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Date:   Thu, 30 Mar 2023 16:01:44 -0700
From:   Ira Weiny <ira.weiny@intel.com>
To:     Zhao Liu <zhao1.liu@linux.intel.com>,
        Jani Nikula <jani.nikula@linux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@linux.intel.com>,
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        Matthew Auld <matthew.auld@intel.com>,
        Thomas =?iso-8859-1?Q?Hellstr=F6m?= 
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CC:     Ira Weiny <ira.weiny@intel.com>,
        "Fabio M . De Francesco" <fmdefrancesco@gmail.com>,
        Zhenyu Wang <zhenyu.z.wang@intel.com>,
        Zhao Liu <zhao1.liu@intel.com>,
        Dave Hansen <dave.hansen@intel.com>
Subject: Re: [PATCH v2 2/9] drm/i915: Use memcpy_[from/to]_page() in
 gem/i915_gem_pyhs.c
Message-ID: <642614d888783_375f7e294a5@iweiny-mobl.notmuch>
References: <20230329073220.3982460-1-zhao1.liu@linux.intel.com>
 <20230329073220.3982460-3-zhao1.liu@linux.intel.com>
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Zhao Liu wrote:
> 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() + 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@intel.com
> 
> 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@intel.com>
> Suggested-by: Ira Weiny <ira.weiny@intel.com>

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