linux-kernel - [RESEND PATCH 0/3] drm/i915: Replace kmap() with kmap_local

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Date:   Sun, 16 Oct 2022 20:09:35 +0200
From:   "Fabio M. De Francesco" <fmdefrancesco@...il.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>,
        Zhenyu Wang <zhenyuw@...ux.intel.com>,
        Zhi Wang <zhi.a.wang@...el.com>,
        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>,
        Christian König <christian.koenig@....com>,
        Matt Roper <matthew.d.roper@...el.com>,
        John Harrison <John.C.Harrison@...el.com>,
        Daniele Ceraolo Spurio <daniele.ceraolospurio@...el.com>,
        Lucas De Marchi <lucas.demarchi@...el.com>,
        intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, intel-gvt-dev@...ts.freedesktop.org
Cc:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>,
        "Venkataramanan, Anirudh" <anirudh.venkataramanan@...el.com>,
        Ira Weiny <ira.weiny@...el.com>
Subject: [RESEND PATCH 0/3] drm/i915: Replace kmap() with kmap_local_page()

kmap() is being deprecated in favor of kmap_local_page().

There are two main problems with kmap(): (1) It comes with an overhead as
mapping space is restricted and protected by a global lock for
synchronization and (2) it also requires global TLB invalidation when the
kmap’s pool wraps and it might block when the mapping space is fully
utilized until a slot becomes available.

With kmap_local_page() the mappings are per thread, CPU local, can take
page faults, and can be called from any context (including interrupts).
It is faster than kmap() in kernels with HIGHMEM enabled. Furthermore,
the tasks can be preempted and, when they are scheduled to run again, the
kernel virtual addresses are restored and still valid.

Since its use in drm/i915 is safe everywhere, it should be preferred.

Therefore, replace kmap() with kmap_local_page() in drm/i915.

These changes should be tested in an 32 bits system, booting a kernel
with HIGHMEM enabled. Unfortunately I have no i915 based hardware,
therefore any help with testing would be greatly appreciated.

I'm resending this little series because I suspect that it has been
lost, since it was submitted on Aug 11, 2022. In the meantime I'm
adding one more recipient (Anirudh) who is helping, along with others, Ira
and me with these conversions / removals of kmap() and kmap_atomic() 

Cc: "Venkataramanan, Anirudh" <anirudh.venkataramanan@...el.com>
Suggested-by: Ira Weiny <ira.weiny@...el.com>
Signed-off-by: Fabio M. De Francesco <fmdefrancesco@...il.com>

Fabio M. De Francesco (3):
  drm/i915: Replace kmap() with kmap_local_page()
  drm/i915/gt: Replace kmap() with kmap_local_page()
  drm/i915/gem: Replace kmap() with kmap_local_page()

 drivers/gpu/drm/i915/gem/i915_gem_shmem.c          |  6 ++----
 drivers/gpu/drm/i915/gem/selftests/i915_gem_mman.c |  8 ++++----
 drivers/gpu/drm/i915/gt/intel_ggtt_fencing.c       |  4 ++--
 drivers/gpu/drm/i915/gt/shmem_utils.c              | 11 ++++-------
 drivers/gpu/drm/i915/i915_gem.c                    |  8 ++++----
 5 files changed, 16 insertions(+), 21 deletions(-)

-- 
2.37.1