Re: [PATCH] mfd: intel-lpss: use devm_ioremap_uc for mmio

[Luis Chamberlain]

On Mon, Sep 30, 2019 at 02:05:22PM +0300, Andy Shevchenko wrote:
> On Fri, Sep 27, 2019 at 11:55:13AM -0600, Tuowen Zhao wrote:
> > Write-combining BAR for intel-lpss-pci in MTRR causes system hangs
> > during boot.
> > 
> > This patch adds devm_ioremap_uc as a new managed wrapper to ioremap_uc
> > and with it forces the use of strongly uncachable mmio in intel-lpss.
> > 
> > This bahavior is seen on Dell XPS 13 7390 2-in-1:
> > 
> > [    0.001734]   5 base 4000000000 mask 6000000000 write-combining
> > 
> > 4000000000-7fffffffff : PCI Bus 0000:00
> >   4000000000-400fffffff : 0000:00:02.0 (i915)
> >   4010000000-4010000fff : 0000:00:15.0 (intel-lpss-pci)
> 
> +Cc: Luis as author of UC flavour of ioremap.
> 
> Luis, some BIOSes in the wild have wrong MTRR setting for PCI resource window
> and thus when Linux tries to allocate 64-bit MMIO address space (and in
> opposite to Windows, which does this from the end of available space towards
> beginning, Linux do this from the beginning towards end). Ideally we have to
> push vendors to fix firmware.
> 
> This patch AFAIU overrides MTTR/PAT settings for those pages and makes it
> possible to workaround firmware bug.
> 
> What do you think is the best approach here?

Indeed, such cases can come up, and yes _uc can be a workaround for such
cases.

> > +EXPORT_SYMBOL(devm_ioremap_uc);

EXPORT_SYMBOL_GPL() would be my preference. But other than that, this
makes sense.

  Luis