[PATCH v2 2/3] drm/i915/gt: Replace kmap() with kmap_local_page()

[Fabio M. De Francesco]

kmap() s been deprecated in favor of kmap_local_page().

There are two main problems with kmap(): (1) It comes with an overhead as
mapping space is restricted and protected by a global lock for
synchronization and (2) it also requires global TLB invalidation when the
kmap’s pool wraps and it might block when the mapping space is fully
utilized until a slot becomes available.

With kmap_local_page() the mappings are per thread, CPU local, can take
page faults, and can be called from any context (including interrupts).
It is faster than kmap() in kernels with HIGHMEM enabled. Furthermore,
the tasks can be preempted and, when they are scheduled to run again, the
kernel virtual addresses are restored and are still valid.

Obviously, thread locality implies that the kernel virtual addresses are
only valid in the context of the callers. The use of kmap_local_page() in
i915/gt doesn't break the above-mentioned constraint, so it should be
preferred to kmap().

Therefore, replace kmap() with kmap_local_page() in i915/gt.

Suggested-by: Ira Weiny <ira.weiny@xxxxxxxxx>
Signed-off-by: Fabio M. De Francesco <fmdefrancesco@xxxxxxxxx>
---
 drivers/gpu/drm/i915/gt/intel_ggtt_fencing.c |  4 ++--
 drivers/gpu/drm/i915/gt/shmem_utils.c        | 11 ++++-------
 2 files changed, 6 insertions(+), 9 deletions(-)

diff --git a/drivers/gpu/drm/i915/gt/intel_ggtt_fencing.c b/drivers/gpu/drm/i915/gt/intel_ggtt_fencing.c
index 37d0b0fe791d..89295c6921d6 100644
--- a/drivers/gpu/drm/i915/gt/intel_ggtt_fencing.c
+++ b/drivers/gpu/drm/i915/gt/intel_ggtt_fencing.c
@@ -749,7 +749,7 @@ static void swizzle_page(struct page *page)
 	char *vaddr;
 	int i;

-	vaddr = kmap(page);
+	vaddr = kmap_local_page(page);

 	for (i = 0; i < PAGE_SIZE; i += 128) {
 		memcpy(temp, &vaddr[i], 64);
@@ -757,7 +757,7 @@ static void swizzle_page(struct page *page)
 		memcpy(&vaddr[i + 64], temp, 64);
 	}

-	kunmap(page);
+	kunmap_local(vaddr);
 }

 /**
diff --git a/drivers/gpu/drm/i915/gt/shmem_utils.c b/drivers/gpu/drm/i915/gt/shmem_utils.c
index 449c9ed44382..be809839a241 100644
--- a/drivers/gpu/drm/i915/gt/shmem_utils.c
+++ b/drivers/gpu/drm/i915/gt/shmem_utils.c
@@ -101,22 +101,19 @@ static int __shmem_rw(struct file *file, loff_t off,
 		unsigned int this =
 			min_t(size_t, PAGE_SIZE - offset_in_page(off), len);
 		struct page *page;
-		void *vaddr;

 		page = shmem_read_mapping_page_gfp(file->f_mapping, pfn,
 						   GFP_KERNEL);
 		if (IS_ERR(page))
 			return PTR_ERR(page);

-		vaddr = kmap(page);
 		if (write) {
-			memcpy(vaddr + offset_in_page(off), ptr, this);
+			memcpy_to_page(page, offset_in_page(off), ptr, this);
 			set_page_dirty(page);
 		} else {
-			memcpy(ptr, vaddr + offset_in_page(off), this);
+			memcpy_from_page(ptr, page, offset_in_page(off), this);
 		}
 		mark_page_accessed(page);
-		kunmap(page);
 		put_page(page);

 		len -= this;
@@ -143,11 +140,11 @@ int shmem_read_to_iosys_map(struct file *file, loff_t off,
 		if (IS_ERR(page))
 			return PTR_ERR(page);

-		vaddr = kmap(page);
+		vaddr = kmap_local_page(page);
 		iosys_map_memcpy_to(map, map_off, vaddr + offset_in_page(off),
 				    this);
 		mark_page_accessed(page);
-		kunmap(page);
+		kunmap_local(vaddr);
 		put_page(page);

 		len -= this;
--
2.40.0