As far as I know, we haven't gotten to the arm implementation yet, since
we are trying to get i915 compile for arm without using random ifdefs
and dummy functions.
"Noob question - why is i915 the only driver calling it? Do other GPUs
never need to flush CPU cache?"
Unfortunately I don't have enough expertise to comfortable answer this
question. Maybe someone else can chime in here? Lucas? Matt?
On 2022-02-01 8:32 a.m., Tvrtko Ursulin wrote:
On 01/02/2022 15:41, Michael Cheng wrote:
Ah, thanks for the clarification! While discussion goes on about the
route you suggested, could we land these patches (after addressing
the reviews) to unblock compiling i915 on arm?
I am 60-40 to no, since follow up can be hard. I'd prefer a little bit
of discussion before merging.
Also, what will be the Arm implementation of drm_clflush_virt_range?
Noob question - why is i915 the only driver calling it? Do other GPUs
never need to flush CPU cache?
Regards,
Tvrtko
On 2022-02-01 1:25 a.m., Tvrtko Ursulin wrote:
On 31/01/2022 17:02, Michael Cheng wrote:
Hey Tvrtko,
Are you saying when adding drm_clflush_virt_range(addr,
sizeof(addr), this function forces an x86 code path only? If that
is the case, drm_clflush_virt_range(addr, sizeof(addr) currently
has ifdefs that seperate out x86 and powerpc, so we can add an
ifdef for arm in the near future when needed.
No, I was noticing that the change you are making in this patch,
while it indeed fixes a build failure, it is a code path which does
not get executed on Arm at all.
So what effectively happens is a single assembly instruction gets
replaced with a function call on all integrated GPUs up to and
including Tigerlake.
That was the slightly annoying part I was referring to and asking
whether it was discussed before.
Sadly I don't think there is a super nice solution apart from
duplicating drm_clflush_virt_range as for example i915_clflush_range
and having it static inline. That would allow the integrated GPU
code path to remain of the same performance profile, while solving
the Arm problem. However it would be code duplication so might be
frowned upon.
I'd be tempted to go that route but it is something which needs a
bit of discussion if that hasn't happened already.
Regards,
Tvrtko
On 2022-01-31 6:55 a.m., Tvrtko Ursulin wrote:
On 28/01/2022 22:10, Michael Cheng wrote:
Use drm_clflush_virt_range instead of clflushopt and remove the
memory
barrier, since drm_clflush_virt_range takes care of that.
Signed-off-by: Michael Cheng <michael.cheng@xxxxxxxxx>
---
drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c | 8 +++-----
1 file changed, 3 insertions(+), 5 deletions(-)
diff --git a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
index 498b458fd784..0854276ff7ba 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
@@ -1332,10 +1332,8 @@ static void *reloc_vaddr(struct i915_vma
*vma,
static void clflush_write32(u32 *addr, u32 value, unsigned int
flushes)
{
if (unlikely(flushes & (CLFLUSH_BEFORE | CLFLUSH_AFTER))) {
- if (flushes & CLFLUSH_BEFORE) {
- clflushopt(addr);
- mb();
- }
+ if (flushes & CLFLUSH_BEFORE)
+ drm_clflush_virt_range(addr, sizeof(addr));
*addr = value;
@@ -1347,7 +1345,7 @@ static void clflush_write32(u32 *addr,
u32 value, unsigned int flushes)
* to ensure ordering of clflush wrt to the system.
*/
if (flushes & CLFLUSH_AFTER)
- clflushopt(addr);
+ drm_clflush_virt_range(addr, sizeof(addr));
} else
*addr = value;
}
Slightly annoying thing here (maybe in some other patches from the
series as well) is that the change adds a function call to x86
only code path, because relocations are not supported on discrete
as per:
static in
eb_validate_vma(...)
/* Relocations are disallowed for all platforms after
TGL-LP. This
* also covers all platforms with local memory.
*/
if (entry->relocation_count &&
GRAPHICS_VER(eb->i915) >= 12 && !IS_TIGERLAKE(eb->i915))
return -EINVAL;
How acceptable would be, for the whole series, to introduce a
static inline i915 cluflush wrapper and so be able to avoid
functions calls on x86? Is this something that has been discussed
and discounted already?
Regards,
Tvrtko
P.S. Hmm I am now reminded of my really old per platform build
patches. With them you would be able to compile out large portions
of the driver when building for ARM. Probably like a 3rd if my
memory serves me right.
