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:   Tue, 25 Oct 2016 19:31:29 +0200
From:   "Luis R. Rodriguez" <mcgrof@...nel.org>
To:     Dave Airlie <airlied@...hat.com>
Cc:     torvalds@...ux-foundation.org, dan.j.williams@...el.com,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, Toshi Kani <toshi.kani@...com>,
        Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Brian Gerst <brgerst@...il.com>
Subject: Re: [PATCH 1/2] x86/io: add interface to reserve io memtype for a
 resource range. (v1.1)

On Mon, Oct 24, 2016 at 04:31:45PM +1000, Dave Airlie wrote:
> A recent change to the mm code in:
> 87744ab3832b83ba71b931f86f9cfdb000d07da5
> mm: fix cache mode tracking in vm_insert_mixed()
> 
> started enforcing checking the memory type against the registered list for
> amixed pfn insertion mappings. It happens that the drm drivers for a number
> of gpus relied on this being broken. Currently the driver only inserted
> VRAM mappings into the tracking table when they came from the kernel,
> and userspace mappings never landed in the table. This led to a regression
> where all the mapping end up as UC instead of WC now.

Eek.

> I've considered a number of solutions but since this needs to be fixed
> in fixes and not next, and some of the solutions were going to introduce
> overhead that hadn't been there before I didn't consider them viable at
> this stage. These mainly concerned hooking into the TTM io reserve APIs,
> but these API have a bunch of fast paths I didn't want to unwind to add
> this to.
> 
> The solution I've decided on is to add a new API like the arch_phys_wc
> APIs (these would have worked but wc_del didn't take a range), and
> use them from the drivers to add a WC compatible mapping to the table
> for all VRAM on those GPUs. This means we can then create userspace
> mapping that won't get degraded to UC.

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.

  Luis