linux-kernel - Re: [PATCH v2 28/76] ARC: I/O and DMA Mappings

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Date:	Fri, 18 Jan 2013 16:18:11 +0000
From:	Arnd Bergmann <arnd@...db.de>
To:	Paul Mundt <lethal@...ux-sh.org>
Cc:	Vineet Gupta <Vineet.Gupta1@...opsys.com>,
	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 28/76] ARC: I/O and DMA Mappings

On Friday 18 January 2013, Paul Mundt wrote:
> On Fri, Jan 18, 2013 at 03:55:43PM +0000, Arnd Bergmann wrote:
> > On Friday 18 January 2013, Vineet Gupta wrote:
> > > TBD: Do we need early ioremap support like openrisc
> > > 
> > > Signed-off-by: Vineet Gupta <vgupta@...opsys.com>
> > 
> > Can you explain why early ioremap is necessary for you?
> > 
> > I wasn't aware that openrisc has it. In a lot of cases,
> > you can use fixmap as a replacement.
> > 
> The two don't need to be mutually exclusive, either. sh implements early
> ioremap transparently via fixmaps for example:
> 
>         arch/sh/mm/ioremap.c
>         arch/sh/mm/ioremap_fixed.c

Ah, right. It also seems that openrisc uses the same approach
for its early ioremap.

I usually recommend doing as few things as possible "early"
though, that's why I was asking. If it's only one or two drivers
that need an early mapping and can't be moved to a later stage
of bootup, using only hardcoded fixmaps would make it harder
to add more users, which one might consider a good thing.

Allowing ioremap to be used before memory_init() adds flexibility
but could lead to code being moved into an early stage by accident
rather than by necessity.

	Arnd
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