linux-kernel - Re: [PATCH 4/8] tty/serial: add generic serial earlycon

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Date:	Mon, 24 Mar 2014 16:42:02 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	Rob Herring <robherring2@...il.com>
Cc:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Russell King <linux@....linux.org.uk>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"x86@...nel.org" <x86@...nel.org>,
	Will Deacon <Will.Deacon@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
	Jiri Slaby <jslaby@...e.cz>
Subject: Re: [PATCH 4/8] tty/serial: add generic serial earlycon

On Monday 24 March 2014 08:36:46 Rob Herring wrote:
> On Mon, Mar 24, 2014 at 6:29 AM, Arnd Bergmann <arnd@...db.de> wrote:
> > On Monday 24 March 2014 11:22:03 Catalin Marinas wrote:
> >> On Fri, Mar 21, 2014 at 09:08:44PM +0000, Rob Herring wrote:
> >> > diff --git a/drivers/tty/serial/earlycon.c b/drivers/tty/serial/earlycon.c
> >> > new file mode 100644
> >> > index 0000000..241757a
> >> > --- /dev/null
> >> > +++ b/drivers/tty/serial/earlycon.c
> >> [...]
> >> > +static void __iomem * __init earlycon_map(unsigned long paddr, size_t size)
> >> > +{
> >> > +   void __iomem *base;
> >> > +#ifdef CONFIG_FIX_EARLYCON_MEM
> >> > +   set_fixmap_nocache(FIX_EARLYCON_MEM_BASE, paddr & PAGE_MASK);
> >> > +   base = (void __iomem *)__fix_to_virt(FIX_EARLYCON_MEM_BASE);
> >> > +   base += paddr & ~PAGE_MASK;
> >> > +#else
> >> > +   base = ioremap_nocache(paddr, size);
> >> > +#endif
> >>
> >> Just curious why not set_fixmap_io (and plain ioremap)?
> >
> > Good point. Note that ioremap_nocache() is the same as ioremap()
> > on *all* architectures.
> 
> I investigated this before adding this to arm64. set_fixmap_io and
> set_fixmap_nocache are not the same settings on x86. Whether the
> mapping type really matters on x86 or not, I don't know. So I added
> the nocache variant to arm64 to avoid a change to x86.

My best guess is that it's an x86 bug. ioremap() always uses an uncached
mapping on x86, so it's strange to see early_ioremap() and set_fixmap_io()
use a cached mapping. It probably doesn't matter as long as the mtrr is
set up to treat all MMIO registers as non-cacheable, but I think there
should not be a difference.

	Arnd
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