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

quoted

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?

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?

quoted

quoted

+		atsd_start = start;

Which would also require:

      	    atsd_start = ALIGN_DOWN(start, PAGE_64K);

quoted

+		atsd_psize = MMU_PAGE_64K;
+	} else if (ALIGN_DOWN(start, PAGE_2M) == ALIGN_DOWN(end, PAGE_2M)) {

Wouldn't this lead to under invalidation in ranges which happen to cross a 2M
boundary? For example invalidating a 128K (ie. 2x64K pages) range with start ==
0x1f0000 and end == 0x210000 would result in an invalidation of the range 0x0 -
0x200000 incorrectly leaving 0x200000 - 0x210000 in the GPU TLB.

In this example:
start                         0x1f0000
size                          0x020000
end (start + size - 1)        0x20ffff
ALIGN_DOWN(start, PAGE_2M)    0x000000
ALIGN_DOWN(end, PAGE_2M)      0x200000

Since ALIGN_DOWN(start, PAGE_2M) != ALIGN_DOWN(end, PAGE_2M), the
condition fails and we move to the 1G clause. Then
ALIGN_DOWN(start, PAGE_1G) == ALIGN_DOWN(end, PAGE_1G) == 0, so we
invalidate the range [0, 1G).

Oh yeah, sorry that makes sense and looks good to me.

- Alistair