[Intel-gfx] i915 clear-residuals BYT + IVB rendering issue + possible fix

[Hans de Goede]

Hi All,

We (Fedora) received a bug-report about rendering issues
on BYT and IVB caused by the i915 clear-residuals work
which landed in 5.10.y :

The Fedora bug:
https://bugzilla.redhat.com/show_bug.cgi?id=1959581

Points to these i915 bugs:
https://gitlab.freedesktop.org/drm/intel/-/issues/3071
https://gitlab.freedesktop.org/drm/intel/-/issues/3081#note_890606

With the second link containing a possible fix which looks promising
if someone can take a look at this, then that would be great:

diff --git a/drivers/gpu/drm/i915/gt/gen7_renderclear.c b/drivers/gpu/drm/i915/gt/gen7_renderclear.c
index de575fdb0..054a0f5b8 100644
--- a/drivers/gpu/drm/i915/gt/gen7_renderclear.c
+++ b/drivers/gpu/drm/i915/gt/gen7_renderclear.c
@@ -397,7 +397,7 @@ static void emit_batch(struct i915_vma * const vma,
        gen7_emit_pipeline_invalidate(&cmds);
        batch_add(&cmds, MI_LOAD_REGISTER_IMM(2));
        batch_add(&cmds, i915_mmio_reg_offset(CACHE_MODE_0_GEN7));
-       batch_add(&cmds, 0xffff0000);
+       batch_add(&cmds, 0xfffb0000);
        batch_add(&cmds, i915_mmio_reg_offset(CACHE_MODE_1));
        batch_add(&cmds, 0xffff0000 | PIXEL_SUBSPAN_COLLECT_OPT_DISABLE);
        gen7_emit_pipeline_invalidate(&cmds);

Note the:
https://gitlab.freedesktop.org/drm/intel/-/issues/3081#note_890606

Link contains details about the what and why of this change.

Regards,

Hans