Re: [PATCH 2/3] powerpc/powernv/npu: Use size-based ATSD invalidates

On Wed, 3 Oct 2018, Alistair Popple wrote:

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We also support 4K page sizes on PPC. If I am not mistaken this means every ATSD
would invalidate the entire GPU TLB for a the given PID on those systems. Could
we change the above check to `if (size <= PAGE_64K)` to avoid this?

PPC supports 4K pages but the GPU ATS implementation does not. For that
reason I didn't bother handling invalidates smaller than 64K. I'll add a
comment on that.

Interesting, I was not aware of that limitation. Do you know if it is a
SW/driver limitation or a HW limitation?

HW.

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I don't know that this requirement is enforced anywhere though. I could
add a PAGE_SIZE == 64K check to pnv_npu2_init_context if you think it
would be useful.

Given it's a static kernel build parameter perhaps it makes more sense to do the
check as part of the driver build in a conftest rather than a runtime failure?

Sure, we can do it in the driver. I'll just stick with a comment for the
kernel.