Performance regression in ast drm driver

[David Airlie]

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.

Have we seen the problem across a number of AST devices?

Dave.