Hi, Michael, others.
This is the response from Linus last time we copied that already
pre-existing wbinvd_on_all_cpus() macro to another place in the driver:
https://lists.freedesktop.org/archives/dri-devel/2021-November/330928.html
My first interpretation of this is that even if there currently are
similar patterns in drm_cache.c, we shouldn't introduce more,
encouraging other drivers to use incoherent IO.
Other than that I think we should move whatever wbinvds we can over to
the ranged versions, unless that is proven to be a performance drop.
Finally for any wbinvds left in our driver, ensure that they are never
executed for any gpu where we provide full coherency. That is all
discrete gpus (and to be discussed integrated gpus moving forward).
Might be that drm maintainers want to chime in here with other views.
Thanks,
Thomas
On 3/15/22 17:59, Michael Cheng wrote:
+Daniel for additional feedback!
On 2022-03-14 4:06 p.m., Michael Cheng wrote:
On 2022-03-08 10:58 a.m., Lucas De Marchi wrote:
On Tue, Feb 22, 2022 at 08:24:31PM +0100, Thomas Hellström (Intel)
wrote:
Hi, Michael,
On 2/22/22 18:26, Michael Cheng wrote:
This patch removes logic for wbinvd_on_all_cpus and brings in
drm_cache.h. This header has the logic that outputs a warning
when wbinvd_on_all_cpus when its being used on a non-x86 platform.
Signed-off-by: Michael Cheng <michael.cheng@xxxxxxxxx>
Linus has been pretty clear that he won't accept patches that add
macros that works on one arch and warns on others anymore in i915
and I figure even less so in drm code.
So we shouldn't try to move this out to drm. Instead we should
restrict the wbinvd() inside our driver to integrated and X86 only.
For discrete on all architectures we should be coherent and hence
not be needing wbinvd().
the warn is there to guarantee we don't forget a code path. However
simply adding the warning is the real issue: we should rather guarantee
we can't take that code path. I.e., as you said refactor the code to
guarantee it works on discrete without that logic.
$ git grep wbinvd_on_all_cpus -- drivers/gpu/drm/
drivers/gpu/drm/drm_cache.c: if (wbinvd_on_all_cpus())
drivers/gpu/drm/drm_cache.c: if (wbinvd_on_all_cpus())
drivers/gpu/drm/drm_cache.c: if (wbinvd_on_all_cpus())
drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c: * Currently we
just do a heavy handed wbinvd_on_all_cpus() here since
drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c: wbinvd_on_all_cpus();
It looks like we actually go through this on other discrete
graphics. Is
this missing an update like s/IS_DG1/IS_DGFX/? Or should we be doing
something else?
drivers/gpu/drm/i915/gem/i915_gem_pm.c:#define
wbinvd_on_all_cpus() \
drivers/gpu/drm/i915/gem/i915_gem_pm.c: wbinvd_on_all_cpus();
Those are for suspend. Revert ac05a22cd07a ("drm/i915/gem:
Almagamate clflushes on suspend")
or extract that part to a helper function and implement it differently
for arches != x86?
drivers/gpu/drm/i915/gem/i915_gem_pm.c: wbinvd_on_all_cpus();
Probably take a similar approach to the suspend case?
drivers/gpu/drm/i915/gt/intel_ggtt.c: wbinvd_on_all_cpus();
For a helper function, I have a #define for all non x86 architecture
that gives a warn on [1] within drm_cache.h Or would it be better to
implement a helper function instead?
[1]. https://patchwork.freedesktop.org/patch/475750/?series=99991&rev=5
This one comes from 64b95df91f44 ("drm/i915: Assume exclusive access
to objects inside resume")
Shouldn't that be doing the invalidate if the write domain is
I915_GEM_DOMAIN_CPU
In the end I think the warning would be ok if it was the cherry on top,
to guarantee we don't take those paths. We should probably have a
warn_once() to avoid spamming the console. But we also have to rework
the code to guarantee we are the only ones who may eventually get that
warning, and not the end user.
Could we first add the helper function/#define for now, and rework
the code in a different patch series?
Lucas De Marchi
Thanks,
/Thomas
