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From: Dave Airlie <airlied@gmail.com>
Date: Wed, 26 Oct 2016 16:12:15 +1000
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Subject: Re: [PATCH 1/2] x86/io: add interface to reserve io memtype for a
 resource range. (v1.1)
To: "Luis R. Rodriguez" <mcgrof@kernel.org>,
        Dave Airlie <airlied@redhat.com>, Toshi Kani <toshi.kani@hp.com>,
        Brian Gerst <brgerst@gmail.com>, x86@kernel.org,
        LKML <linux-kernel@vger.kernel.org>,
        dri-devel <dri-devel@lists.freedesktop.org>,
        Borislav Petkov <bp@alien8.de>, Andy Lutomirski <luto@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>, Denys Vlasenko <dvlasenk@redhat.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Linus Torvalds <torvalds@linux-foundation.org>
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>>
>> Is anything on a driver to be able to tell when this is actually needed ?
>> How will driver developers know? Can you add a bit of documentation to
>> the API? If its transitive towards a secondary solution indicating so
>> would help driver developers.
>
> I'll plug the io-mapping stuff again here, and more specifically the
> userspace pte wrangling stuff we've added in 4.9 to i915_mm.c. Should
> probably move that one to the core. That way io_mapping takes care of the
> full reservartion, and allows you to on-demand kmap (for kernel) and write
> ptes. All nicely fast and all, and for bonus, also nicely encapsulated.

Yeah I think ideally we'd want to move towards that, however we don't tend
to want to ioremap the full range even on 64-bit, which is what io-mapping does.

At least on most GPUs with VRAM we rarely want to map VRAM for much,
I think page tables and fbcon are probably the main two uses for touch
it at all.

So I don't think we need to be as efficient as i915 in this area.

Dave.