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

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Message-ID: <CAL_JsqJwKQ_KuWWQNZHrMh8NwwiubB2jR+g-Y2n_dH4kJ5zVow@mail.gmail.com>
Date:	Thu, 17 Apr 2014 13:27:46 -0500
From:	Rob Herring <robherring2@...il.com>
To:	Arnd Bergmann <arnd@...db.de>, Alan Cox <alan@...rguk.ukuu.org.uk>
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 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"? Why are there 2 flavors if they are
always the same.


>> 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.

At some point it was believed to be needed on the 8250 driver. Perhaps
Alan can comment since the commit message tells us nothing:

commit 6f441fe99814f64315b8c11890744230b990c460
Author: Alan Cox <alan@...rguk.ukuu.org.uk>
Date:   Thu May 1 04:34:59 2008 -0700

    8250: switch 8250 drivers to use _nocache ioremaps

    Signed-off-by: Alan Cox <alan@...hat.com>
    Cc: Russell King <rmk@....linux.org.uk>
    Cc: Ingo Molnar <mingo@...e.hu>
    Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
    Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>

Rob
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