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

On Fri, Mar 20, 2015 at 4:17 PM, Luis R. Rodriguez
[off-list ref] wrote:

From: "Luis R. Rodriguez" <redacted>

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