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    Subject[PATCH v2 8/9] drm/i915: Use kmap_local_page() in i915_cmd_parser.c
    Date
    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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