Re: [Xen-devel] [PATCH v1 06/47] mtrr: add __arch_phys_wc_add()

["Luis R. Rodriguez" <mcgrof@xxxxxxxx>]

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@xxxxxxxx> 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@xxxxxxxxxxxxxxxx> wrote:
> >> > From: "Luis R. Rodriguez" <mcgrof@xxxxxxxx>
> >> >
> >> > 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?

  Luis

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