-----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 directions
[..]
you can no longer operate on devices independently across threads (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 thread-safety properties of the hwcontext API have been lost
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.
* 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. 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. That's why I said: yes, I understand the case you are talking about. But it would require two separate patches, one for enabling extension options and another one for matching extension options when deriving a device. This would make sense, but: It has nothing to do with this patchset. (it could be done before or afterwards) I'm open to discuss this (separately), because it opens up a range of questions how it could be done.
diff --git a/libavutil/hwcontext.c b/libavutil/hwcontext.c index ab9ad3703e..1aea7dd5c3 100644 --- a/libavutil/hwcontext.c +++ b/libavutil/hwcontext.c @@ -123,6 +123,7 @@ static const AVClass hwdevice_ctx_class = { static void hwdevice_ctx_free(void *opaque, uint8_t *data) { AVHWDeviceContext *ctx = (AVHWDeviceContext*)data; + int i;
/* uninit might still want access the hw context and the user * free() callback might destroy it, so uninit has to be called first */ @@ -133,6 +134,8 @@ static void hwdevice_ctx_free(void *opaque, uint8_t *data) ctx->free(ctx);
av_buffer_unref(&ctx->internal->source_device); + for (i = 0; i < AV_HWDEVICE_TYPE_NB; i++) + av_buffer_unref(&ctx->internal->derived_devices[i]);
av_freep(&ctx->hwctx); av_freep(&ctx->internal->priv); @@ -644,10 +647,31 @@ fail: return ret; }
-int av_hwdevice_ctx_create_derived_opts(AVBufferRef **dst_ref_ptr, - enum AVHWDeviceType type, - AVBufferRef *src_ref, - AVDictionary *options, int flags) +static AVBufferRef* find_derived_hwdevice_ctx(AVBufferRef *src_ref, enum AVHWDeviceType type) +{ + AVBufferRef *tmp_ref; + AVHWDeviceContext *src_ctx; + int i; + + src_ctx = (AVHWDeviceContext*)src_ref->data; + if (src_ctx->type == type) + return src_ref; + + for (i = 0; i < AV_HWDEVICE_TYPE_NB; i++) + if (src_ctx->internal->derived_devices[i]) { + tmp_ref = find_derived_hwdevice_ctx(src_ctx- internal- derived_devices[i], type); + if (tmp_ref) + return tmp_ref; + } + + return NULL; +} + +static int hwdevice_ctx_create_derived(AVBufferRef **dst_ref_ptr, + enum AVHWDeviceType type, + AVBufferRef *src_ref, + AVDictionary *options, int flags, + int get_existing) { AVBufferRef *dst_ref = NULL, *tmp_ref; AVHWDeviceContext *dst_ctx, *tmp_ctx; @@ -667,6 +691,18 @@ int av_hwdevice_ctx_create_derived_opts(AVBufferRef **dst_ref_ptr, tmp_ref = tmp_ctx->internal->source_device; }
+ if (get_existing) { + tmp_ref = find_derived_hwdevice_ctx(src_ref, type); + if (tmp_ref) { + dst_ref = av_buffer_ref(tmp_ref); + if (!dst_ref) { + ret = AVERROR(ENOMEM); + goto fail; + } + goto done; + } + } + dst_ref = av_hwdevice_ctx_alloc(type); if (!dst_ref) { ret = AVERROR(ENOMEM); @@ -688,6 +724,13 @@ int av_hwdevice_ctx_create_derived_opts(AVBufferRef **dst_ref_ptr, ret = AVERROR(ENOMEM); goto fail; } + if (!tmp_ctx->internal->derived_devices[type]) {
I wonder whether you only want to do this when the user made the new call, not the old one?
The semantics would perhaps feel clearer as "get or create the shared derived device" rather than "get the first device derived or create a new one if not".
I've been there for a moment, and then I thought that when the API consumer would mix API calls, e.g. first without 'get' and second with 'get', then the second call would not produce the expected result.
Let me know what you think, I have no strong opinion about this.
Can you explain your example further?
Maybe we should get clear about what this patchset does exactly. Let's look at the following derivation chain of devices: A ├─ X │ └─ Y ├─ B │ └─ C └─ V └─ W The meaning is: - Y is derived from X, X is derived from A - C is derived from B, B is derived from A - W is derived from V, V is derived from A In the existing implementation, each device "knows" its parent (via the 'source_device' field). When you call av_hwdevice_ctx_create_derived() and specify "C" as the source device, then it will iterate the tree upwards, so when B is of the requested type, it will return B or if A is of the requested type, it will return A. Otherwise, it will create a new device of the requested type and sets C as its parent. But it doesn't return X, Y, V or W (when any would match the requested type). This is the current behavior. What this patchset does is that we also store the derived children for each device (derived_devices array). In the example above, it means hat A has references to X, B and V. X to Y, B to C and V to W. The behavior of the new function is as follows: When you call av_hwdevice_ctx_get_or_create_derived() and specify "C" as the source device, then it will iterate the tree upwards, so when B is of the requested type, it will return B or if A is of the requested type, it will return A (like before). Additionally, it will also iterate all through other children of B and other children of A. Which means that if X, Y, V or W matches the requested type, it would return it. Otherwise, it will create a new device of the requested type and sets C as its parent. This is the behavior of the new function. All that it changes is that it searches the full tree instead of the parents only. Now back to your original question:
+ if (!tmp_ctx->internal->derived_devices[type]) {
I wonder whether you only want to do this when the user made the new call, not the old one?
The semantics would perhaps feel clearer as "get or create the shared derived device" rather than "get the first device derived or create a new one if not".
The line of code you commented on, is about adding a newly created derived device to the child collection of its parent.
I wonder whether you only want to do this when the user made the new call, not the old one?
The difference between old and new is that old searches only parents and new searches the full tree. But should calling the old function also control whether a newly created derived device is added to the child collection of its parent? It might be confusing behavior when you first call the old function and later call the new function but you would get the device only when it's a parent but not when it's a down-tree-child of any parent. And that's where I said:
Let me know what you think, I have no strong opinion about this.
Best regards, softworkz