[FFmpeg-devel] [PATCH] libavcodec/qsv.c: Re-design session control and internal allocation

wm4 nfxjfg at googlemail.com
Mon Oct 26 12:44:50 CET 2015

On Mon, 26 Oct 2015 11:22:38 +0100
Gwenole Beauchesne <gb.devel at gmail.com> wrote:

> >> /**
> >>  * Hardware Accelerator identifier.
> >>  *
> >>  * @note
> >>  * A hardware accelerator can be device-less. This means that only the
> >>  * underlying hardware resource, e.g. a Linux dma_buf handle, is being
> >>  * transported in the AVFrame. That hardware resource could be mapped
> >>  * through standard OS-dependent calls, e.g. mmap() on Linux.
> >>  */
> >> enum AVHWAccelId {
> >>     AV_HWACCEL_ID_NONE = -1,
> >>     AV_HWACCEL_ID_NB,           ///< Not part of ABI
> >> };
> >>
> >> Name to be improved if people have better suggestions, as this really
> >> is to be seen as HW resource, not necessarily attached to a particular
> >> HW device. i.e. this could be a dma_buf handle from a V4L2 buffer or
> >> VA surface.  
> >
> > OK. (Minor nit: if ID_NONE is valid and means HW API without context,
> > maybe it should be 0, not -1. Also, if it was meant this way, maybe
> > these should still have their own ID for other purposes.)  
> In my current model, ID_NONE is not meant to be valid because the
> hwaccel side data shall only exist for hwaccel purposes. Besides,
> having ID_NONE set to -1 is consistent with other liavu enums and
> convenient to have ID_NB express directly the exact number of
> hwaccels.

OK, this makes sense to me.

> >> I am reworking the patch series as I changed my mind again: current
> >> map strategy was overly complex (and required to be). There were at
> >> am now preferring a unique av_hwaccel_frame_get_pixels() defined as
> >> follow:
> >>
> >> /**
> >>  * Returns AVFrame pixels into linear memory
> >>  *
> >>  * This function takes a snapshot of the underlying HW surface and
> >>  * exposes it to SW backed memory. This may involve a copy from GPU
> >>  * memory to CPU memory.
> >>  *
> >>  * @note
> >>  * There is no effort underway to commit the modified pixels back to
> >>  * GPU memory when the \ref dst AVFrame is released.
> >>  *
> >>  * @param[in] src       the source frame to read
> >>  * @param[inout] dst    the target frame to update, or create if NULL
> >>  * @param[in] flags     an optional combination of AV_FRAME_FLAG_xxx flags
> >>  * @return 0 on success, an AVERROR code on failure.
> >>  */
> >> int
> >> av_hwaccel_frame_get_pixels(AVFrame *src, AVFrame **dst, unsigned flags);
> >>
> >> i.e. the cost of allocating and copying AVFrame metadata should be
> >> less than the actual work needed behind the scene. So, it looks like a
> >> better interim solution for starters.  
> >
> > So this is for read-access only, right? If it can map data, there
> > also needs to be an unmap function, and the user would have to know
> > about when to use it.  
> Well, put can be implementing by reversing src/dst in this function. :)
> Actually, this can be av_hwaccel_frame_copy(), but the benefit of
> having get_pixels() is to leave out the allocation business to lavu
> and just having the user to bother about _unref().

Also makes sense to me.

What is a problem is that mapped frames and CPU frames (let's call pure
CPU-allocated surfaces that) are not exactly the same thing. If the API
user assumes the frame is a CPU frame, it might reference it for a long
time, which would cause various problems. On the other hand, you don't
want the user to force copying a frame if it's really a CPU frame.

Maybe this is not really a problem. I'm just mentioning it as another

> >> For compatibility, that's also the idea behind another generic
> >> AV_PIX_FMT_HWACCEL that would enforce data[i] to be clear of any
> >> user-supplied pointers, and buf[i] shall be filled in by appropriate
> >> accessors, or while creating the side-data, e.g.
> >> av_vaapi_frame_create_side_data(). i.e. when lavc swallows up an
> >> AVFrame with new-style hwaccel, this is where the AV_PIX_FMT_VAAPI
> >> would be replaced with AV_PIX_FMT_HWACCEL. Replace "swallows up" with
> >> e.g. av_vaapi_frame_convert_in_place() if you prefer. Otherwise, IMHO,
> >> the old-style fields should live untouched, hence the need to keep the
> >> hwaccel side-data around.  
> >
> > Isn't the problem more about output?
> >
> > Again, there's the problem with the current hwaccel API selecting the
> > hwaccel with get_format(), just using the hwaccel-specific pixfmt.  
> I also envision a need for AVCodecContext.hwaccel_id field + possibly
> .get_hwaccel(). Just so that to depart from that pixfmt tie.

There are some of us who would like this. Of course it makes the API
change larger. Also, I do find it useful to have pixfmt distinguish
between underlying surface types (i.e. the hwaccel API). For example,
if we add support for hw filters to libavfilter, how would you prevent
that a vdpau filter takes vaapi surfaces as input?

So I'm not sure if a single AV_PIX_FMT_HWACCEL is the way to go, even
if we make access to hwaccel AVFrames somewhat more uniform.

> > Also, AVFrame.buf[] should cover the memory referenced by
> > AVFrame.data[]. It's merely a refcount helper for AVFrame.data[], and
> > should not do additional things.
> >
> > I think using AVFrame side data for this would be a bit awkward.
> > Possibly it _could_ be used to store things like VADisplay if we don't
> > find a better way, but I think having a AVHWAccelFrame would be better.  
> Side data is quite simple to use, and ref-counted easily. I didn't
> want to touch to AVFrame fields. Though, it's of course possible to
> extend it with either public or external fields.

Side data is really just for things by the "side", not information that
is critical and central.

Maybe we agree that there's no technical issue here, and it's only a
matter of API design and "taste".

> >> >  
> >> >> PS: other benefit of the AVHWAccelFrame side-data is that I can stuff
> >> >> crop information into there. Since this is only useful to hwaccel, no
> >> >> need to populate AVFrame with additional fields IMHO.  
> >> >
> >> > IMHO, crop information should be generally available, even for software
> >> > surfaces. What we currently do are terrible hacks: align the X/Y
> >> > coordinates to chroma boundaries and adjust the pointer (meaning
> >> > everyone has to do with possibly unaligned pointers, and non-mod-2
> >> > crops don't work correctly), and this also means you can have a
> >> > non-mod-2 width/height, which doesn't really make sense for chroma.  
> >>
> >> I don't really see why this would be needed for SW. AVFrame.buf[] will
> >> hold the buffers as in "the original allocation". Then AVFrame.data[]
> >> shall be filled in to fit the actual cropped/decoded region. Isn't it?  
> >
> > Yes, AVFrame.buf[] does this, but you still don't know e.g. the
> > original width, or even the pointer to a plane's original (0, 0) pixel
> > if AVFrame.buf[0] covers all planes.
> >
> > I think doing cropping as metadata would also simplify code a lot. For
> > example, nobody has to worry about non-mod-2 yuv420 anymore, and how it
> > should be handled. It's less tricky, more correct, more efficient.  
> OK. A crop side-data is desired then. I somehow was convinced that it
> wouldn't matter for SW. Though, it would actually be really need that

At least this is my opinion. I would like to have such cropping side
data, instead of half-broken ad-hoc cropping in the decoder and things
like coded_width.

I don't know what most others think about this. I suspect most would
find such a change too intrusive. At least for hwaccel it's mandatory
though. (What we currently do just barely works, and I need hacks in my
own code to reconstruct the real surface size.)

> the user doesn't have to care about anything and just use .data[]
> appropriately. So, probably have internal_data[] fields for that SIMD
> alignment needs?

This reminds me of AVFrame.base[], which was removed 2 years ago.

I'm fairly sure a cropping rect would be cleaner.

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