hwdec/vaapi: improve probing of supported sw formats

The logic around trying to establish what formats are supported by
vaapi is confusing, and it turns out that we've been doing it wrong.

Up until now, we've been going through the list of decoding profile
endpoints and checking the formats declared in those profiles to build
our list.

However, the set of formats that the vaapi driver can actually support
is potentially a superset of those supported by any given profile
endpoint. This master list is exposed by libavutil via the
av_hwframe_transfer_get_formats() function, and so we should use that
list as our starting point.

Perhaps surprisingly, this change actually removes no code, because we
still want the logic that enumerates endpoints and finds supported
formats for endpoints. We need this because we must have at least one
known sw format to initialise the hwframe_ctx with.

Additionally, while in the general case,
av_hwframe_transfer_get_formats can return different formats depending
on what format you initialise the hwframe_ctx with, I happen to have
read the libavutil code, and it doesn't care, so we just need to call
it once, we know we'll get back all known formats.

In practice, with an Intel vaapi driver, this will result in us
supporting a handful of extra formats for hwuploads - particularly
yuv420p (so no need to convert to nv12 first) and various orderings
of rgb(a).

video/out/hwdec/hwdec_vaapi.c

@@ -380,16 +380,51 @@ err:
 static void try_format_config(struct ra_hwdec *hw, AVVAAPIHWConfig *hwconfig)
 {
     struct priv_owner *p = hw->priv;
+    enum AVPixelFormat *fmts = NULL;
+
     AVHWFramesConstraints *fc =
             av_hwdevice_get_hwframe_constraints(p->ctx->av_device_ref, hwconfig);
     if (!fc) {
         MP_WARN(hw, "failed to retrieve libavutil frame constraints\n");
         return;
     }
-    for (int n = 0; fc->valid_sw_formats &&
-                    fc->valid_sw_formats[n] != AV_PIX_FMT_NONE; n++)
-        try_format_pixfmt(hw, fc->valid_sw_formats[n]);
+
+    /*
+     * We need a hwframe_ctx to be able to get the valid formats, but to
+     * initialise it, we need a format, so we get the first format from the
+     * hwconfig. We don't care about the other formats in the config because the
+     * transfer formats list will already include them.
+     */
+    AVBufferRef *fref = NULL;
+    fref = av_hwframe_ctx_alloc(p->ctx->av_device_ref);
+    if (!fref) {
+        MP_WARN(hw, "failed to alloc libavutil frame context\n");
+        goto err;
+    }
+    AVHWFramesContext *fctx = (void *)fref->data;
+    fctx->format = AV_PIX_FMT_VAAPI;
+    fctx->sw_format = fc->valid_sw_formats[0];
+    fctx->width = 128;
+    fctx->height = 128;
+    if (av_hwframe_ctx_init(fref) < 0) {
+        MP_WARN(hw, "failed to init libavutil frame context\n");
+        goto err;
+    }
+
+    int ret = av_hwframe_transfer_get_formats(fref, AV_HWFRAME_TRANSFER_DIRECTION_TO, &fmts, 0);
+    if (ret) {
+        MP_WARN(hw, "failed to get libavutil frame context supported formats\n");
+        goto err;
+    }
+
+    for (int n = 0; fmts &&
+                    fmts[n] != AV_PIX_FMT_NONE; n++)
+        try_format_pixfmt(hw, fmts[n]);
+
+err:
     av_hwframe_constraints_free(&fc);
+    av_buffer_unref(&fref);
+    av_free(fmts);
 }
 
 static void determine_working_formats(struct ra_hwdec *hw)
@@ -416,6 +451,12 @@ static void determine_working_formats(struct ra_hwdec *hw)
     if (!CHECK_VA_STATUS(hw, "vaQueryConfigProfiles()"))
         num_profiles = 0;
 
+    /*
+     * We need to find one declared format to bootstrap probing. So find a valid
+     * decoding profile and use its config. If try_format_config() finds any
+     * formats, they will be all the supported formats, and we don't need to
+     * look at any other profiles.
+     */
     for (int n = 0; n < num_profiles; n++) {
         VAProfile profile = profiles[n];
         if (profile == VAProfileNone) {
@@ -448,6 +489,9 @@ static void determine_working_formats(struct ra_hwdec *hw)
             try_format_config(hw, hwconfig);
 
             vaDestroyConfig(p->display, config);
+            if (p->formats && p->formats[0]) {
+                goto done;
+            }
         }
     }