linux-kernel - Re: [PATCH] video: fbdev: atyfb: only use ioremap

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Date:   Tue, 12 Nov 2019 15:06:31 +0100
From:   Christoph Hellwig <hch@....de>
To:     Daniel Vetter <daniel.vetter@...ll.ch>
Cc:     Christoph Hellwig <hch@....de>, Arnd Bergmann <arnd@...db.de>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        X86 ML <x86@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, linux-ia64@...r.kernel.org,
        Tony Luck <tony.luck@...el.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Souptick Joarder <jrdr.linux@...il.com>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Linux Fbdev development list <linux-fbdev@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Tuowen Zhao <ztuowen@...il.com>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Subject: Re: [PATCH] video: fbdev: atyfb: only use ioremap_uc() on i386 and
 ia64

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?

With that we could kill ioremap_uc entirely.