[v3,4/9] drm/i915: Use kmap_local_page() in gem/selftests/huge_pages.c

Commit Message

Zhao Liu Dec. 3, 2023, 1:29 p.m. UTC

From: Zhao Liu <[e-mail]>

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 or preemption disables.

In drm/i915/gem/selftests/huge_pages.c, function __cpu_check_shmem()
mainly uses mapping to flush cache and check the value. There're
2 reasons why __cpu_check_shmem() doesn't need to disable pagefaults
and preemption for mapping:

1. The flush operation is safe. Function __cpu_check_shmem() 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, __cpu_check_shmem() 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().

[1]: https://lore.kernel.org/all/[e-mail]

Suggested-by: Dave Hansen <[e-mail]>
Suggested-by: Ira Weiny <[e-mail]>
Suggested-by: Fabio M. De Francesco <[e-mail]>
Signed-off-by: Zhao Liu <[e-mail]>
Reviewed-by: Ira Weiny <[e-mail]>
Reviewed-by: Fabio M. De Francesco <[e-mail]>
---
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/huge_pages.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

Patch

diff --git a/drivers/gpu/drm/i915/gem/selftests/huge_pages.c b/drivers/gpu/drm/i915/gem/selftests/huge_pages.c
index 6b9f6cf50bf6..c9e6d77abab0 100644
--- a/drivers/gpu/drm/i915/gem/selftests/huge_pages.c
+++ b/drivers/gpu/drm/i915/gem/selftests/huge_pages.c
@@ -1082,7 +1082,7 @@  __cpu_check_shmem(struct drm_i915_gem_object *obj, u32 dword, u32 val)
 		goto err_unlock;
 
 	for (n = 0; n < obj->base.size >> PAGE_SHIFT; ++n) {
-		u32 *ptr = kmap_atomic(i915_gem_object_get_page(obj, n));
+		u32 *ptr = kmap_local_page(i915_gem_object_get_page(obj, n));
 
 		if (needs_flush & CLFLUSH_BEFORE)
 			drm_clflush_virt_range(ptr, PAGE_SIZE);
@@ -1090,12 +1090,12 @@  __cpu_check_shmem(struct drm_i915_gem_object *obj, u32 dword, u32 val)
 		if (ptr[dword] != val) {
 			pr_err("n=%lu ptr[%u]=%u, val=%u\n",
 			       n, dword, ptr[dword], val);
-			kunmap_atomic(ptr);
+			kunmap_local(ptr);
 			err = -EINVAL;
 			break;
 		}
 
-		kunmap_atomic(ptr);
+		kunmap_local(ptr);
 	}
 
 	i915_gem_object_finish_access(obj);