linux-kernel - Re: [PATCH 1/2] x86/io: add interface to reserve io memtype for a resource range. (v1.1)

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Date:   Wed, 26 Oct 2016 16:12:15 +1000
From:   Dave Airlie <airlied@...il.com>
To:     "Luis R. Rodriguez" <mcgrof@...nel.org>,
        Dave Airlie <airlied@...hat.com>,
        Toshi Kani <toshi.kani@...com>,
        Brian Gerst <brgerst@...il.com>, x86@...nel.org,
        LKML <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 1/2] x86/io: add interface to reserve io memtype for a
 resource range. (v1.1)

>>
>> 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.