linux-kernel - Re: [PATCH v1 06/47] mtrr: add __arch_phys_wc

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Date:	Fri, 20 Mar 2015 16:48:46 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"Luis R. Rodriguez" <mcgrof@...not-panic.com>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, Juergen Gross <jgross@...e.com>,
	Jan Beulich <JBeulich@...e.com>, Borislav Petkov <bp@...e.de>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	venkatesh.pallipadi@...el.com, Dave Airlie <airlied@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linux Fbdev development list <linux-fbdev@...r.kernel.org>,
	X86 ML <x86@...nel.org>,
	"xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
	"Luis R. Rodriguez" <mcgrof@...e.com>, Ingo Molnar <mingo@...e.hu>,
	Daniel Vetter <daniel.vetter@...ll.ch>,
	Antonino Daplas <adaplas@...il.com>,
	Jean-Christophe Plagniol-Villard <plagnioj@...osoft.com>,
	Tomi Valkeinen <tomi.valkeinen@...com>
Subject: Re: [PATCH v1 06/47] mtrr: add __arch_phys_wc_add()

On Fri, Mar 20, 2015 at 4:17 PM, Luis R. Rodriguez
<mcgrof@...not-panic.com> wrote:
> From: "Luis R. Rodriguez" <mcgrof@...e.com>
>
> Ideally on systems using PAT we can expect a swift
> transition away from MTRR. There can be a few exceptions
> to this, one is where device drivers are known to exist
> on PATs with errata, another situation is observed on
> old device drivers where devices had combined MMIO
> register access with whatever area they typically
> later wanted to end up using MTRR for on the same
> PCI BAR. This situation can still be addressed by
> splitting up ioremap'd PCI BAR into two ioremap'd
> calls, one for MMIO registers, and another for whatever
> is desirable for write-combining -- in order to
> accomplish this though quite a bit of driver
> restructuring is required.
>
> Device drivers which are known to require large
> amount of re-work in order to split ioremap'd areas
> can use __arch_phys_wc_add() to avoid regressions
> when PAT is enabled.
>
> For a good example driver where things are neatly
> split up on a PCI BAR refer the infiniband qib
> driver. For a good example of a driver where good
> amount of work is required refer to the infiniband
> ipath driver.
>
> This is *only* a transitive API -- and as such no new
> drivers are ever expected to use this.

What's the exact layout that this helps?  I'm sceptical that this can
ever be correct.

Is there some awful driver that has a large ioremap that's supposed to
contain multiple different memtypes?  If so, can we ioremap +
set_page_xyz instead?

--Andy
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