[PATCH v2 1/9] drm/i915: Use kmap_local_page() in gem/i915_gem_object.c

From: Zhao Liu <zhao1.liu@linux.intel.com>
To: "Jani Nikula" <jani.nikula@linux.intel.com>,
	"Joonas Lahtinen" <joonas.lahtinen@linux.intel.com>,
	"Rodrigo Vivi" <rodrigo.vivi@intel.com>,
	"Tvrtko Ursulin" <tvrtko.ursulin@linux.intel.com>,
	"David Airlie" <airlied@gmail.com>,
	"Daniel Vetter" <daniel@ffwll.ch>,
	"Matthew Auld" <matthew.auld@intel.com>,
	"Thomas Hellström" <thomas.hellstrom@linux.intel.com>,
	"Nirmoy Das" <nirmoy.das@intel.com>,
	"Maarten Lankhorst" <maarten.lankhorst@linux.intel.com>,
	"Chris Wilson" <chris@chris-wilson.co.uk>,
	"Christian König" <christian.koenig@amd.com>,
	intel-gfx@lists.freedesktop.org, dri-devel@lists.freedesktop.org,
	linux-kernel@vger.kernel.org
Cc: Ira Weiny <ira.weiny@intel.com>,
	"Fabio M . De Francesco" <fmdefrancesco@gmail.com>,
	Zhenyu Wang <zhenyu.z.wang@intel.com>,
	Zhao Liu <zhao1.liu@intel.com>,
	Dave Hansen <dave.hansen@intel.com>
Subject: [PATCH v2 1/9] drm/i915: Use kmap_local_page() in gem/i915_gem_object.c
Date: Wed, 29 Mar 2023 15:32:12 +0800	[thread overview]
Message-ID: <20230329073220.3982460-2-zhao1.liu@linux.intel.com> (raw)
In-Reply-To: <20230329073220.3982460-1-zhao1.liu@linux.intel.com>

From: Zhao Liu <zhao1.liu@intel.com>

The use of kmap_atomic() is being deprecated in favor of
kmap_local_page()[1], and this patch converts the call from
kmap_atomic() to kmap_local_page().

The main difference between atomic and local mappings is that local
mappings doesn't disable page faults or preemption (the preemption is
disabled for !PREEMPT_RT case, otherwise it only disables migration).

With kmap_local_page(), we can avoid the often unwanted side effect of
unnecessary page faults and preemption disables.

There're 2 reasons why i915_gem_object_read_from_page_kmap() doesn't
need to disable pagefaults and preemption for mapping:

1. The flush operation is safe. In drm/i915/gem/i915_gem_object.c,
i915_gem_object_read_from_page_kmap() calls drm_clflush_virt_range() 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 flush
operation is global.

2. Any context switch caused by preemption or page faults (page fault
may cause sleep) doesn't affect the validity of local mapping.

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.

Convert the calls of kmap_atomic() / kunmap_atomic() to
kmap_local_page() / kunmap_local().

And remove the redundant variable that stores the address of the mapped
page since kunmap_local() can accept any pointer within the page.

[1]: https://lore.kernel.org/all/20220813220034.806698-1-ira.weiny@intel.com

v2:
* Dropped hot plug related description since it has nothing to do with
  kmap_local_page().
* Rebased on f47e630 (drm/i915/gem: Typecheck page lookups) to keep
  the "idx" variable of type pgoff_t here.
* Added description of the motivation of using kmap_local_page().

Suggested-by: Dave Hansen <dave.hansen@intel.com>
Suggested-by: Ira Weiny <ira.weiny@intel.com>
Suggested-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
---
Suggested by credits:
  Dave: Referred to his explanation about cache flush.
  Ira: Referred to his task document, review comments and explanation
       about cache flush.
  Fabio: Referred to his boiler plate commit message and his description
         about why kmap_local_page() should be preferred.
---
 drivers/gpu/drm/i915/gem/i915_gem_object.c | 8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/drivers/gpu/drm/i915/gem/i915_gem_object.c b/drivers/gpu/drm/i915/gem/i915_gem_object.c
index e6d4efde4fc5..c0bfdd7784f7 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_object.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_object.c
@@ -428,17 +428,15 @@ static void
 i915_gem_object_read_from_page_kmap(struct drm_i915_gem_object *obj, u64 offset, void *dst, int size)
 {
 	pgoff_t idx = offset >> PAGE_SHIFT;
-	void *src_map;
 	void *src_ptr;
 
-	src_map = kmap_atomic(i915_gem_object_get_page(obj, idx));
-
-	src_ptr = src_map + offset_in_page(offset);
+	src_ptr = kmap_local_page(i915_gem_object_get_page(obj, idx))
+	          + offset_in_page(offset);
 	if (!(obj->cache_coherent & I915_BO_CACHE_COHERENT_FOR_READ))
 		drm_clflush_virt_range(src_ptr, size);
 	memcpy(dst, src_ptr, size);
 
-	kunmap_atomic(src_map);
+	kunmap_local(src_ptr);
 }
 
 static void
-- 
2.34.1

