive and is better than explain people that
> > there is -
> > qsv_device for that purpose. Niceness of -hwaccel_device is that it
> > is not
> > tight to particular acceleration, hence easier to work with for end
> > users.
> 
> In case of QSV, the usage of hwaccel_device is already defined:
> 
> device selects a value in ‘MFX_IMPL_*’. Allowed values are:
> auto sw  hw  auto_any hw_any hw2 hw3 hw4
> 
> I agree that it would be more intuitive, but you can't just give it a
> different meaning from one day to another and break functionality.

Adding new values does not break functionality! No one  suggests to
drop existing set of values (sw, hw, etc.) - these are remaining. But
basically suggestion is to extend allowed set to accept drm device
specification on Linux, that's it. Honestly, I don't see problem here.
Some background story: hw,hw2,hw3 allows to select adapter for msdk
library on Windows, but they don't allow to do that on Linux. On Linux
all these hw options are basically the same for the reason that device
is being specified externally via SetHandle call. On Linux only sw/hw
makes a difference (other story that sw msdk library is not actually
released currently on Linux). That's the origin of the request: we need
a good way to specify adapter on Linux.

> 
> Kind regards,
> softworkz
> 
> 
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