Messages in this thread Patch in this message | | | From | Zhao Liu <> | Subject | [PATCH v2 8/9] drm/i915: Use kmap_local_page() in i915_cmd_parser.c | Date | Wed, 29 Mar 2023 15:32:19 +0800 |
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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 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
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/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
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