gallium/st: add a back buffer fallback for front rendering

Unlike front buffer used by big GL API for front rendering,
EGL_KHR_mutable_render_buffer together with ES redirects GL_BACK to the
front buffer.

This patch adds a fallback to use back buffer and ensures no behavior
change for unrelated frontends.

Signed-off-by: Yiwei Zhang <zzyiwei@chromium.org>
Reviewed-by: Emma Anholt <emma@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10685>

src/gallium/frontends/dri/dri2.c

@@ -672,7 +672,7 @@ dri2_allocate_textures(struct dri_context *ctx,
    }
 }
 
-static void
+static bool
 dri2_flush_frontbuffer(struct dri_context *ctx,
                        struct dri_drawable *drawable,
                        enum st_attachment_type statt)
@@ -683,7 +683,7 @@ dri2_flush_frontbuffer(struct dri_context *ctx,
    struct pipe_context *pipe = ctx->st->pipe;
 
    if (statt != ST_ATTACHMENT_FRONT_LEFT)
-      return;
+      return false;
 
    if (drawable->stvis.samples > 1) {
       /* Resolve the front buffer. */
@@ -704,6 +704,8 @@ dri2_flush_frontbuffer(struct dri_context *ctx,
    else if (loader->flushFrontBuffer) {
       loader->flushFrontBuffer(dri_drawable, dri_drawable->loaderPrivate);
    }
+
+   return true;
 }
 
 /**

src/gallium/frontends/dri/dri_drawable.c

@@ -124,9 +124,7 @@ dri_st_framebuffer_flush_front(struct st_context_iface *stctx,
       (struct dri_drawable *) stfbi->st_manager_private;
 
    /* XXX remove this and just set the correct one on the framebuffer */
-   drawable->flush_frontbuffer(ctx, drawable, statt);
-
-   return true;
+   return drawable->flush_frontbuffer(ctx, drawable, statt);
 }
 
 /**

src/gallium/frontends/dri/dri_drawable.h

@@ -70,7 +70,7 @@ struct dri_drawable
 
    void (*update_drawable_info)(struct dri_drawable *drawable);
 
-   void (*flush_frontbuffer)(struct dri_context *ctx,
+   bool (*flush_frontbuffer)(struct dri_context *ctx,
                              struct dri_drawable *drawable,
                              enum st_attachment_type statt);
 

src/gallium/frontends/dri/drisw.c

@@ -290,15 +290,15 @@ drisw_copy_sub_buffer(__DRIdrawable *dPriv, int x, int y,
    }
 }
 
-static void
+static bool
 drisw_flush_frontbuffer(struct dri_context *ctx,
                         struct dri_drawable *drawable,
                         enum st_attachment_type statt)
 {
    struct pipe_resource *ptex;
 
-   if (!ctx)
-      return;
+   if (!ctx || statt != ST_ATTACHMENT_FRONT_LEFT)
+      return false;
 
    if (drawable->stvis.samples > 1) {
       /* Resolve the front buffer. */
@@ -311,6 +311,8 @@ drisw_flush_frontbuffer(struct dri_context *ctx,
    if (ptex) {
       drisw_copy_to_front(ctx->st->pipe, ctx->dPriv, ptex);
    }
+
+   return true;
 }
 
 /**

src/gallium/frontends/glx/xlib/xm_st.c

@@ -268,6 +268,9 @@ xmesa_st_framebuffer_flush_front(struct st_context_iface *stctx,
    struct xmesa_st_framebuffer *xstfb = xmesa_st_framebuffer(stfbi);
    bool ret;
 
+   if (statt != ST_ATTACHMENT_FRONT_LEFT)
+      return false;
+
    ret = xmesa_st_framebuffer_display(stfbi, stctx, statt, NULL);
 
    if (ret && xmesa_strict_invalidate)

src/gallium/frontends/hgl/hgl.c

@@ -65,6 +65,9 @@ hgl_st_framebuffer_flush_front(struct st_context_iface* stctxi,
 	struct hgl_buffer* buffer = hgl_st_framebuffer(stfbi);
 	struct pipe_resource* ptex = buffer->textures[statt];
 
+	if (statt != ST_ATTACHMENT_FRONT_LEFT)
+		return false;
+
 	if (!ptex)
 		return true;
 

src/gallium/frontends/osmesa/osmesa.c

@@ -366,6 +366,9 @@ osmesa_st_framebuffer_flush_front(struct st_context_iface *stctx,
    unsigned bpp;
    int dst_stride;
 
+   if (statt != ST_ATTACHMENT_FRONT_LEFT)
+      return false;
+
    if (osmesa->pp) {
       struct pipe_resource *zsbuf = NULL;
       unsigned i;

src/mesa/state_tracker/st_manager.c

@@ -1167,15 +1167,22 @@ st_manager_flush_frontbuffer(struct st_context *st)
        !stfb->Base.Visual.doubleBufferMode)
       return;
 
+   /* Check front buffer used at the GL API level. */
+   enum st_attachment_type statt = ST_ATTACHMENT_FRONT_LEFT;
    strb = st_renderbuffer(stfb->Base.Attachment[BUFFER_FRONT_LEFT].
                           Renderbuffer);
+   if (!strb) {
+       /* Check back buffer redirected by EGL_KHR_mutable_render_buffer. */
+       statt = ST_ATTACHMENT_BACK_LEFT;
+       strb = st_renderbuffer(stfb->Base.Attachment[BUFFER_BACK_LEFT].
+                              Renderbuffer);
+   }
 
    /* Do we have a front color buffer and has it been drawn to since last
     * frontbuffer flush?
     */
-   if (strb && strb->defined) {
-      stfb->iface->flush_front(&st->iface, stfb->iface,
-                               ST_ATTACHMENT_FRONT_LEFT);
+   if (strb && strb->defined &&
+       stfb->iface->flush_front(&st->iface, stfb->iface, statt)) {
       strb->defined = GL_FALSE;
 
       /* Trigger an update of strb->defined on next draw */