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To:     Jani Nikula <jani.nikula@linux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@linux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@intel.com>,
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        intel-gfx@lists.freedesktop.org, dri-devel@lists.freedesktop.org,
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Cc:     Ira Weiny <ira.weiny@intel.com>,
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        Zhao Liu <zhao1.liu@intel.com>
Subject: Re: [PATCH 0/9] drm/i915: Replace kmap_atomic() with kmap_local_page()
Date:   Sat, 29 Oct 2022 09:12:27 +0200
Message-ID: <10176905.nUPlyArG6x@suse>
In-Reply-To: <20221017093726.2070674-1-zhao1.liu@linux.intel.com>
References: <20221017093726.2070674-1-zhao1.liu@linux.intel.com>
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On luned=EC 17 ottobre 2022 11:37:16 CEST Zhao Liu wrote:
> From: Zhao Liu <zhao1.liu@intel.com>
>=20
> The use of kmap_atomic() is being deprecated in favor of
> kmap_local_page()[1].

Some words to explain why kmap_atomic was deprecated won't hurt. Many=20
maintainers and reviewers, and also casual readers might not yet be aware o=
f=20
the reasons behind that deprecation.
=20
> In the following patches, we can convert the calls of kmap_atomic() /
> kunmap_atomic() to kmap_local_page() / kunmap_local(), which can
> instead do the mapping / unmapping regardless of the context.

Readers are probably much more interested in what you did in the following=
=20
patches and why you did it, instead of being informed about what "we can" d=
o.

I would suggest something like "The following patches convert the calls to=
=20
kmap_atomic() to kmap_local_page() [the rest looks OK]".

This could also be the place to say something about why we prefer=20
kmap_local_page() to kmap_atomic().=20

Are you sure that the reasons that motivates your conversions are merely=20
summarized to kmap_local_page() being able to do mappings regardless of=20
context? I think you are missing the real reasons why.=20

What about avoiding the often unwanted side effect of unnecessary page faul=
ts=20
disables?

>=20
> With kmap_local_page(), the mapping is per thread, CPU local and not
> globally visible.

No news here. kmap_atomic() is "per thread, CPU local and not glocally=20
visible". I cannot see any difference here between kmap_atomic() and=20
kmap_local_page().

>=20
> [1]: https://lore.kernel.org/all/20220813220034.806698-1-ira.weiny@intel.=
com
> ---
> Zhao Liu (9):
>   drm/i915: Use kmap_local_page() in gem/i915_gem_object.c
>   drm/i915: Use kmap_local_page() in gem/i915_gem_pyhs.c
>   drm/i915: Use kmap_local_page() in gem/i915_gem_shmem.c
>   drm/i915: Use kmap_local_page() in gem/selftests/huge_pages.c
>   drm/i915: Use kmap_local_page() in gem/selftests/i915_gem_coherency.c
>   drm/i915: Use kmap_local_page() in gem/selftests/i915_gem_context.c
>   drm/i915: Use memcpy_from_page() in gt/uc/intel_uc_fw.c
>   drm/i915: Use kmap_local_page() in i915_cmd_parser.c
>   drm/i915: Use kmap_local_page() in gem/i915_gem_execbuffer.c
>=20
>  drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c       | 10 +++++-----
>  drivers/gpu/drm/i915/gem/i915_gem_object.c           |  8 +++-----
>  drivers/gpu/drm/i915/gem/i915_gem_phys.c             |  8 ++++----
>  drivers/gpu/drm/i915/gem/i915_gem_shmem.c            |  6 ++++--
>  drivers/gpu/drm/i915/gem/selftests/huge_pages.c      |  6 +++---
>  .../gpu/drm/i915/gem/selftests/i915_gem_coherency.c  | 12 ++++--------
>  .../gpu/drm/i915/gem/selftests/i915_gem_context.c    |  8 ++++----
>  drivers/gpu/drm/i915/gt/uc/intel_uc_fw.c             |  5 +----
>  drivers/gpu/drm/i915/i915_cmd_parser.c               |  4 ++--
>  9 files changed, 30 insertions(+), 37 deletions(-)

Thanks for helping with kmap_atomic() conversions to kmap_local_page().

=46abio