Mesa (main): wgl, d3d12: Handle front buffer writes for double-buffered MSAA surfaces backed by swapchains

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Module: Mesa
Branch: main
Commit: 1ef329b825c27871cec86b17bf49d9e4d0eb1637
URL:    http://cgit.freedesktop.org/mesa/mesa/commit/?id=1ef329b825c27871cec86b17bf49d9e4d0eb1637

Author: Jesse Natalie <jenatali at microsoft.com>
Date:   Thu Apr 28 13:53:58 2022 -0700

wgl, d3d12: Handle front buffer writes for double-buffered MSAA surfaces backed by swapchains

We don't need to go as far as the fake front thing when MSAA is being used, because the
swapchain (single-sampled) is already decoupled from the app render buffers. But we do
need to direct the frontbuffer flush to the single-sampled back buffer, and then present
the back buffer. We also need to swap the buffers when we do this, so the next blit
targets the former front buffer.

Reviewed-by: Louis-Francis Ratté-Boulianne <lfrb at collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16227>

---

 src/gallium/frontends/wgl/stw_st.c | 34 +++++++++++++++++++++++-----------
 1 file changed, 23 insertions(+), 11 deletions(-)

diff --git a/src/gallium/frontends/wgl/stw_st.c b/src/gallium/frontends/wgl/stw_st.c
index 3ff97338eb3..7050a4e24b9 100644
--- a/src/gallium/frontends/wgl/stw_st.c
+++ b/src/gallium/frontends/wgl/stw_st.c
@@ -160,11 +160,15 @@ stw_st_framebuffer_validate_locked(struct st_context_iface *stctx,
 
    /* As of now, the only stw_winsys_framebuffer implementation is
     * d3d12_wgl_framebuffer and it doesn't support front buffer
-    * drawing. A fake front texture is needed to handle that scenario */
+    * drawing. A fake front texture is needed to handle that scenario.
+    * For MSAA, we just need to make sure that the back buffer also
+    * exists, so we can blt to it during flush_frontbuffer. */
    if (mask & ST_ATTACHMENT_FRONT_LEFT_MASK &&
-       stwfb->fb->winsys_framebuffer &&
-       stwfb->stvis.samples <= 1) {
-      stwfb->needs_fake_front = true;
+       stwfb->fb->winsys_framebuffer) {
+      if (stwfb->stvis.samples <= 1)
+         stwfb->needs_fake_front = true;
+      else
+         mask |= ST_ATTACHMENT_BACK_LEFT_MASK;
    }
 
    /* remove outdated textures */
@@ -440,6 +444,7 @@ stw_st_framebuffer_flush_front(struct st_context_iface *stctx,
    struct pipe_context *pipe = stctx->pipe;
    bool ret;
    HDC hDC;
+   bool need_swap_textures = false;
 
    if (statt != ST_ATTACHMENT_FRONT_LEFT)
       return false;
@@ -448,17 +453,24 @@ stw_st_framebuffer_flush_front(struct st_context_iface *stctx,
 
    /* Resolve the front buffer. */
    if (stwfb->stvis.samples > 1) {
-      stw_pipe_blit(pipe, stwfb->textures[statt], stwfb->msaa_textures[statt]);
+      enum st_attachment_type blit_target = statt;
+      if (stwfb->fb->winsys_framebuffer) {
+         blit_target = ST_ATTACHMENT_BACK_LEFT;
+         need_swap_textures = true;
+      }
+
+      stw_pipe_blit(pipe, stwfb->textures[blit_target],
+                    stwfb->msaa_textures[statt]);
    } else if (stwfb->needs_fake_front) {
-      struct pipe_resource *ptex;
+      /* fake front texture is now invalid */
+      p_atomic_inc(&stwfb->base.stamp);
+      need_swap_textures = true;
+   }
 
-      /* swap the textures */
-      ptex = stwfb->textures[ST_ATTACHMENT_FRONT_LEFT];
+   if (need_swap_textures) {
+      struct pipe_resource *ptex = stwfb->textures[ST_ATTACHMENT_FRONT_LEFT];
       stwfb->textures[ST_ATTACHMENT_FRONT_LEFT] = stwfb->textures[ST_ATTACHMENT_BACK_LEFT];
       stwfb->textures[ST_ATTACHMENT_BACK_LEFT] = ptex;
-
-      /* fake front texture is now invalid */
-      p_atomic_inc(&stwfb->base.stamp);
    }
 
    if (stwfb->textures[statt])