Re: [PATCH v2 5/9] drm/i915: Use kmap_local_page() in gem/selftests/i915_gem_coherency.c

[Ira Weiny]

Zhao Liu wrote:
> From: Zhao Liu <zhao1.liu@xxxxxxxxx>
> 
> 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 or preemption disables.
> 
> In drm/i915/gem/selftests/i915_gem_coherency.c, functions cpu_set()
> and cpu_get() mainly uses mapping to flush cache and assign the value.
> There're 2 reasons why cpu_set() and cpu_get() don't need to disable
> pagefaults and preemption for mapping:
> 
> 1. The flush operation is safe. cpu_set() and cpu_get() 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_set() and cpu_get() 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@xxxxxxxxx
> 
> 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@xxxxxxxxx>
> Suggested-by: Ira Weiny <ira.weiny@xxxxxxxxx>

Reviewed-by: Ira Weiny <ira.weiny@xxxxxxxxx>