drm/i915: Use kmap_local_page() in i915_cmd_parser.c

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@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>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231203132947.2328805-9-zhao1.liu@linux.intel.com

diff --git a/drivers/gpu/drm/i915/i915_cmd_parser.c b/drivers/gpu/drm/i915/i915_cmd_parser.c
index ddf49c2..2905df8 100644 (file)
--- 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;