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

2023-03-30 Thread Ira Weiny
Zhao Liu wrote:
> From: Zhao Liu 
> 
> 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.we...@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 
> Suggested-by: Ira Weiny 

Reviewed-by: Ira Weiny 


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

2023-03-29 Thread Zhao Liu
From: Zhao Liu 

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.we...@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 
Suggested-by: Ira Weiny 
Suggested-by: Fabio M. De Francesco 
Signed-off-by: Zhao Liu 
---
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