[PATCH v3 1/9] drm/i915: Use kmap_local_page() in gem/i915_gem_object.c

Zhao Liu
2023-12-03T13:18:14Z

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

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>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Reviewed-by: Fabio M. De Francesco <fmdefrancesco@gmail.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 c26d87555825..a2a7e5005415 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_object.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_object.c
@@ -493,17 +493,15 @@ 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