[Mesa-dev] [v2 02/17] i965/blorp: Skip redundant re-fast clear for non-compressed

[Pohjolainen, Topi]

On Wed, Nov 23, 2016 at 09:05:39AM -0800, Jason Ekstrand wrote:
>    On Wed, Nov 23, 2016 at 1:16 AM, Topi Pohjolainen
>    <[1]topi.pohjolainen at gmail.com> wrote:
> 
>      Originally re-clears where skipped but when lossless compression
>      was introduced the re-clears where errorneously enabled also for
>      non-compressed fast clears.
>      Signed-off-by: Topi Pohjolainen <[2]topi.pohjolainen at intel.com>
>      CC: Ben Widawsky <[3]benjamin.widawsky at intel.com>
>      CC: Kenneth Graunke <[4]kenneth at whitecape.org>
>      CC: Harri Syrja <[5]harri.syrja at intel.com>
>      Cc: Chad Versace <[6]chad at kiwitree.net>
>      ---
>       src/mesa/drivers/dri/i965/brw_blorp.c | 19 ++++++++++++++++---
>       1 file changed, 16 insertions(+), 3 deletions(-)
>      diff --git a/src/mesa/drivers/dri/i965/brw_blorp.c
>      b/src/mesa/drivers/dri/i965/brw_blorp.c
>      index 556f2c0..9a849f5 100644
>      --- a/src/mesa/drivers/dri/i965/brw_blorp.c
>      +++ b/src/mesa/drivers/dri/i965/brw_blorp.c
>      @@ -825,10 +825,23 @@ do_single_blorp_clear(struct brw_context *brw,
>      struct gl_framebuffer *fb,
>                                           brw, &irb->mt->gen9_fast_clear_
>      color,
>                                           &override_color);
>      -      /* If the buffer is already in INTEL_FAST_CLEAR_STATE_CLEAR,
>      the clear
>      -       * is redundant and can be skipped.
>      +      /* If the buffer is already in INTEL_FAST_CLEAR_STATE_CLEAR,
>      and the
>      +       * buffer isn't compressed, the clear is redundant and can be
>      skipped.
>      +       *
>      +       * Without compression fast clear only operates on the mcs
>      buffer
>      +       * recording if color values are cleared. The hardware,
>      however,
>      +       * doesn't write the actual color value into the mcs or color
>      +       * buffer. Only by the time of render (inclucing color
>      resolve) does the
>      +       * hardware read the _current_ color value in the surface
>      state and
>      +       * write the actual pixel values in the color buffer
>      accordingly.
>      +       *
>      +       * This seems to be the reason why sampler engine cannot
>      handle
>      +       * non-compressed fast clear - it doesn't know how to read
>      the color
>      +       * value from the surface state. With compression the color
>      value is
>      +       * recorded in the color buffer (only not for every pixel)
>      and therefore
>      +       * it is available without consulting the surface state.
> 
>    This doesn't jive with my understanding of fast clears on gen9.
>    Everything I've seen so far indicates that the clear color in the
>    surface state *does* matter.  Otherwise, why would it be there?  In
>    particular, my understanding of the 2-bit CCS values is that 0 means
>    resolved, 1 means compressed and 3 means clear where "clear" means "go
>    look at the clear color".  Have you done experiments that lead you to
>    some other conclusion?

Right, you are correct. This is actually a patch from really early days when I
didn't know any better. We might want to drop this for now, there is the 0/1
color thing for sampler engine that we probably need to fix first anyway.

What do you think?

> 
>              */
>      -      if (!color_updated &&
>      +      if ((!color_updated || !is_lossless_compressed) &&
>                 irb->mt->fast_clear_state ==
>      INTEL_FAST_CLEAR_STATE_CLEAR)
>                return true;
>      --
>      2.5.5
>      _______________________________________________
>      mesa-dev mailing list
>      [7]mesa-dev at lists.freedesktop.org
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> 
> References
> 
>    1. mailto:topi.pohjolainen at gmail.com
>    2. mailto:topi.pohjolainen at intel.com
>    3. mailto:benjamin.widawsky at intel.com
>    4. mailto:kenneth at whitecape.org
>    5. mailto:harri.syrja at intel.com
>    6. mailto:chad at kiwitree.net
>    7. mailto:mesa-dev at lists.freedesktop.org
>    8. https://lists.freedesktop.org/mailman/listinfo/mesa-dev