On Mon, 30 Jan 2017 10:10:11 +0000
Mark Thompson <[email protected]> wrote:
> >> + *
> >> + * If set by libavcodec:
> >> + *
> >> + * - decoding: The hw_device_ctx field should be set by the user
> >> before
> >> + * calling avcodec_open2(). If at some later point a
> >> hardware
> >> + * pixel format is selected by get_format(), then a new
> >> + * AVHWFramesContext will be created by the decoder and
> >> put
> >> + * in this field.
> >> + *
> >
> >> + * It can be read only inside the get_buffer2() callback
> >> + * (which must return a buffer from the given context).
> >
> > Any reason for this specific restriction?
> >
> > Shouldn't it be ok to access it any time - just that it might not be
> > set to something useful? (Could be NULL or wrong format.)
>
> If the decoder is single-threaded, yes. If it isn't, then I think that
> inside get_buffer2() is the only place where you can guarantee there isn't a
> data race (which is undefined behaviour in C11).
Good point about threading. I'm not sure how that works with hwaccel.
Normally, each thread has a "shadow" AVCodecContext, and the generic
thread code copies fields between them at defined times. Maybe elenril
knows how this affects hwaccel.
> >> + * A reference to the AVHWDeviceContext describing the device which
> >> will
> >> + * be used by a hardware encoder/decoder. The reference is set by the
> >> + * caller and afterwards owned (and freed) by libavcodec.
> >> + *
> >> + * This should only be used if either the codec device does not
> >> require
> >> + * hardware frames or any that are used are allocated internally by
> >> + * libavcodec. If the frames may be supplied by the user, then
> >> + * hw_frames_ctx must be used instead.
> >
> > I'm not sure about the word "may". For some decoders, the user can
> > either set hw_device_ctx, or he can set a hw_frames_ctx (but they
> > can't both be set). Some decoders support hw_frames_ctx only. Shouldn't
> > it say "have to" instead of "may"? Maybe the wording is fine - I'm not a
> > native speaker.
>
> It's trying to say "if you ever want to supply the frames, you need to use
> hw_frames_ctx" - that is, even if you only want to do it some of the time you
> still need to make the frames context all the time.
Oh ok.
> This should only be used if either the codec device does not require
> hardware frames or any that are used are allocated internally by
> libavcodec. If the user ever intends to supply any of the frames used
> for decoding then hw_frames_ctx must be used instead.
Sounds ok. Maybe the exclusiveness of both fields could be emphasized
in a more explicit way:
The user must never set both hw_frames_ctx and hw_device_ctx. The
API supports setting hw_frames_ctx for manual frame allocation, and
hw_device_ctx for automatic management. Both modes are exclusive to
each other.
> >> + *
> >> + * If the same device is to be used throughout the lifetime of a
> >> decoder,
> >> + * this field should be set before avcodec_open2() is called. Some
> >> + * decoders can only use one device and therefore require this
> >> behaviour.
> >> + *
> >> + * For other decoders, the hw_device_ctx field can be set inside the
> >> + * get_format() callback to indicate the device to use for decoding a
> >> + * particular section of the stream. Any previous value in the field
> >> + * must be unreferenced before being overwritten. This also supports
> >> + * changing between use of hw_device_ctx and hw_frames_ctx - when
> >> doing
> >> + * so, the other field must always be set to NULL.
>
> Should mention encoders here too:
>
> For encoders, this field must be set before avcodec_open2() is called.
> (Encoders can only use a single device.)
Sounds good.
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