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

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Message-Id: <201404191332.24392.arnd@arndb.de>
Date:	Sat, 19 Apr 2014 13:32:24 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Rob Herring <robherring2@...il.com>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>,
	"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 Thursday 17 April 2014, Rob Herring wrote:
> On Mon, Mar 24, 2014 at 10:42 AM, Arnd Bergmann <arnd@...db.de> wrote:
> > 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.
> 
> Did you mean "not the same"?

No

> Why are there 2 flavors if they are
> always the same.

I assume it's just a mistake someone made a long time ago and nobody
ever changed it because they didn't want to break obscure code.

x86 seems to be a bit odd here, because is has both page-table
and MTRR method of setting up whether an access can be cached or not.
This may have led to the introduction of the _nocache variant, when
the regular ioremap was already uncached on all other architectures
but cached on x86.

	Arnd
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