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

From: Luis Chamberlain <mcgrof@kernel.org>
To: Daniel Vetter <daniel.vetter@ffwll.ch>, Juergen Gross <jgross@suse.com>
Cc: Linux Fbdev development list <linux-fbdev@vger.kernel.org>,
	linux-ia64@vger.kernel.org,
	dri-devel <dri-devel@lists.freedesktop.org>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Lee Jones <lee.jones@linaro.org>, Christoph Hellwig <hch@lst.de>,
	X86 ML <x86@kernel.org>, Ingo Molnar <mingo@redhat.com>,
	Tuowen Zhao <ztuowen@gmail.com>,
	Fenghua Yu <fenghua.yu@intel.com>, Arnd Bergmann <arnd@arndb.de>,
	Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>,
	"Luis R. Rodriguez" <mcgrof@suse.com>,
	Borislav Petkov <bp@alien8.de>,
	Thomas Gleixner <tglx@linutronix.de>,
	Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
	Mika Westerberg <mika.westerberg@linux.intel.com>,
	Tony Luck <tony.luck@intel.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	AceLan Kao <acelan.kao@canonical.com>,
	Souptick Joarder <jrdr.linux@gmail.com>,
	Roman Gilg <subdiff@gmail.com>
Subject: Re: [PATCH] video: fbdev: atyfb: only use ioremap_uc() on i386 and ia64
Date: Tue, 12 Nov 2019 22:24:23 +0000	[thread overview]
Message-ID: <20191112222423.GO11244@42.do-not-panic.com> (raw)
In-Reply-To: <CAKMK7uFaA607rOS6x_FWjXQ2+Qdm8dQ1dQ+Oi-9if_Qh_wHWPg@mail.gmail.com>

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