On Wed, 6 Sep 2017 18:55:46 +0100
Mark Thompson <[email protected]> wrote:

> On 06/09/17 17:29, wm4 wrote:
> > On Wed, 6 Sep 2017 16:53:04 +0100
> > Mark Thompson <[email protected]> wrote:
> >   
> >> On 06/09/17 15:37, wm4 wrote:  
> >>> On Tue,  5 Sep 2017 23:59:31 +0100
> >>> Mark Thompson <[email protected]> wrote:
> >>>     
> >>>> ---
> >>>>  doc/APIchanges       |  3 +++
> >>>>  libavcodec/avcodec.h | 23 +++++++++++++++++++++++
> >>>>  2 files changed, 26 insertions(+)
> >>>>
> >>>> diff --git a/doc/APIchanges b/doc/APIchanges
> >>>> index 6f70f3c96..f21dc4db0 100644
> >>>> --- a/doc/APIchanges
> >>>> +++ b/doc/APIchanges
> >>>> @@ -14,6 +14,9 @@ libavutil:     2017-03-23
> >>>>  API changes, most recent first:
> >>>>  
> >>>>  2017-xx-xx - xxxxxxx - lavc 58.x+1.0 - avcodec.h
> >>>> +  Add AVCodecHWConfig and AVCodec.hw_configs.
> >>>> +
> >>>> +2017-xx-xx - xxxxxxx - lavc 58.x+1.0 - avcodec.h
> >>>>    Add AV_HWACCEL_FLAG_REUSE_CONTEXT.
> >>>>  
> >>>>  2017-xx-xx - xxxxxxx - lavfi 7.x+1.0 - avfilter.h
> >>>> diff --git a/libavcodec/avcodec.h b/libavcodec/avcodec.h
> >>>> index 89e432d16..4f6b76203 100644
> >>>> --- a/libavcodec/avcodec.h
> >>>> +++ b/libavcodec/avcodec.h
> >>>> @@ -35,6 +35,7 @@
> >>>>  #include "libavutil/cpu.h"
> >>>>  #include "libavutil/dict.h"
> >>>>  #include "libavutil/frame.h"
> >>>> +#include "libavutil/hwcontext.h"
> >>>>  #include "libavutil/log.h"
> >>>>  #include "libavutil/pixfmt.h"
> >>>>  #include "libavutil/rational.h"
> >>>> @@ -2773,6 +2774,19 @@ typedef struct AVProfile {
> >>>>      const char *name; ///< short name for the profile
> >>>>  } AVProfile;
> >>>>  
> >>>> +typedef struct AVCodecHWConfig {
> >>>> +    /**
> >>>> +     * A device type which can be supplied to the codec.
> >>>> +     */
> >>>> +    enum AVHWDeviceType device_type;
> >>>> +    /**
> >>>> +     * The matching hardware pixel format which the codec can produce.
> >>>> +     *
> >>>> +     * Set to AV_PIX_FMT_NONE if no hardware pixel format is available.
> >>>> +     */
> >>>> +    enum AVPixelFormat pix_fmt;
> >>>> +} AVCodecHWConfig;
> >>>> +
> >>>>  typedef struct AVCodecDefault AVCodecDefault;
> >>>>  
> >>>>  struct AVSubtitle;
> >>>> @@ -2808,6 +2822,15 @@ typedef struct AVCodec {
> >>>>      const AVClass *priv_class;              ///< AVClass for the 
> >>>> private context
> >>>>      const AVProfile *profiles;              ///< array of recognized 
> >>>> profiles, or NULL if unknown, array is terminated by {FF_PROFILE_UNKNOWN}
> >>>>  
> >>>> +    /**
> >>>> +     * Array of hardware configurations supported by the codec, or NULL
> >>>> +     * if no hardware supported.
> >>>> +     *
> >>>> +     * The array is terminated by a configuration with a device_type of
> >>>> +     * AV_HW_DEVICE_TYPE_NONE.
> >>>> +     */
> >>>> +    const AVCodecHWConfig *hw_configs;
> >>>> +
> >>>>      /*****************************************************************
> >>>>       * No fields below this line are part of the public API. They
> >>>>       * may not be used outside of libavcodec and can be changed and    
> >>>
> >>> What if a decoder provides hardware decoding, but has no associated
> >>> AVHWDeviceType? (Such as mmal.)    
> >>
> >> They are in the pix_fmts list.  What other information do you want?  
> > 
> > They don't necessarily output a hw format. For example, crystalhd
> > (ffmpeg only) doesn't. It would also not be very uniform.  
> 
> If it doesn't require any additional setup and it outputs software frames 
> then it's indistinguishable from a normal software decoder; therefore, treat 
> it as such.
> 
> MMAL is harder - I'm only treating the real hwaccels here because they now 
> have a consistent form we can make use of.
> 
> I haven't really thought about this properly, but how about something like:
> 
> enum AVCodecHWConfigMethod {
>   AV_CODEC_HW_CONFIG_METHOD_HW_DEVICE = 1,
>   AV_CODEC_HW_CONFIG_METHOD_HW_FRAMES = 2,
>   AV_CODEC_HW_CONFIG_METHOD_LEGACY_SHIT = 4,
>   AV_CODEC_HW_CONFIG_METHOD_MAGIC = 8,
> };
> 
> struct AVCodecHWConfig {
>   int methods;
>   enum AVHWDeviceType device_type;
>   enum AVPixelFormat pix_fmt;
> };
> 
> Then you would label MMAL as MAGIC, because it can just output the MMAL 
> frames without additional setup.  VAAPI/VDPAU would be HW_DEVICE | HW_FRAMES 
> | LEGACY_SHIT.  QSV would be HW_FRAMES | LEGACY_SHIT.

That sounds OK too. Maybe we should include the information whether it's
a hwaccel, i.e. something that has an actual software decoder under the
hood, and that dynamic fallback/resuming HW decoding is supported?

What is LEGACY_SHIT exactly?

> Is there then sufficient information for ff_get_format() to look at this list 
> to decide on whether it needs an AVHWAccel, and therefore eliminate dummy 
> hwaccels?

I suspect it is. I wonder why this check is even being done at all.
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