va: allocator: Use derived images only if not mapped for reading.

Derived images are direct maps to surfaces bits, but in Intel Gen7 to
Gen9, that memory is not cachable, thus reading can be very slow (it
might produce timeout is tests such as fluster).

This patch tries first to define if derived images are possible, and
later use them only if mapping is not for reading.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/2128>

sys/va/gstvaallocator.c

@@ -1012,6 +1012,7 @@ struct _GstVaAllocator
   guint32 fourcc;
   guint32 rt_format;
 
+  GstVideoInfo derived_info;
   GstVideoInfo info;
   guint usage_hint;
 
@@ -1146,13 +1147,24 @@ _ensure_image (GstVaDisplay * display, VASurfaceID surface,
   return ret;
 }
 
+static inline void
+_update_info (GstVideoInfo * info, const VAImage * image)
+{
+  guint i;
+
+  for (i = 0; i < image->num_planes; i++) {
+    GST_VIDEO_INFO_PLANE_OFFSET (info, i) = image->offsets[i];
+    GST_VIDEO_INFO_PLANE_STRIDE (info, i) = image->pitches[i];
+  }
+
+  GST_VIDEO_INFO_SIZE (info) = image->data_size;
+}
+
 static inline gboolean
 _update_image_info (GstVaAllocator * va_allocator)
 {
   VASurfaceID surface;
   VAImage image = {.image_id = VA_INVALID_ID, };
-  gboolean derived;
-  guint i;
 
   /* Create a test surface first */
   if (!_create_surfaces (va_allocator->display, va_allocator->rt_format,
@@ -1169,13 +1181,16 @@ _update_image_info (GstVaAllocator * va_allocator)
 
   /* Try derived first, but different formats can never derive */
   if (va_allocator->surface_format == va_allocator->img_format) {
-    derived = TRUE;
+    if (_get_derive_image (va_allocator->display, surface, &image)) {
-    if (_get_derive_image (va_allocator->display, surface, &image))
+      va_allocator->use_derived = TRUE;
-      goto update;
+      va_allocator->derived_info = va_allocator->info;
+      _update_info (&va_allocator->derived_info, &image);
+      _destroy_image (va_allocator->display, image.image_id);
+    }
+    image.image_id = VA_INVALID_ID;     /* reset it */
   }
 
   /* Then we try to create a image. */
-  derived = FALSE;
   if (!_create_image (va_allocator->display, va_allocator->img_format,
           GST_VIDEO_INFO_WIDTH (&va_allocator->info),
           GST_VIDEO_INFO_HEIGHT (&va_allocator->info), &image)) {
@@ -1183,16 +1198,7 @@ _update_image_info (GstVaAllocator * va_allocator)
     return FALSE;
   }
 
-update:
+  _update_info (&va_allocator->info, &image);
-  va_allocator->use_derived = derived;
-
-  for (i = 0; i < image.num_planes; i++) {
-    GST_VIDEO_INFO_PLANE_OFFSET (&va_allocator->info, i) = image.offsets[i];
-    GST_VIDEO_INFO_PLANE_STRIDE (&va_allocator->info, i) = image.pitches[i];
-  }
-
-  GST_VIDEO_INFO_SIZE (&va_allocator->info) = image.data_size;
-
   _destroy_image (va_allocator->display, image.image_id);
   _destroy_surfaces (va_allocator->display, &surface, 1);
 
@@ -1203,8 +1209,10 @@ static gpointer
 _va_map_unlocked (GstVaMemory * mem, GstMapFlags flags)
 {
   GstAllocator *allocator = GST_MEMORY_CAST (mem)->allocator;
+  GstVideoInfo *info;
   GstVaAllocator *va_allocator;
   GstVaDisplay *display;
+  gboolean use_derived;
 
   g_return_val_if_fail (mem->surface != VA_INVALID_ID, NULL);
   g_return_val_if_fail (GST_IS_VA_ALLOCATOR (allocator), NULL);
@@ -1230,11 +1238,20 @@ _va_map_unlocked (GstVaMemory * mem, GstMapFlags flags)
     goto success;
   }
 
-  if (!_ensure_image (display, mem->surface, &va_allocator->info, &mem->image,
+  /* On Gen7-Gen9 Intel graphics the memory is mappable but not
-          va_allocator->use_derived))
+   * cached, so normal memcpy() access is very slow to read, but it's
+   * ok for writing. So let's assume that users won't prefer
+   * direct-mapped memory if they request read access. */
+  use_derived = va_allocator->use_derived && !(flags & GST_MAP_READ);
+  if (use_derived)
+    info = &va_allocator->derived_info;
+  else
+    info = &va_allocator->info;
+
+  if (!_ensure_image (display, mem->surface, info, &mem->image, use_derived))
     return NULL;
 
-  mem->is_derived = va_allocator->use_derived;
+  mem->is_derived = use_derived;
 
   if (!mem->is_derived) {
     if (!_get_image (display, mem->surface, &mem->image))
@@ -1359,8 +1376,6 @@ gst_va_allocator_init (GstVaAllocator * self)
   allocator->mem_unmap = (GstMemoryUnmapFunction) _va_unmap;
   allocator->mem_share = _va_share;
 
-  self->use_derived = TRUE;
-
   gst_va_memory_pool_init (&self->pool);
 
   GST_OBJECT_FLAG_SET (self, GST_ALLOCATOR_FLAG_CUSTOM_ALLOC);