linux-kernel - [PATCH 2/9] drm/i915: Use kmap_local_page() in gem/i915_gem

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Date:   Mon, 17 Oct 2022 17:37:18 +0800
From:   Zhao Liu <zhao1.liu@...ux.intel.com>
To:     Jani Nikula <jani.nikula@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Matthew Auld <matthew.auld@...el.com>,
        Thomas Hellström 
        <thomas.hellstrom@...ux.intel.com>,
        Nirmoy Das <nirmoy.das@...el.com>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        Christian König <christian.koenig@....com>,
        intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org
Cc:     Ira Weiny <ira.weiny@...el.com>,
        "Fabio M . De Francesco" <fmdefrancesco@...il.com>,
        Zhenyu Wang <zhenyu.z.wang@...el.com>,
        Zhao Liu <zhao1.liu@...el.com>,
        Dave Hansen <dave.hansen@...el.com>
Subject: [PATCH 2/9] drm/i915: Use kmap_local_page() in gem/i915_gem_pyhs.c

From: Zhao Liu <zhao1.liu@...el.com>

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.

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
these 2 reasons:

1. The flush operation is safe for CPU hotplug when preemption is not
disabled. In drm/i915/gem/i915_gem_object.c, the functions
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 and any issue
with cpu's being added or removed can be handled safely.

2. Any context switch caused by preemption or sleep (pagefault 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 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

Suggested-by: Dave Hansen <dave.hansen@...el.com>
Suggested-by: Ira Weiny <ira.weiny@...el.com>
Suggested-by: Fabio M. De Francesco <fmdefrancesco@...il.com>
Signed-off-by: Zhao Liu <zhao1.liu@...el.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.
---
 drivers/gpu/drm/i915/gem/i915_gem_phys.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/i915/gem/i915_gem_phys.c b/drivers/gpu/drm/i915/gem/i915_gem_phys.c
index 0d0e46dae559..d602ba19ecb2 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_phys.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_phys.c
@@ -66,10 +66,10 @@ static int i915_gem_object_get_pages_phys(struct drm_i915_gem_object *obj)
 		if (IS_ERR(page))
 			goto err_st;
 
-		src = kmap_atomic(page);
+		src = kmap_local_page(page);
 		memcpy(dst, src, PAGE_SIZE);
 		drm_clflush_virt_range(dst, PAGE_SIZE);
-		kunmap_atomic(src);
+		kunmap_local(src);
 
 		put_page(page);
 		dst += PAGE_SIZE;
@@ -114,10 +114,10 @@ i915_gem_object_put_pages_phys(struct drm_i915_gem_object *obj,
 			if (IS_ERR(page))
 				continue;
 
-			dst = kmap_atomic(page);
+			dst = kmap_local_page(page);
 			drm_clflush_virt_range(src, PAGE_SIZE);
 			memcpy(dst, src, PAGE_SIZE);
-			kunmap_atomic(dst);
+			kunmap_local(dst);
 
 			set_page_dirty(page);
 			if (obj->mm.madv == I915_MADV_WILLNEED)
-- 
2.34.1