kmsgrab: Use invalid modifier if modifiers weren't used.

The kernel defaults to initializing the field to 0 when modifiers
are not used and this happens to be linear. If we end up actually
passing the modifier to a driver, tiling issues happen.

So if the kernel doesn't return a modifier set it explicitly to
INVALID. That way later processing knows there is no explicit
modifier.

Signed-off-by: Mark Thompson <sw@jkqxz.net>

libavdevice/kmsgrab.c

@@ -160,6 +160,7 @@ static int kmsgrab_get_fb2(AVFormatContext *avctx,
     KMSGrabContext *ctx = avctx->priv_data;
     drmModeFB2 *fb;
     int err, i, nb_objects;
+    uint64_t modifier = ctx->drm_format_modifier;
 
     fb = drmModeGetFB2(ctx->hwctx->fd, plane->fb_id);
     if (!fb) {
@@ -195,6 +196,9 @@ static int kmsgrab_get_fb2(AVFormatContext *avctx,
         goto fail;
     }
 
+    if (fb->flags & DRM_MODE_FB_MODIFIERS)
+        modifier = fb->modifier;
+
     *desc = (AVDRMFrameDescriptor) {
         .nb_layers = 1,
         .layers[0] = {
@@ -243,7 +247,7 @@ static int kmsgrab_get_fb2(AVFormatContext *avctx,
             desc->objects[obj] = (AVDRMObjectDescriptor) {
                 .fd              = fd,
                 .size            = size,
-                .format_modifier = fb->modifier,
+                .format_modifier = modifier,
             };
             desc->layers[0].planes[i] = (AVDRMPlaneDescriptor) {
                 .object_index = obj,
@@ -557,6 +561,8 @@ static av_cold int kmsgrab_read_header(AVFormatContext *avctx)
             err = AVERROR(EINVAL);
             goto fail;
         }
+
+        if (fb2->flags & DRM_MODE_FB_MODIFIERS) {
             if (ctx->drm_format_modifier != DRM_FORMAT_MOD_INVALID &&
                 ctx->drm_format_modifier != fb2->modifier) {
                 av_log(avctx, AV_LOG_ERROR, "Framebuffer format modifier "
@@ -567,6 +573,7 @@ static av_cold int kmsgrab_read_header(AVFormatContext *avctx)
             } else {
                 ctx->drm_format_modifier = fb2->modifier;
             }
+        }
         av_log(avctx, AV_LOG_VERBOSE, "Format is %s, from "
                "DRM format %"PRIx32" modifier %"PRIx64".\n",
                av_get_pix_fmt_name(ctx->format),