[PATCH v2 8/9] drm/i915: Use kmap_local_page() in i915_cmd_parser.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 (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 function copy_batch() doesn't need to disable
pagefaults and preemption for mapping:

1. The flush operation is safe. In i915_cmd_parser.c, copy_batch() 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, copy_batch() 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@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/i915_cmd_parser.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_cmd_parser.c b/drivers/gpu/drm/i915/i915_cmd_parser.c
index ddf49c2dbb91..2905df83e180 100644
--- a/drivers/gpu/drm/i915/i915_cmd_parser.c
+++ b/drivers/gpu/drm/i915/i915_cmd_parser.c
@@ -1211,11 +1211,11 @@ static u32 *copy_batch(struct drm_i915_gem_object *dst_obj,
 		for (n = offset >> PAGE_SHIFT; remain; n++) {
 			int len = min(remain, PAGE_SIZE - x);
 
-			src = kmap_atomic(i915_gem_object_get_page(src_obj, n));
+			src = kmap_local_page(i915_gem_object_get_page(src_obj, n));
 			if (src_needs_clflush)
 				drm_clflush_virt_range(src + x, len);
 			memcpy(ptr, src + x, len);
-			kunmap_atomic(src);
+			kunmap_local(src);
 
 			ptr += len;
 			remain -= len;
-- 
2.34.1