[PATCH v3 6/9] drm/i915: Use kmap_local_page() in gem/selftests/i915_gem_context.c

Zhao Liu
2023-12-03T13:18:27Z

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.

With kmap_local_page(), we can avoid the often unwanted side effect of
unnecessary page faults or preemption disables.

In drm/i915/gem/selftests/i915_gem_context.c, functions cpu_fill() and
cpu_check() mainly uses mapping to flush cache and check/assign the
value.

There're 2 reasons why cpu_fill() and cpu_check() don't need to disable
pagefaults and preemption for mapping:

1. The flush operation is safe. cpu_fill() and cpu_check() call
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, cpu_fill() and cpu_check() are functions 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>
---
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/selftests/i915_gem_context.c | 8 ++++----
1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/i915/gem/selftests/i915_gem_context.c b/drivers/gpu/drm/i915/gem/selftests/i915_gem_context.c
index 7021b6e9b219..89d4dc8b60c6 100644
--- a/drivers/gpu/drm/i915/gem/selftests/i915_gem_context.c
+++ b/drivers/gpu/drm/i915/gem/selftests/i915_gem_context.c
@@ -489,12 +489,12 @@ static int cpu_fill(struct drm_i915_gem_object *obj, u32 value)
for (n = 0; n < real_page_count(obj); n++) {
u32 *map;

- map = kmap_atomic(i915_gem_object_get_page(obj, n));
+ map = kmap_local_page(i915_gem_object_get_page(obj, n));
for (m = 0; m < DW_PER_PAGE; m++)
map[m] = value;
if (!has_llc)
drm_clflush_virt_range(map, PAGE_SIZE);
- kunmap_atomic(map);
+ kunmap_local(map);
}

i915_gem_object_finish_access(obj);
@@ -520,7 +520,7 @@ static noinline int cpu_check(struct drm_i915_gem_object *obj,
for (n = 0; n < real_page_count(obj); n++) {
u32 *map, m;

- map = kmap_atomic(i915_gem_object_get_page(obj, n));
+ map = kmap_local_page(i915_gem_object_get_page(obj, n));
if (needs_flush & CLFLUSH_BEFORE)
drm_clflush_virt_range(map, PAGE_SIZE);

@@ -546,7 +546,7 @@ static noinline int cpu_check(struct drm_i915_gem_object *obj,
}

out_unmap:
- kunmap_atomic(map);
+ kunmap_local(map);
if (err)
break;
}
--
2.34.1