[PATCH v2 6/9] drm/i915: Use kmap_local_page() in gem/selftests/i915_gem_context.c

[Zhao Liu]

From: Zhao Liu <zhao1.liu@xxxxxxxxx>

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.

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

In drm/i915/gem/selftests/i915_gem_context.c, functions cpu_fill() and
cpu_check() mainly uses mapping to flush cache and check/assign the
value.

There're 2 reasons why cpu_fill() and cpu_check() don't need to disable
pagefaults and preemption for mapping:

1. The flush operation is safe. cpu_fill() and cpu_check() call
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, cpu_fill() and cpu_check() are functions 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().

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

v2:
* Dropped hot plug related description since it has nothing to do with
  kmap_local_page().
* No code change since v1, and added description of the motivation of
  using kmap_local_page().

Suggested-by: Dave Hansen <dave.hansen@xxxxxxxxx>
Suggested-by: Ira Weiny <ira.weiny@xxxxxxxxx>
Suggested-by: Fabio M. De Francesco <fmdefrancesco@xxxxxxxxx>
Signed-off-by: Zhao Liu <zhao1.liu@xxxxxxxxx>
---
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/selftests/i915_gem_context.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/i915/gem/selftests/i915_gem_context.c b/drivers/gpu/drm/i915/gem/selftests/i915_gem_context.c
index a81fa6a20f5a..dcbc0b8e3323 100644
--- a/drivers/gpu/drm/i915/gem/selftests/i915_gem_context.c
+++ b/drivers/gpu/drm/i915/gem/selftests/i915_gem_context.c
@@ -481,12 +481,12 @@ static int cpu_fill(struct drm_i915_gem_object *obj, u32 value)
 	for (n = 0; n < real_page_count(obj); n++) {
 		u32 *map;
 
-		map = kmap_atomic(i915_gem_object_get_page(obj, n));
+		map = kmap_local_page(i915_gem_object_get_page(obj, n));
 		for (m = 0; m < DW_PER_PAGE; m++)
 			map[m] = value;
 		if (!has_llc)
 			drm_clflush_virt_range(map, PAGE_SIZE);
-		kunmap_atomic(map);
+		kunmap_local(map);
 	}
 
 	i915_gem_object_finish_access(obj);
@@ -512,7 +512,7 @@ static noinline int cpu_check(struct drm_i915_gem_object *obj,
 	for (n = 0; n < real_page_count(obj); n++) {
 		u32 *map, m;
 
-		map = kmap_atomic(i915_gem_object_get_page(obj, n));
+		map = kmap_local_page(i915_gem_object_get_page(obj, n));
 		if (needs_flush & CLFLUSH_BEFORE)
 			drm_clflush_virt_range(map, PAGE_SIZE);
 
@@ -538,7 +538,7 @@ static noinline int cpu_check(struct drm_i915_gem_object *obj,
 		}
 
 out_unmap:
-		kunmap_atomic(map);
+		kunmap_local(map);
 		if (err)
 			break;
 	}
-- 
2.34.1