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() + memcpy() to memcpy_[from/to]_page(), which use > kmap_local_page() to build local mapping and then do memcpy(). > > 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. > > In drm/i915/gem/i915_gem_phys.c, the functions > i915_gem_object_get_pages_phys() and i915_gem_object_put_pages_phys() > don't need to disable pagefaults and preemption for mapping because of > 2 reasons: > > 1. The flush operation is safe. In drm/i915/gem/i915_gem_object.c, > i915_gem_object_get_pages_phys() and i915_gem_object_put_pages_phys() > 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, i915_gem_object_get_pages_phys() and > i915_gem_object_put_pages_phys() are two functions where the uses of > local mappings in place of atomic mappings are correctly suited. > > Convert the calls of kmap_atomic() / kunmap_atomic() + memcpy() to > memcpy_from_page() and memcpy_to_page(). > > [1]: https://lore.kernel.org/all/20220813220034.806698-1-ira.weiny@xxxxxxxxx > > v2: > * Used memcpy_from_page() and memcpy_to_page() to replace > kmap_local_page() + memcpy(). > * Dropped hot plug related description since it has nothing to do with > kmap_local_page(). > * 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>
