On Thu, Nov 03, 2022 at 09:51:23AM -0700, Ira Weiny wrote: > Date: Thu, 3 Nov 2022 09:51:23 -0700 > From: Ira Weiny <ira.weiny@xxxxxxxxx> > Subject: Re: [PATCH 1/9] drm/i915: Use kmap_local_page() in > gem/i915_gem_object.c > > On Sat, Oct 29, 2022 at 01:17:03PM +0200, Fabio M. De Francesco wrote: > > On luned? 17 ottobre 2022 11:37:17 CEST Zhao Liu wrote: > > > From: Zhao Liu <zhao1.liu@xxxxxxxxx> > > > > > > The use of kmap_atomic() is being deprecated in favor of > > > kmap_local_page()[1]. > > > > > > The main difference between atomic and local mappings is that local > > > mappings doesn't disable page faults or preemption. > > > > You are right about about page faults which are never disabled by > > kmap_local_page(). However kmap_atomic might not disable preemption. It > > depends on CONFIG_PREEMPT_RT. > > > > Please refer to how kmap_atomic_prot() works (this function is called by > > kmap_atomic() when kernels have HIGHMEM enabled). > > > > > > > > There're 2 reasons why i915_gem_object_read_from_page_kmap() doesn't > > > need to disable pagefaults and preemption for mapping: > > > > > > 1. The flush operation is safe for CPU hotplug when preemption is not > > > disabled. > > > > I'm confused here. Why are you talking about CPU hotplug? > > I agree with Fabio here. I'm not making the connection between cpu hotplug and > this code path. Sorry, my misunderstanding. Will delete this wrong explanation. Thanks, Zhao
