vaapi dec.: deduce SW format to reported valid

Set AVHWFramesContext::sw_format to first of av_hwframe_transfer_get_formats().

This is consistent how MPV does that. Fixes NV12 being transmitted
despite AVHWFramesContext::sw_format was set to yuv420p causing chroma
channels corruption (because the nv12 data was misinterpreted as the
latter one) occuring on AMD cards, steps to reproduce:
```
uv -t testcard -c lavc:enc=libx264:safe -d gl --param use-hw-accel=vaapi
```

See also:
<mpv-player/mpv@66e30e7>

src/hwaccel_vaapi.c

@@ -48,18 +48,20 @@
 #include "config_win32.h"
 #endif // defined HAVE_CONFIG_H
 
-#include "hwaccel_vaapi.h"
-
-#include "debug.h"
-
-#include "hwaccel_libav_common.h"
 #include <libavcodec/version.h>
+#include <libavutil/pixdesc.h>
 #if LIBAVCODEC_VERSION_INT < AV_VERSION_INT(57, 74, 100)
 #include <libavcodec/vaapi.h>
 #endif
 #include <libavutil/hwcontext_vaapi.h>
 
+#include "debug.h"
+#include "hwaccel_libav_common.h"
+#include "hwaccel_vaapi.h"
+#include "libavcodec/lavc_common.h"
+
 #define DEFAULT_SURFACES 20
+#define MOD_NAME "[vaapi] "
 
 struct vaapi_ctx {
         AVBufferRef          *device_ref;
@@ -196,6 +198,84 @@ static int vaapi_create_context(struct vaapi_ctx *ctx,
 }
 #endif //LIBAVCODEC_VERSION_INT < AV_VERSION_INT(57, 74, 100)
 
+/**
+ * Returns first SW format from valid_sw_formats. This is usually
+ * AV_PIX_FMT_YUV420P or AV_PIX_FMT_NV12.
+ *
+ * The code borrows heavily from mpv
+ * <https://github.com/mpv-player/mpv/blob/master/video/out/hwdec/hwdec_vaapi.c>
+ * namely from function try_format_config().
+ */
+static enum AVPixelFormat
+get_sw_format(VADisplay display, AVBufferRef *device_ref,
+              enum AVPixelFormat fallback_fmt)
+{
+        enum AVPixelFormat     ret       = AV_PIX_FMT_NONE;
+        AVVAAPIHWConfig       *hwconfig  = NULL;
+        VAConfigID             config_id = 0;
+        AVHWFramesConstraints *fc        = NULL;
+
+        VAStatus status = vaCreateConfig(
+            display, VAProfileNone, VAEntrypointVideoProc, NULL, 0, &config_id);
+        if (status != VA_STATUS_SUCCESS) {
+                MSG(ERROR, "cannot create config\n");
+                goto fail;
+        }
+        fc = av_hwdevice_get_hwframe_constraints(device_ref, hwconfig);
+        if (!fc) {
+                MSG(ERROR, "failed to retrieve libavutil frame constraints\n");
+                goto fail;
+        }
+
+        /*
+         * 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(device_ref);
+        if (!fref) {
+                MSG(ERROR, "failed to alloc libavutil frame context\n");
+                goto fail;
+        }
+        AVHWFramesContext *fctx = (void *) fref->data;
+        enum {
+                INIT_SIZE = 128, ///< just some valid size
+        };
+        fctx->format    = AV_PIX_FMT_VAAPI;
+        fctx->sw_format = fc->valid_sw_formats[0];
+        fctx->width     = INIT_SIZE;
+        fctx->height    = INIT_SIZE;
+        if (av_hwframe_ctx_init(fref) < 0) {
+                MSG(ERROR, "failed to init libavutil frame context\n");
+                goto fail;
+        }
+
+        enum AVPixelFormat *fmts = NULL;
+        int                 rc   = av_hwframe_transfer_get_formats(
+            fref, AV_HWFRAME_TRANSFER_DIRECTION_FROM, &fmts, 0);
+        if (rc) {
+                MSG(ERROR, "failed to get libavutil frame context supported "
+                           "formats\n");
+                goto fail;
+        }
+        MSG(DEBUG, "Available HW layouts: %s\n", get_avpixfmts_names(fmts));
+        ret = fmts[0];
+
+fail:
+        av_hwframe_constraints_free(&fc);
+        av_buffer_unref(&fref);
+        if (ret == AV_PIX_FMT_NONE) {
+                MSG(WARNING, "Using fallback HW frames layout: %s\n",
+                    av_get_pix_fmt_name(ret));
+                ret = fallback_fmt;
+        }
+        MSG(VERBOSE, "Selected HW frames layout: %s\n",
+            av_get_pix_fmt_name(ret));
+        return ret;
+}
+
 int vaapi_init(struct AVCodecContext *s,
                 struct hw_accel_state *state,
                 codec_t out_codec)
@@ -224,11 +304,13 @@ int vaapi_init(struct AVCodecContext *s,
         if (s->active_thread_type & FF_THREAD_FRAME)
                 decode_surfaces += s->thread_count;
 
+        const enum AVPixelFormat sw_format = get_sw_format(
+            ctx->device_vaapi_ctx->display, ctx->device_ref, s->sw_pix_fmt);
         ret = create_hw_frame_ctx(ctx->device_ref,
                         s->coded_width,
                         s->coded_height,
                         AV_PIX_FMT_VAAPI,
-                        s->sw_pix_fmt,
+                        sw_format,
                         decode_surfaces,
                         &ctx->hw_frames_ctx);
         if(ret < 0)

src/libavcodec/lavc_common.c

@@ -56,6 +56,7 @@
 
 #include "host.h"
 #include "libavcodec/lavc_common.h"
+#include "utils/macros.h"
 #include "video.h"
 
 #define MOD_NAME "[lavc_common] "
@@ -356,4 +357,25 @@ audio_bps_to_av_sample_fmt(int bps, bool planar)
                 abort();
         }
 }
+
+/**
+ * Prints space-separated nammes of AVPixelFormats in AV_PIX_FMT_NONE-terminated
+ * pixfmts list to given buf and returns pointer to given buf.
+ */
+const char *
+get_avpixfmts_names(const enum AVPixelFormat *pixfmts)
+{
+        _Thread_local static char buf[STR_LEN];
+        if (pixfmts == NULL || *pixfmts == AV_PIX_FMT_NONE) {
+                snprintf(buf, sizeof buf, " (none)");
+                return buf;
+        }
+        const enum AVPixelFormat *it = pixfmts;
+        while (*it != AV_PIX_FMT_NONE) {
+                snprintf(buf + strlen(buf), sizeof buf - strlen(buf), "%s%s",
+                         it != pixfmts ? " " : "", av_get_pix_fmt_name(*it));
+                it++;
+        }
+        return buf;
+}
 /* vi: set expandtab sw=8: */

src/libavcodec/lavc_common.h

@@ -123,6 +123,7 @@ void lavd_flush(AVCodecContext *codec_ctx);
 const char *lavc_thread_type_to_str(int thread_type);
 struct audio_desc audio_desc_from_av_frame(const AVFrame *frm);
 enum AVSampleFormat audio_bps_to_av_sample_fmt(int bps, bool planar);
+const char         *get_avpixfmts_names(const enum AVPixelFormat *pixfmts);
 
 #ifdef __cplusplus
 }

src/video_compress/libavcodec.cpp

@@ -672,16 +672,10 @@ static int vaapi_init(struct AVCodecContext *s){
 #endif
 
 void print_codec_supp_pix_fmts(const enum AVPixelFormat *first) {
-        char out[STR_LEN] = MOD_NAME "Codec supported pixel formats:" TERM_BOLD;
-        if (first == nullptr) {
-                snprintf(out + strlen(out), sizeof out - strlen(out),
-                         " (none)");
-        }
-        const enum AVPixelFormat *it = first;
-        while (it != nullptr && *it != AV_PIX_FMT_NONE) {
-                snprintf(out + strlen(out), sizeof out - strlen(out), " %s",
-                         av_get_pix_fmt_name(*it++));
-        }
+        char out[STR_LEN];
+        snprintf(out, sizeof out,
+                 MOD_NAME "Codec supported pixel formats: " TBOLD("%s"),
+                 get_avpixfmts_names(first));
         LOG(LOG_LEVEL_VERBOSE) << wrap_paragraph(out) << TERM_RESET "\n";
 }