-----Original Message----- From: ffmpeg-devel <ffmpeg-devel-bounces@ffmpeg.org> On Behalf Of Mark Thompson Sent: Tuesday, May 3, 2022 1:57 AM To: ffmpeg-devel@ffmpeg.org Subject: Re: [FFmpeg-devel] [PATCH 1/3] avutils/hwcontext: add derive- device function which searches for existing devices in both directions
-----Original Message----- From: ffmpeg-devel <ffmpeg-devel-bounces@ffmpeg.org> On Behalf Of Mark Thompson Sent: Tuesday, May 3, 2022 12:12 AM To: ffmpeg-devel@ffmpeg.org Subject: Re: [FFmpeg-devel] [PATCH 1/3] avutils/hwcontext: add derive- device function which searches for existing devices in both
-----Original Message----- From: ffmpeg-devel <ffmpeg-devel-bounces@ffmpeg.org> On Behalf Of Mark Thompson Sent: Monday, May 2, 2022 12:01 AM To: ffmpeg-devel@ffmpeg.org Subject: Re: [FFmpeg-devel] [PATCH 1/3] avutils/hwcontext: add derive- device function which searches for existing devices in both
On 02/05/2022 09:14, Soft Works wrote: directions
[..]
> * The thread-safety properties of the hwcontext API have been
lost
-
> you can no longer operate on devices independently across
> (insofar as the underlying API allows that). > Maybe there is an argument that derivation is something which > should happen early on and therefore documenting it as thread- unsafe > is ok, but when hwupload/hwmap can use it inside filtergraphs that > just isn't going to happen (and will be violated in the FFmpeg utility > if filters get threaded, as is being worked on).
From my understanding there will be a single separate thread which handles all filtergraph operations. I don't think it would even be possible (without massive changes) to arbitrate filter processing in parallel. But even if this would be implemented: the filtergraph setup (init, uninit, query_formats, etc.) would surely happen on a single thread.
The ffmpeg utility creates filtergraphs dynamically when the first frame is available from their inputs, so I don't see why you wouldn't allow multiple of them to be created in parallel in that case.
If you create all devices at the beginning and then give references to them to the various filters which need them (so never manipulate devices dynamically within the graph) then it would be ok, but I think you've already rejected that approach.
Please let's not break out of the scope of this patchset. This patchset is not about re-doing device derivation. The only small change that it does is that it returns an existing device instead of creating a new one when such device already exists in the same(!) chain.
The change it makes has really nothing to do with threading or anything around it.
The change makes existing thread-safe hwcontext APIs unsafe. That is definitely not "nothing to do with threading", and has to be resolved since they can currently be called from contexts which expect
On 02/05/2022 23:59, Soft Works wrote: directions threads thread-
safety (such as making filtergraphs).
As I said, I'm not aware that filtergraph creation would be a context which requires thread-safety, even when considering frames which get pushed into a filtergraph (like from a decoder).
User programs can do whatever they like within the API constraints.
Could you please show a command line which causes a situation where device_derive is being called from multiple threads?
Given that the ffmpeg utility is currently entirely single-threaded, this will only happen once the intended plans for adding threading to it are complete.
As mentioned, I don't think that would be possible easily, and from what I have understood, the plan is to have separate threads for decoding, encoding and filtering but not multiple threads for filtering.
With that, it will be true for something which makes two filtergraphs and uses derivation in both of them. For example:, this currently makes two independent VAAPI devices, but equally could reuse the same device:
# ffmpeg -y -f kmsgrab -i /dev/dri/card0 -vf hwmap=derive_device=vaapi,scale_vaapi=format=nv12 -c:v h264_vaapi out1.mp4 -vf hwmap=derive_device=vaapi,scale_vaapi=w=1280:h=720:format=nv12 -c:v h264_vaapi out2.mp4
Well, multi-threading is not an issue right now, and I don't expect it to be then. But on a more practical take: what do you want me to do? Guard that function with a lock? I can do this, no problem. But none of any of the device control function does any synchronization, so why would exactly this - and only this function need synchronization?
> * I'm not sure that it is reasonable to ignore options. If an > unrelated component derived a device before you with special options, > you might get that device even if you have incompatible different > options.
I understand what you mean, but this is outside the scope of this patchset, because when you would want to do this, it would need to be implemented for derivation in general, not in this patchset which only adds reverse-search to the existing derivation functionality.
I'm not sure what you mean by that? The feature already exists; here is a concrete example of where you would get the wrong result:
Start with VAAPI device A.
Component P derives Vulkan device B with some extension options X.
Component Q wants the same device as P, so it derives again with extension options X and gets B.
Everything works fine for a while.
Later, unrelated component R is inserted before P and Q. It wants a Vulkan device C with extension options Y, so it derives that.
Now component Q is broken because it gets C instead of B and has the wrong extensions enabled.
As per your request, this patchset's changes are now limited to use ffmpeg cli. And there is currently no support for "extension options" when deriving a device.
Yes there is - see the "instance_extensions" and "device_extensions" options to Vulkan device derivation. (Hence the VAAPI+Vulkan example.)
OK, but when deriving a device via command line, it doesn't consider such parameters in the current device_derive function, and hence it's not something that can be burdened onto this patchset.
Then it sounds like you want this function to be part of the ffmpeg utility, not inside one of the libraries which have other users.
No. I say, matching device parameters when searching for an existing device isn't done right now, and it's outside the scope of this patchset.
What I meant above is this:
Assume this patchset wouldn't be applied, but a patchset would be applied that allows to specify "extension options". Then, even without this patchset, I could construct a similar example, where you would get the same device when deriving two times from the same device but with different extension options.
How?
VAAPI >> QSV(Param1) >> OpenCL
Now, from OpenCL, you want to derive QSV but with different parameters (Param2). You won't get a new device, you get the existing QSV device.
This doesn't make sense - remember that device derivation is unidirectional, and OpenCL is a leaf API which can only derive from other things. The "derive" operation there has to be interpreted as "return the device this was derived from", in which case options don't make sense.
(Indeed, it seems like a good idea to disallow options in that case. I will prepare a patch to that effect.)
OK.
No it doesn't? Your new function find_derived_hwdevice_ctx() is called only for the original source device, and recurses into its (transitive) children. It can't return any of X, Y, V or W when starting from C.
OK, that's my mistake. It's been a while... What I described is the original behavior. There were reasons to limit it this way.
Well, which one is is actually intended then?
This patchset. It is used by us and by Intel for quite a while already. Thanks, softworkz