Message-ID: <CALCETrXVL=5O2_LwAgO5vFZRdEpR8WbyRE5-C-NBEER4ZQrE8Q@mail.gmail.com>
Date:	Fri, 27 Mar 2015 14:23:16 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"Luis R. Rodriguez" <mcgrof@...e.com>
Cc:	Bjorn Helgaas <bhelgaas@...gle.com>,
	Ville Syrjälä <syrjala@....fi>,
	Mauro Carvalho Chehab <mchehab@....samsung.com>,
	Mike Marciniszyn <infinipath@...el.com>,
	"Luis R. Rodriguez" <mcgrof@...not-panic.com>,
	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>,
	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 27, 2015 at 1:30 PM, Luis R. Rodriguez <mcgrof@...e.com> wrote:
> On Fri, Mar 27, 2015 at 12:58:02PM -0700, Andy Lutomirski wrote:
>> On Fri, Mar 27, 2015 at 12:53 PM, Luis R. Rodriguez <mcgrof@...e.com> wrote:
>> > On Fri, Mar 20, 2015 at 04:48:46PM -0700, Andy Lutomirski wrote:
>> >> 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?
>> >
>> > Yes, I cc'd you just now on one where I made changes on a driver which uses one
>> > PCI with mixed memtypes and uses MTRR to hole in WC. A transition to
>> > arch_phys_wc_add() is therefore not possible if PAT is enabled as it would
>> > regress those drivers by making the MTRR WC hole trick non functional.
>> > The changes are non trivial and so in this series I supplied changes on
>> > one driver only to show the effort required. The other drivers which
>> > required this were:
>> >
>> > Driver          File
>> > ------------------------------------------------------------
>> > fusion          drivers/message/fusion/mptbase.c
>> > ivtv            drivers/media/pci/ivtv/ivtvfb.c
>> > ipath           drivers/infiniband/hw/ipath/ipath_driver.c
>> >
>> > This series makes those drivers use __arch_phys_wc_add() more as a
>> > transitory phase in hopes we can address the proper split as with the
>> > atyfb illustrates. For ipath the changes required have a nice template
>> > with the qib driver as they share very similar driver structure, the
>> > qib driver *did* do the nice split.
>> >
>> >> If so, can we ioremap + set_page_xyz instead?
>> >
>> > I'm not sure I see which call we'd use.  Care to provide an example patch
>> > alternative for the atyfb as a case in point alternative to the work required
>> > to do the split?
>> >
>>
>> I'm still confused.  Would it be insufficient to ioremap_nocache the
>> whole thing and then call set_memory_wc on parts of it?  (Sorry,
>> set_page_xyz was a typo.)
>
> I think that would be a sexy alternative.
>
> In this driver's case the thing is a bit messy as it not only used
> the WC MTRR for a hole but it also then used a UC MTRR on top of
> it all, so since I already tried to address the split, and if we address
> the power of 2 woes, I think it'd be best to try to remove the UC MTRR
> and just avoid set_page_wc() in this driver's case, but for the other cases
> (fusion, ivtv, ipath) I think this makes sense.
>
> Thoughts?

Once that WC MTRR is in place, I think you really need UC and not UC-
if you want to override it.  Otherwise I agree with all of this.

--Andy
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