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Date:   Wed, 21 Jun 2023 20:51:34 +0200
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Subject: Re: [PATCH v2] drm/i915: Replace kmap() with kmap_local_page()
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To:     Ira Weiny <ira.weiny@intel.com>,
        Sumitra Sharma <sumitraartsy@gmail.com>
Cc:     Jani Nikula <jani.nikula@linux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@linux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@intel.com>,
        Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>,
        David Airlie <airlied@gmail.com>,
        Daniel Vetter <daniel@ffwll.ch>,
        intel-gfx@lists.freedesktop.org, dri-devel@lists.freedesktop.org,
        linux-kernel@vger.kernel.org, Fabio <fmdefrancesco@gmail.com>,
        Deepak R Varma <drv@mailo.com>
References: <20230620180759.GA419158@sumitra.com>
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 <649326df1b895_1c0dd29486@iweiny-mobl.notmuch>
From:   =?UTF-8?Q?Thomas_Hellstr=c3=b6m_=28Intel=29?= 
        <thomas_os@shipmail.org>
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On 6/21/23 18:35, Ira Weiny wrote:
> Thomas Hellström (Intel) wrote:
>> I think one thing worth mentioning in the context of this patch is that
>> IIRC kmap_local_page() will block offlining of the mapping CPU until
>> kunmap_local(), so while I haven't seen any guidelines around the usage
>> of this api for long-held mappings, I figure it's wise to keep the
>> mapping duration short, or at least avoid sleeping with a
>> kmap_local_page() map active.
>>
>> I figured, while page compression is probably to be considered "slow"
>> it's probably not slow enough to motivate kmap() instead of
>> kmap_local_page(), but if anyone feels differently, perhaps it should be
>> considered.
> What you say is all true.  But remember the mappings are only actually
> created on a HIGHMEM system.  HIGHMEM systems are increasingly rare.  Also
> they must suffer such performance issues because there is just no other
> way around supporting them.
>
> Also Sumitra, and our kmap conversion project in general, is focusing on
> not using kmap* if at all possible.  Thus the reason V1 tried to use
> page_address().
>
> Could we guarantee the i915 driver is excluded from all HIGHMEM systems?

The i915 maintainers might want to chime in here, but I would say no, we 
can't, although we don't care much about optimizing for them. Same for 
the new xe driver.

Thanks,

/Thomas


>
>> With that said, my Reviewed-by: still stands.
> Thanks!
> Ira