[Bug 110395] Shadows are flickering in SuperTuxKart

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=110395

            Bug ID: 110395
           Summary: Shadows are flickering in SuperTuxKart
           Product: Mesa
           Version: git
          Hardware: Other
                OS: All
            Status: NEW
          Severity: normal
          Priority: medium
         Component: Drivers/DRI/i965
          Assignee: intel-3d-bugs at lists.freedesktop.org
          Reporter: deveee at gmail.com
        QA Contact: intel-3d-bugs at lists.freedesktop.org

In current git STK grass shadows are constantly flickering.

Here is original bug report:
https://github.com/supertuxkart/stk-code/issues/3824

Basically it is related to the file:
https://github.com/supertuxkart/stk-code/blob/master/data/shaders/sp_grass_shadow.vert

I can "fix" this by changing order of the "layer" and "wind_direction" uniform
variables. Maybe it's just a case.

And it can be reproduced on any track with grass/trees when shadows are enabled
in options, eg. in Nessie's Pond.

Not sure which graphics cards are affected. Maybe <= gen8, because original bug
report is from haswell and I was testing it on ivybridge.

Git bisect shows:

eca4a6548d07bbbb02a7768edb397bad7b72cfc2 is the first bad commit
commit eca4a6548d07bbbb02a7768edb397bad7b72cfc2
Author: Danylo Piliaiev <danylo.piliaiev at gmail.com>
Date:   Mon Jul 2 17:04:23 2018 +0300

    i965: Disable dual source blending when shader doesn't support it on gen8+

    Dual source blending behaviour is undefined when shader doesn't
    have second color output, dismissing fragment in such situation
    leads to a hang on gen8+ if depth test in enabled.

    Since blending cannot be gracefully fixed in such case and the result
    is undefined - blending is simply disabled.

    v2 (Kenneth Graunke):
     - Listen to BRW_NEW_FS_PROG_DATA in 3DSTATE_PS_BLEND
     - Also whack BLEND_STATE[] to keep the two in sync, since we're not
       sure exactly which copy of the redundant info the hardware will use.

    Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=107088
    Signed-off-by: Danylo Piliaiev <danylo.piliaiev at globallogic.com>
    Reviewed-by: Jason Ekstrand <jason at jlekstrand.net>
    Reviewed-by: Kenneth Graunke <kenneth at whitecape.org>

:040000 040000 e4887675a8acfdd017f7022f553a8bfeefc2b31b
8eb8649a23e09c70cb3088873249763e05f5452e M      src

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