diff mbox series

[v2,4/9] drm/i915: Use kmap_local_page() in gem/selftests/huge_pages.c

Message ID 20230329073220.3982460-5-zhao1.liu@linux.intel.com
State New
Headers
Series drm/i915: Replace kmap_atomic() with kmap_local_page() |

Commit Message

Zhao Liu March 29, 2023, 7:32 a.m. UTC 
  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 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/20220813220034.806698-1-ira.weiny@intel.com

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@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/selftests/huge_pages.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

Comments

Ira Weiny March 31, 2023, 3:04 a.m. UTC | #1 
Zhao Liu wrote:
> 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 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/20220813220034.806698-1-ira.weiny@intel.com
> 
> 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@intel.com>
> Suggested-by: Ira Weiny <ira.weiny@intel.com>

Reviewed-by: Ira Weiny <ira.weiny@intel.com>
diff mbox series

Patch

diff --git a/drivers/gpu/drm/i915/gem/selftests/huge_pages.c b/drivers/gpu/drm/i915/gem/selftests/huge_pages.c
index defece0bcb81..3f9ea48a48d0 100644
--- a/drivers/gpu/drm/i915/gem/selftests/huge_pages.c
+++ b/drivers/gpu/drm/i915/gem/selftests/huge_pages.c
@@ -1026,7 +1026,7 @@  __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);
@@ -1034,12 +1034,12 @@  __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);