[Mesa-dev] [RFC v2] etnaviv: flush color cache and depth cache together before resolves

[Philipp Zabel]

Before resolving a rendertarget or a depth/stencil resource into a
texture, flush both the color cache and the depth cache together.

It is unclear whether this is necessary for the following stall to
work properly, or whether the depth flush just adds enough time
for the color cache flush to finish before the resolver is started,
but this change removes artifacts that otherwise appear if a texture
is sampled directly after rendering into it.

The test case is a simple QML scene graph with a QtWebEngine based
WebView rendered on top of a blue background:

	import QtQuick 2.0
	import QtQuick.Window 2.2
	import QtWebView 1.1

	Window {
		Rectangle {
			id: background
			anchors.fill: parent
			color: "blue"
		}

		WebView {
			id: webView
			anchors.fill: parent
		}

		Component.onCompleted: {
			webView.url = "<some animated website>"
		}
	}

If the website is animated, the WebView renders the site contents into
texture tiles and immediately afterwards samples from them to draw the
tiles into the Qt renderbuffer. Without this patch, a small irregular
triangle in the lower right of each browser tile appears solid blue, as
if the texture sampler samples zeroes instead of the website contents,
and the previously rendered blue Rectangle shows through.

Other attempts such as adding a pipeline stall before the color flush or
a TS cache flush afterwards or flushing multiple times, with stalls
before and after each flush, have shown no effect.

Signed-off-by: Philipp Zabel <p.zabel at pengutronix.de>
---
Changes since v1:
 - Add a comment explaining why we flush color and depth cache together.
---
 src/gallium/drivers/etnaviv/etnaviv_clear_blit.c | 22 +++++++++++++---------
 1 file changed, 13 insertions(+), 9 deletions(-)

diff --git a/src/gallium/drivers/etnaviv/etnaviv_clear_blit.c b/src/gallium/drivers/etnaviv/etnaviv_clear_blit.c
index e4620a3015..d9538907fe 100644
--- a/src/gallium/drivers/etnaviv/etnaviv_clear_blit.c
+++ b/src/gallium/drivers/etnaviv/etnaviv_clear_blit.c
@@ -465,15 +465,19 @@ etna_try_rs_blit(struct pipe_context *pctx,
       ts_mem_config |= VIVS_TS_MEM_CONFIG_MSAA | msaa_format;
    }
 
-   uint32_t to_flush = 0;
-
-   if (src->base.bind & PIPE_BIND_RENDER_TARGET)
-      to_flush |= VIVS_GL_FLUSH_CACHE_COLOR;
-   if (src->base.bind & PIPE_BIND_DEPTH_STENCIL)
-      to_flush |= VIVS_GL_FLUSH_CACHE_DEPTH;
-
-   if (to_flush) {
-      etna_set_state(ctx->stream, VIVS_GL_FLUSH_CACHE, to_flush);
+   /* Always flush color and depth cache together before resolving. This works
+    * around artifacts that appear in some cases when scanning out a texture
+    * directly after it has been rendered to, such as rendering an animated web
+    * page in a QtWebEngine based WebView on GC2000. The artifacts look like
+    * the texture sampler samples zeroes instead of texture data in a small,
+    * irregular triangle in the lower right of each browser tile quad. Other
+    * attempts to avoid these artifacts, including a pipeline stall before the
+    * color flush or a TS cache flush afterwards, or flushing multiple times,
+    * with stalls before and after each flush, have shown no effect. */
+   if (src->base.bind & PIPE_BIND_RENDER_TARGET ||
+       src->base.bind & PIPE_BIND_DEPTH_STENCIL) {
+      etna_set_state(ctx->stream, VIVS_GL_FLUSH_CACHE,
+		     VIVS_GL_FLUSH_CACHE_COLOR | VIVS_GL_FLUSH_CACHE_DEPTH);
       etna_stall(ctx->stream, SYNC_RECIPIENT_RA, SYNC_RECIPIENT_PE);
    }
 
-- 
2.11.0