Re: [PATCH] video: fbdev: atyfb: only use ioremap_uc() on i386 and ia64

[Daniel Vetter]

On Tue, Nov 12, 2019 at 3:06 PM Christoph Hellwig <hch@xxxxxx> wrote:
> On Tue, Nov 12, 2019 at 02:04:16PM +0100, Daniel Vetter wrote:
> > Wut ... Maybe I'm missing something, but from how we use mtrr in other
> > gpu drivers it's a) either you use MTRR because that's all you got or
> > b) you use pat. Mixing both sounds like a pretty bad idea, since if
> > you need MTRR for performance (because you dont have PAT) then you
> > can't fix the wc with the PAT-based ioremap_uc. And if you have PAT,
> > then you don't really need an MTRR to get wc.
> >
> > So I'd revert this patch from Luis and ...
>
> Sounds great to me..
>
> > ... apply this one. Since the same reasoning should apply to anything
> > that's running on any cpu with PAT.
>
> Can you take a look at "mfd: intel-lpss: Use devm_ioremap_uc for MMIO"
> in linux-next, which also looks rather fishy to me?  Can't we use
> the MTRR APIs to override the broken BIOS MTRR setup there as well?

Hm so that's way out of my knowledge, but I think mtrr_cleanup() was
supposed to fix up messy/broken MTRR setups by the bios. So maybe they
simply didn't enable that in their .config with CONFIG_MTRR_SANITIZER.

An explicit cleanup is currently not possible for drivers, since the
only interface exported to drivers is arch_phys_wc_add/del (which
short-circuits if pat works since you don't need mtrr in that case).
Adding everyone from that commit, plus Luis. Drivers really shouldn't
assume/work around the bios setting up superflous/wrong MTRR.

> With that we could kill ioremap_uc entirely.

So yeah removing that seems definitely like the right thing.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch