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From:   Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Tue, 12 Nov 2019 15:26:35 +0100
Message-ID: <CAKMK7uFaA607rOS6x_FWjXQ2+Qdm8dQ1dQ+Oi-9if_Qh_wHWPg@mail.gmail.com>
Subject: Re: [PATCH] video: fbdev: atyfb: only use ioremap_uc() on i386 and ia64
To:     Christoph Hellwig <hch@lst.de>, Tuowen Zhao <ztuowen@gmail.com>,
        AceLan Kao <acelan.kao@canonical.com>,
        Mika Westerberg <mika.westerberg@linux.intel.com>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Roman Gilg <subdiff@gmail.com>,
        Lee Jones <lee.jones@linaro.org>,
        "Luis R. Rodriguez" <mcgrof@suse.com>
Cc:     Arnd Bergmann <arnd@arndb.de>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>,
        X86 ML <x86@kernel.org>, Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>, linux-ia64@vger.kernel.org,
        Tony Luck <tony.luck@intel.com>,
        Fenghua Yu <fenghua.yu@intel.com>,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Souptick Joarder <jrdr.linux@gmail.com>,
        dri-devel <dri-devel@lists.freedesktop.org>,
        Linux Fbdev development list <linux-fbdev@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Luis Chamberlain <mcgrof@kernel.org>
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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.

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