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

[Luis Chamberlain <mcgrof@kernel.org>]

On Tue, Nov 12, 2019 at 03:26:35PM +0100, Daniel Vetter wrote:
> On Tue, Nov 12, 2019 at 3:06 PM Christoph Hellwig <hch@lst.de> 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.

I had originally suggested to just make the driver build on x86, but an
atlternative was to provide the call for the missing architecture.

> 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).

Right, the goal was to not call MTRR directly.

> Adding everyone from that commit, plus Luis. Drivers really shouldn't
> assume/work around the bios setting up superflous/wrong MTRR.

Such things are needed, otherwise some systems may not boot...

> > With that we could kill ioremap_uc entirely.
> 
> So yeah removing that seems definitely like the right thing.

I think this would be possible if we could flop ioremap_nocache() to UC
instead of UC- on x86. Otherwise, I can't see how we can remove this by
still not allowing direct MTRR calls.

  Luis