[Mesa-dev] [PATCH 0.5/5] i965/gen9/fast-clear: Handle linear→SRGB conversion

[Neil Roberts]

If GL_FRAMEBUFFER_SRGB is enabled when writing to an SRGB-capable
framebuffer then the color will be converted from linear to SRGB
before being written. There is no chance for the hardware to do this
itself because it can't modify the clear color that is programmed in
the surface state so it seems pretty clear that the driver should be
handling this itself.

Note that this wasn't a problem before Gen9 because previously we were
only able to do fast clears to 0 or 1 and those values are the same in
linear and SRGB space.
---

I noticed this after posting the patch series but I think it belongs
in this series because it only causes a problem once we enable fast
clears for MSRTs. I made a test case to demonstrate this here:

http://patchwork.freedesktop.org/patch/66222/

I tried to think what other similar problems there could be such as
whether we need to clamp the values to [0,1] for normalised formats.
However this seems to just work so I guess the sampler hardware does
look at the surface format enough to determine it needs to clamp the
clear color. It's a bit odd what the sampler does and doesn't do with
the fast clear color. I made a test case for this as well:

http://patchwork.freedesktop.org/patch/66223/

Annoyingly it doesn't clamp the value correctly for
GL_R11F_G11F_B10F_EXT. That format is floating-point but it has no
signed bits so I think it should clamp to a minimum of 0.

I have a github branch with all of my SKL fast clear patches here:

https://github.com/bpeel/mesa/commits/skl-fast-clear

 src/mesa/drivers/dri/i965/brw_meta_fast_clear.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/src/mesa/drivers/dri/i965/brw_meta_fast_clear.c b/src/mesa/drivers/dri/i965/brw_meta_fast_clear.c
index 1b2ea42..f1920b2 100644
--- a/src/mesa/drivers/dri/i965/brw_meta_fast_clear.c
+++ b/src/mesa/drivers/dri/i965/brw_meta_fast_clear.c
@@ -41,6 +41,8 @@
 #include "main/api_validate.h"
 #include "main/state.h"
 
+#include "util/format_srgb.h"
+
 #include "vbo/vbo_context.h"
 
 #include "drivers/common/meta.h"
@@ -424,6 +426,15 @@ set_fast_clear_color(struct brw_context *brw,
          override_color.f[3] = 1.0f;
    }
 
+   /* Handle linear→SRGB conversion */
+   if (brw->ctx.Color.sRGBEnabled &&
+       _mesa_get_srgb_format_linear(mt->format) != mt->format) {
+      for (int i = 0; i < 3; i++) {
+         override_color.f[i] =
+            util_format_linear_to_srgb_float(override_color.f[i]);
+      }
+   }
+
    if (brw->gen >= 9) {
       mt->gen9_fast_clear_color = override_color;
    } else {
-- 
1.9.3