Quoting Tvrtko Ursulin (2019-07-17 19:06:21)
> From: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
>
> A couple issues were present in this code:
>
> 1.
> fls() usage was incorrect causing off by one in subslice mask lookup,
> which in other words means subslice mask of all zeroes is always used
> (subslice mask of a slice which is not present, or even out of bounds
> array access), rendering the checks in wa_init_mcr either futile or
> random.
>
> 2.
> Condition in WARN_ON was not correct. It is doing a bitwise and operation
> between a positive (present subslices) and negative mask (disabled L3
> banks).
>
> This means that with corrected fls() usage the assert would always
> incorrectly fail.
>
> We could fix this by inverting the fuse bits in the check, but instead do
> one better and improve the code so it not only asserts, but finds the
> first common index between the two masks and only warns if no such index
> can be found.
>
> v2:
> * Simplify check for logic and redability.
> * Improve commentary explaining what is really happening ie. what the
> assert is really trying to check and why.
>
> v3:
> * Find first common index instead of just asserting.
>
> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
> Fixes: fe864b76c2ab ("drm/i915: Implement WaProgramMgsrForL3BankSpecificMmioReads")
> Reviewed-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> # v1
It's still magic to me, but I can attest that it does what you say, and
should be no worse than before :)
Reviewed-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
-Chris
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