[FFmpeg-devel] [PATCH v2] avcodec: add a get_encoder_buffer() callback to AVCodecContext
Michael Niedermayer
michael at niedermayer.cc
Fri Mar 12 21:30:23 EET 2021
On Fri, Mar 12, 2021 at 02:03:52PM -0300, James Almer wrote:
> On 3/12/2021 1:32 PM, Michael Niedermayer wrote:
> > On Thu, Mar 11, 2021 at 02:18:36PM -0300, James Almer wrote:
> > > On 3/11/2021 1:35 PM, Michael Niedermayer wrote:
> > > > On Wed, Mar 10, 2021 at 05:59:11PM -0300, James Almer wrote:
> > > > > On 3/10/2021 5:18 PM, Michael Niedermayer wrote:
> > > > > > On Mon, Feb 22, 2021 at 07:27:34PM -0300, James Almer wrote:
> > > > > > > On 2/21/2021 6:04 PM, James Almer wrote:
> > > > > > > > On 2/21/2021 5:29 PM, Mark Thompson wrote:
> > > > > > > > > On 21/02/2021 20:00, James Almer wrote:
> > > > > > > > > > On 2/21/2021 4:13 PM, Mark Thompson wrote:
> > > > > > > > > > > On 21/02/2021 17:35, James Almer wrote:
> > > > > > > > > > > > This callback is functionally the same as get_buffer2()
> > > > > > > > > > > > is for decoders, and
> > > > > > > > > > > > implements for the new encode API the functionality of
> > > > > > > > > > > > the old encode API had
> > > > > > > > > > > > where the user could provide their own buffers.
> > > > > > > > > > > >
> > > > > > > > > > > > Signed-off-by: James Almer <jamrial at gmail.com>
> > > > > > > > > > > > ---
> > > > > > > > > > > > Used the names Lynne suggested this time, plus a line
> > > > > > > > > > > > about how the callback
> > > > > > > > > > > > must be thread safe.
> > > > > > > > > > > >
> > > > > > > > > > > > libavcodec/avcodec.h | 45 ++++++++++++++++++++++++++++++++++++
> > > > > > > > > > > > libavcodec/codec.h | 8 ++++---
> > > > > > > > > > > > libavcodec/encode.c | 54
> > > > > > > > > > > > +++++++++++++++++++++++++++++++++++++++++++-
> > > > > > > > > > > > libavcodec/encode.h | 8 +++++++
> > > > > > > > > > > > libavcodec/options.c | 1 +
> > > > > > > > > > > > 5 files changed, 112 insertions(+), 4 deletions(-)
> > > > > > > > > > > >
> > > > > > > > > > > > diff --git a/libavcodec/avcodec.h b/libavcodec/avcodec.h
> > > > > > > > > > > > index 7dbf083a24..e60eb16ce1 100644
> > > > > > > > > > > > --- a/libavcodec/avcodec.h
> > > > > > > > > > > > +++ b/libavcodec/avcodec.h
> > > > > > > > > > > > @@ -513,6 +513,11 @@ typedef struct AVProducerReferenceTime {
> > > > > > > > > > > > */
> > > > > > > > > > > > #define AV_GET_BUFFER_FLAG_REF (1 << 0)
> > > > > > > > > > > > +/**
> > > > > > > > > > > > + * The encoder will keep a reference to the packet and
> > > > > > > > > > > > may reuse it later.
> > > > > > > > > > > > + */
> > > > > > > > > > > > +#define AV_GET_ENCODER_BUFFER_FLAG_REF (1 << 0)
> > > > > > > > > > > > +
> > > > > > > > > > > > struct AVCodecInternal;
> > > > > > > > > > > > /**
> > > > > > > > > > > > @@ -2346,6 +2351,39 @@ typedef struct AVCodecContext {
> > > > > > > > > > > > * - encoding: set by user
> > > > > > > > > > > > */
> > > > > > > > > > > > int export_side_data;
> > > > > > > > > > > > +
> > > > > > > > > > > > + /**
> > > > > > > > > > > > + * This callback is called at the beginning of each
> > > > > > > > > > > > packet to get a data
> > > > > > > > > > > > + * buffer for it.
> > > > > > > > > > > > + *
> > > > > > > > > > > > + * The following field will be set in the packet
> > > > > > > > > > > > before this callback is
> > > > > > > > > > > > + * called:
> > > > > > > > > > > > + * - size
> > > > > > > > > > > > + * This callback must use the above value to
> > > > > > > > > > > > calculate the required buffer size,
> > > > > > > > > > > > + * which must padded by at least
> > > > > > > > > > > > AV_INPUT_BUFFER_PADDING_SIZE bytes.
> > > > > > > > > > > > + *
> > > > > > > > > > > > + * This callback must fill the following fields in the packet:
> > > > > > > > > > > > + * - data
> > > > > > > > > > >
> > > > > > > > > > > Is the data pointer allowed to be in write-only memory?
> > > > > > > > > >
> > > > > > > > > > I'm not sure what the use case for this would be, so probably no?
> > > > > > > > >
> > > > > > > > > The two use-cases I see for this API are:
> > > > > > > > >
> > > > > > > > > * You want to avoid a copy when combining the output with something
> > > > > > > > > else. E.g. you pass a pointer to the block of memory following
> > > > > > > > > where you are going to put your header data (for something you are
> > > > > > > > > going to send over the network, say).
> > > > > > > > >
> > > > > > > > > * You want to avoid a copy when passing the output directly to
> > > > > > > > > something external. E.g. you pass a pointer to a memory-mapped
> > > > > > > > > device buffer (such as a V4L2 buffer, say).
> > > > > > > > >
> > > > > > > > > In the second case, write-only memory on an external device seems
> > > > > > > > > possible, as does memory which is, say, readable but uncached, so
> > > > > > > > > reading it is a really bad idea.
> > > > > > > >
> > > > > > > > Allowing the second case would depend on how encoders behave. Some may
> > > > > > > > attempt to read data already written to the output packet. It's not like
> > > > > > > > all of them allocate the packet, do a memcpy from an internal buffer,
> > > > > > > > then return.
> > > > > > > > There is also the flag meant to signal that the encoder will keep a
> > > > > > > > reference to the packet around, which more or less implies it will be
> > > > > > > > read later in the encoding process.
> > > > > > > >
> > > > > > > > The doxy for avcodec_encode_video2(), which allowed the user to provide
> > > > > > > > their own buffers in the output packet, does not mention any kind of
> > > > > > > > requirement for the data pointer, so I don't think we can say it's an
> > > > > > > > allowed scenario here either.
> > > > > > > >
> > > > > > > > >
> > > > > > > > > > > Does it have any alignment requirements?
> > > > > > > > > >
> > > > > > > > > > No, just padding. AVPacket doesn't require alignment for the payload.
> > > > > > > > >
> > > > > > > > > I think say that explicitly. avcodec_default_get_encoder_buffer()
> > > > > > > > > does give you aligned memory, even though it isn't needed.
> > > > > > > >
> > > > > > > > Would saying "There's no alignment requirement for the data pointer" add
> > > > > > > > anything of value to the doxy? If i don't mention any kind of alignment
> > > > > > > > requirement, it's because there isn't any, and it's implicit.
> > > > > > > > I listed the requirements the user needs to keep in mind, like the
> > > > > > > > padding and the need for an AVBufferRef. But if you think it's worth
> > > > > > > > adding, then sure.
> > > > > > > >
> > > > > > > > >
> > > > > > > > > > > > + * - buf must contain a pointer to an AVBufferRef
> > > > > > > > > > > > structure. The packet's
> > > > > > > > > > > > + * data pointer must be contained in it.
> > > > > > > > > > > > + * See: av_buffer_create(), av_buffer_alloc(),
> > > > > > > > > > > > and av_buffer_ref().
> > > > > > > > > > > > + *
> > > > > > > > > > > > + * If AV_CODEC_CAP_DR1 is not set then
> > > > > > > > > > > > get_encoder_buffer() must call
> > > > > > > > > > > > + * avcodec_default_get_encoder_buffer() instead of
> > > > > > > > > > > > providing a buffer allocated by
> > > > > > > > > > > > + * some other means.
> > > > > > > > > > > > + *
> > > > > > > > > > > > + * If AV_GET_ENCODER_BUFFER_FLAG_REF is set in
> > > > > > > > > > > > flags then the packet may be reused
> > > > > > > > > > > > + * (read and/or written to if it is writable) later
> > > > > > > > > > > > by libavcodec.
> > > > > > > > > > > > + *
> > > > > > > > > > > > + * This callback must be thread-safe, as when frame
> > > > > > > > > > > > multithreading is used, it may
> > > > > > > > > > > > + * be called from multiple threads simultaneously.
> > > > > > > > > > >
> > > > > > > > > > > Allowing simulatenous calls feels unexpectedly tricky. Is
> > > > > > > > > > > it really necessary?
> > > > > > > > > >
> > > > > > > > > > This was a suggestion by Lynne, i personally don't know. We
> > > > > > > > > > support frame threading encoding (For intra-only codecs), but
> > > > > > > > > > currently ff_alloc_packet2() does not seem to be thread safe,
> > > > > > > > > > seeing it calls av_fast_padded_malloc(), yet it's called by
> > > > > > > > > > frame threaded encoders.
> > > > > > > > > > Should i remove this?
> > > > > > > > >
> > > > > > > > > I don't know, I was asking only because it sounds tricky. For cases
> > > > > > > > > with a limited number of buffers available (like memory-mapped
> > > > > > > > > devices) you are going to need locking anyway, so maybe rentrancy
> > > > > > > > > adds no additional inconvenience.
> > > > > > > > >
> > > > > > > > > > > > + *
> > > > > > > > > > > > + * @see avcodec_default_get_encoder_buffer()
> > > > > > > > > > > > + *
> > > > > > > > > > > > + * - encoding: Set by libavcodec, user can override.
> > > > > > > > > > > > + * - decoding: unused
> > > > > > > > > > > > + */
> > > > > > > > > > > > + int (*get_encoder_buffer)(struct AVCodecContext *s,
> > > > > > > > > > > > AVPacket *pkt, int flags);
> > > > > > > > > > >
> > > > > > > > > > > Can the encoder ask for arbitrarily many packets?
> > > > > > > > > > >
> > > > > > > > > > > Can the user return "not yet" somehow to this if they have a
> > > > > > > > > > > fixed output buffer pool but no buffer is currently
> > > > > > > > > > > available?
> > > > > > > > > >
> > > > > > > > > > No, as is it can't. Return values < 0 are considered errors.
> > > > > > > > > >
> > > > > > > > > > >
> > > > > > > > > > > I don't much like the idea of the user suspending the thread
> > > > > > > > > > > in the callback until they have some available, which might
> > > > > > > > > > > work in some cases but might also deadlock if an
> > > > > > > > > > > avcodec_receive_packet() call is blocked by it.
> > > > > > > > > >
> > > > > > > > > > Can we make what's in essence a malloc() call return something
> > > > > > > > > > like EAGAIN, and this in turn be propagated back to
> > > > > > > > > > encode_receive_packet_internal()?
> > > > > > > > >
> > > > > > > > > Maybe, or if it has many threads maybe it could wait for something
> > > > > > > > > else to finish first.
> > > > > > > > >
> > > > > > > > > > Couldn't this potentially end up in the forbidden scenario of
> > > > > > > > > > avcodec_send_frame() and avcodec_receive_packet() both returning
> > > > > > > > > > EAGAIN?
> > > > > > > > >
> > > > > > > > > Yes. If the forbidden case happens then the encoder is stuck anyway
> > > > > > > > > and can't make any forward progress so we need to error out
> > > > > > > > > properly, but the EAGAIN return isn't needed if there is something
> > > > > > > > > else to do on another thread.
> > > > > > > >
> > > > > > > > Ok, but I'm not familiar or knowledgeable enough with the frame thread
> > > > > > > > encoder code to implement this.
> > > > > > >
> > > > > > > Looked at bit into this. AVCodec->encode2() based encoders don't support
> > > > > > > returning EAGAIN at all, as it completely breaks the frame threading logic.
> > > > > > > It would require a considerable rewrite in order to re-add a task that
> > > > > > > didn't fail but also didn't succeed.
> > > > > > >
> > > > > > > Non frame threading encoders could probably support it with some minimal
> > > > > > > changes, but i don't think suddenly letting an scenario that was until now
> > > > > > > guaranteed to never happen start happening (avcodec_send_frame() and
> > > > > > > avcodec_receive_packet() both returning EAGAIN) is a good idea. It's an API
> > > > > > > break.
> > > > > > > Letting the user's custom get_encode_buffer() callback suspend the thread is
> > > > > > > IMO acceptable. In frame threading scenarios, the other threads are still
> > > > > > > working on their own packets (afaics none depends on the others, since it's
> > > > > > > intra only encoders only).
> > > > > >
> > > > > > I think it was not suggested in the thread so:
> > > > > > if the users allocation fails the code can fallback to the default allocator
> > > > > > That would lead to the relation:
> > > > > > If a users allocator can fail (out of buffers) it must be able to handle
> > > > > > that only some of the returned packets are from its own allocator
> > > > >
> > > > > In general, custom allocators are used when the caller doesn't want to use
> > > > > the default one. But yes, they could use
> > > > > avcodec_default_get_encoder_buffer() as fallback, which is why it was added
> > > > > to begin with. Same applies to get_buffer2() custom implementations, and so
> > > > > far i don't think anybody had issues identifying what allocated a packet
> > > > > buffer.
> > > > >
> > > > > One of the additions to AVPacket people were talking about was a user opaque
> > > > > field that libav* would never touch or look at beyond propagating them
> > > > > around all the way to the output AVFrame, if any. This opaque field could
> > > > > perhaps store such allocator specific information the caller could use to
> > > > > identify packets allocated by their own allocator, or those by
> > > > > avcodec_default_get_encoder_buffer().
> > > > >
> > > > > >
> > > > > > About alignment, we should at least recommand that allocated packets are
> > > > > > aligned not less than what out av_malloc() would align to.
> > > > > > Is there a reason to align less ?
> > > > >
> > > > > There's no alignment requirement for AVPacket->data, and av_new_packet()
> > > > > uses av_buffer_realloc(), which does not guarantee any alignment whatsoever
> > > > > on platforms other than Windows. So basically, packet payload buffers
> > > > > allocated by our own helpers never had any alignment.
> > > >
> > > > for the purpose of exporting raw images, alignment would be "nice to have"
> > > > because later filters may need it or need to memcpy
> > >
> > > Filters don't use AVPackets, they use AVFrames.
> >
> > demuxers return AVPackets, so do encoders.
> > These can contain raw frames.
> >
> > also i see for example in rawdec:
> > frame->buf[0] = av_buffer_ref(avpkt->buf);
>
> I ask again, where are you going with this? The alignment for AVPacket data
> buffers is defined: There is *none*.
I simply stated that 'alignment would be "nice to have"'.
and then showed some cases where it would be usefull.
I guess where iam going with this is, is the API you add extensible?
That is if something is not supported now, can it be added later without
adding a new API.
Thanks
[...]
--
Michael GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB
Does the universe only have a finite lifespan? No, its going to go on
forever, its just that you wont like living in it. -- Hiranya Peiri
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