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[79.43.11.206]) by smtp.gmail.com with ESMTPSA id h19-20020a1ccc13000000b003b49ab8ff53sm780145wmb.8.2022.11.03.12.23.28 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 03 Nov 2022 12:23:30 -0700 (PDT) From: "Fabio M. De Francesco" To: Ira Weiny , Zhao Liu Cc: Jani Nikula , Joonas Lahtinen , Rodrigo Vivi , Tvrtko Ursulin , David Airlie , Daniel Vetter , Matthew Auld , Thomas =?ISO-8859-1?Q?Hellstr=F6m?= , Nirmoy Das , Maarten Lankhorst , Chris Wilson , Christian =?ISO-8859-1?Q?K=F6nig?= , intel-gfx@lists.freedesktop.org, dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org, Zhenyu Wang , Zhao Liu , Dave Hansen Subject: Re: [PATCH 1/9] drm/i915: Use kmap_local_page() in gem/i915_gem_object.c Date: Thu, 03 Nov 2022 20:22:04 +0100 Message-ID: <12087538.O9o76ZdvQC@suse> In-Reply-To: References: <20221017093726.2070674-1-zhao1.liu@linux.intel.com> <2541717.Lt9SDvczpP@suse> MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="iso-8859-1" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On gioved=EC 3 novembre 2022 17:51:23 CET Ira Weiny wrote: > On Sat, Oct 29, 2022 at 01:17:03PM +0200, Fabio M. De Francesco wrote: > > On luned=EC 17 ottobre 2022 11:37:17 CEST Zhao Liu wrote: > > > From: Zhao Liu > > >=20 > > > The use of kmap_atomic() is being deprecated in favor of > > > kmap_local_page()[1]. > > >=20 > > > The main difference between atomic and local mappings is that local > > > mappings doesn't disable page faults or preemption. > >=20 > > You are right about about page faults which are never disabled by > > kmap_local_page(). However kmap_atomic might not disable preemption. It > > depends on CONFIG_PREEMPT_RT. > >=20 > > Please refer to how kmap_atomic_prot() works (this function is called by > > kmap_atomic() when kernels have HIGHMEM enabled). > >=20 > > > There're 2 reasons why i915_gem_object_read_from_page_kmap() doesn't > > > need to disable pagefaults and preemption for mapping: > > >=20 > > > 1. The flush operation is safe for CPU hotplug when preemption is not > > > disabled. > >=20 > > I'm confused here. Why are you talking about CPU hotplug? >=20 > I agree with Fabio here. I'm not making the connection between cpu hotpl= ug=20 and > this code path. >=20 > Ira @Zhao, I'd like to add that I was about to put my reviewed-by tag. The other thing= s I=20 objected are minor nits. Please just clarify this connection. Your code is good and deserves to be applied. =46abio >=20 > > In any case, developers should never rely on implicit calls of > > preempt_disable() for the reasons said above. Therefore, flush operatio= ns > > should be allowed regardless that kmap_atomic() potential side effect. > >=20 > > > In drm/i915/gem/i915_gem_object.c, the function > > > i915_gem_object_read_from_page_kmap() calls drm_clflush_virt_range() > >=20 > > If I recall correctly, drm_clflush_virt_range() can always be called wi= th=20 page > > faults and preemption enabled. If so, this is enough to say that the > > conversion is safe. > >=20 > > Is this code explicitly related to flushing the cache lines before=20 removing / > > adding CPUs? If I recall correctly, there are several other reasons beh= ind=20 the > > need to issue cache lines flushes. Am I wrong about this? > >=20 > > Can you please say more about what I'm missing here? > >=20 > > > to > > > use CLFLUSHOPT or WBINVD to flush. Since CLFLUSHOPT is global on x86 > > > and WBINVD is called on each cpu in drm_clflush_virt_range(), the flu= sh > > > operation is global and any issue with cpu's being added or removed > > > can be handled safely. > >=20 > > Again your main concern is about CPU hotplug. > >=20 > > Even if I'm missing something, do we really need all these details abou= t=20 the > > inner workings of drm_clflush_virt_range()? > >=20 > > I'm not an expert, so may be that I'm wrong about all I wrote above. > >=20 > > Therefore, can you please elaborate a little more for readers with very= =20 little > > knowledge of these kinds of things (like me and perhaps others)? > >=20 > > > 2. Any context switch caused by preemption or sleep (pagefault may > > > cause sleep) doesn't affect the validity of local mapping. > >=20 > > I'd replace "preemption or sleep" with "preemption and page faults" sin= ce > > yourself then added that page faults lead to tasks being put to sleep. > >=20 > > > Therefore, i915_gem_object_read_from_page_kmap() is a function where > > > the use of kmap_local_page() in place of kmap_atomic() is correctly > > > suited. > > >=20 > > > Convert the calls of kmap_atomic() / kunmap_atomic() to > > > kmap_local_page() / kunmap_local(). > > >=20 > > > And remove the redundant variable that stores the address of the mapp= ed > > > page since kunmap_local() can accept any pointer within the page. > > >=20 > > > [1]: https://lore.kernel.org/all/20220813220034.806698-1-ira.weiny@in= tel.com > > >=20 > > > Suggested-by: Dave Hansen > > > Suggested-by: Ira Weiny > > > Suggested-by: Fabio M. De Francesco > > > Signed-off-by: Zhao Liu > > > --- > > >=20 > > > Suggested by credits: > > > Dave: Referred to his explanation about cache flush. > > > Ira: Referred to his task document, review comments and explanation= =20 about > > > =20 > > > cache flush. > > > =20 > > > Fabio: Referred to his boiler plate commit message. > > >=20 > > > --- > > >=20 > > > drivers/gpu/drm/i915/gem/i915_gem_object.c | 8 +++----- > > > 1 file changed, 3 insertions(+), 5 deletions(-) > > >=20 > > > diff --git a/drivers/gpu/drm/i915/gem/i915_gem_object.c > > > b/drivers/gpu/drm/i915/gem/i915_gem_object.c index > >=20 > > 369006c5317f..a0072abed75e 100644 > >=20 > > > --- a/drivers/gpu/drm/i915/gem/i915_gem_object.c > > > +++ b/drivers/gpu/drm/i915/gem/i915_gem_object.c > > > @@ -413,17 +413,15 @@ void=20 __i915_gem_object_invalidate_frontbuffer(struct > > > drm_i915_gem_object *obj, static void > > >=20 > > > i915_gem_object_read_from_page_kmap(struct drm_i915_gem_object *obj,= =20 u64 > >=20 > > offset, void > >=20 > > > *dst, int size) { > > > - void *src_map; > > >=20 > > > void *src_ptr; > > >=20 > > > - src_map =3D kmap_atomic(i915_gem_object_get_page(obj, offset >> > >=20 > > PAGE_SHIFT)); > >=20 > > > - > > > - src_ptr =3D src_map + offset_in_page(offset); > > > + src_ptr =3D kmap_local_page(i915_gem_object_get_page(obj, offset >> > >=20 > > PAGE_SHIFT)) > >=20 > > > + + offset_in_page(offset); > > >=20 > > > if (!(obj->cache_coherent & I915_BO_CACHE_COHERENT_FOR_READ)) > > > =09 > > > drm_clflush_virt_range(src_ptr, size); > > > =09 > > > memcpy(dst, src_ptr, size); > > >=20 > > > - kunmap_atomic(src_map); > > > + kunmap_local(src_ptr); > > >=20 > > > } > > > =20 > > > static void > >=20 > > The changes look good, but I'd like to better understand the commit=20 message. > >=20 > > Thanks, > >=20 > > Fabio