[Intel-gfx] [PATCH 2/2] drm/i915/tgl: Move and restrict Wa_1408615072

[Nick Desaulniers]

Hello Jose,
So this is triggering a very subtle bug in 2 separate CI systems:
- KernelCI: https://storage.kernelci.org/next/master/next-20200305/x86_64/x86_64_defconfig/clang-9/build.log
- our CI: https://travis-ci.com/ClangBuiltLinux/continuous-integration/jobs/293826383

commit 50148a2 ("drm/i915/tgl: Move and restrict Wa_1408615072")

for Clang builds.  It seems that Clang is inlining `wa_masked_en` into
`rcs_engine_wa_init`, which has implications for `__builtin_constant_p`
evaluation.

The relevant call chain looks like:

rcs_engine_wa_init (drivers/gpu/drm/i915/gt/intel_workarounds.c) ->
  wa_masked_en (drivers/gpu/drm/i915/gt/intel_workarounds.c) ->
    _MASKED_BIT_ENABLE (drivers/gpu/drm/i915/i915_reg.h) ->
      _MASKED_FIELD (drivers/gpu/drm/i915/i915_reg.h)

So it looks like there's build time validation that the masks and values have
their bottom 2 bytes unset (mask & 0xffff0000, little endian).  (There doesn't
seem to be equivalent runtime checks that the masks & values obey this
invariant when they're not "integer constant expressions").

This will break should GCC ever decide to inline `wa_masked_en` into
`rcs_engine_wa_init`.

VSUNIT_CLKGATE_DIS_TGL is defined as `REG_BIT(19)` which evaluates to
0x00080000, which would fail the check against
0xffff0000.

Can you please validate that VSUNIT_CLKGATE_DIS_TGL has the correct value, or
if the invariant that the bottom two bytes of values passed to
`_MASKED_BIT_ENABLE` must be set?

I would expect one of those two to be incorrect.

Link: https://github.com/ClangBuiltLinux/linux/issues/918