Performance regression in ast drm driver

[Jean Delvare]

Hi David,

On Fri, 2018-11-09 at 10:04 +1000, David Airlie wrote:
> On Thu, Nov 8, 2018 at 10:05 PM Jean Delvare <jdelvare at suse.de> wrote:
> > 
> > On Thu, 1 Nov 2018 16:27:07 +0100, Jean Delvare wrote:
> > > Hi David,
> > > 
> > > The following commit:
> > > 
> > > commit 7cf321d118a825c1541b43ca45294126fd474efa
> > > Author: Dave Airlie
> > > Date:   Mon Oct 24 15:37:48 2016 +1000
> > > 
> > >     drm/drivers: add support for using the arch wc mapping API.
> > > 
> > > is causing a huge performance regression for the ast drm driver. In a
> > > text console, if I call "cat" on a large text file, it takes almost
> > > twice as much time to be displayed and scrolled completely.
> > > 
> > > Can you please check that the ast driver portion of that commit is both
> > > correct and complete?
> > 
> > And in the meantime, what bad will happen if we just revert the ast
> > portion of that commit?
> 
> This seems likely to be a hw problem with PCI writes to the AST "GPU",
> since it's just some sort of RAM + ARM on the end of a PCIE bus, we've
> definitely seen possible issues in the past with write combining
> around some of the mga GPUs with some CPUs.

Takashi asked me to compare the contents of
/sys/kernel/debug/x86/pat_memtype_list before and after the commit
above. Before the commit, we have:

uncached-minus @ 0xfafe0000-0xfb000000
uncached-minus @ 0xfb000000-0xfb005000
write-combining @ 0xfb005000-0xfb584000

After the commit, we have:

uncached-minus @ 0xfafe0000-0xfb000000
uncached-minus @ 0xfb000000-0xfb005000
uncached-minus @ 0xfb000000-0xfb800000
uncached-minus @ 0xfb005000-0xfb584000

The corresponding lines in /proc/iomem are:

f0000000-fed8ffff : PCI Bus 0000:00
  faf00000-fb7fffff : PCI Bus 0000:01
    fafe0000-faffffff : 0000:01:01.0
    fb000000-fb7fffff : 0000:01:01.0

Does it help? Is the change of type expected? Is it not a problem that
one of the ranges is overlapping with 2 other ranges?

-- 
Jean Delvare
SUSE L3 Support