The value of the AFBC_FEATURES register is required by userspace to
determine AFBC support on Bifrost. A user on our IRC channel (#panfrost)
reported a workload that raised a fault on one system's Mali G31 but
worked flawlessly with another system's Mali G31. We determined the
cause to be missing AFBC support on one vendor's Mali implementation --
it turns out AFBC is optional on Bifrost!
Whether AFBC is supported or not is exposed in the AFBC_FEATURES
register on Bifrost, which reads back as 0 on Midgard. A zero value
indicates AFBC is fully supported, provided the architecture itself
supports AFBC, allowing backwards-compatibility with Midgard. Bits 0 and
15 indicate that AFBC support is absent for texturing and rendering
respectively.
The user experiencing the fault reports that AFBC_FEATURES reads back
0x10001 on their system, confirming the architectural lack of AFBC.
Userspace needs this parameter to know to disable AFBC on that
chip, and perhaps others.
v2: Fix typo from copy-paste fail.
v3: Bump the UABI version. This commit was cherry-picked from another
series so chalking this up to a rebase fail.
Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Cc: Rob Herring <robh@kernel.org>
Cc: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Signed-off-by: Steven Price <steven.price@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210604130011.3203-1-alyssa.rosenzweig@collabora.com
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