linux-kernel - Re: [PATCH v2 6/7] x86, mm: Support huge I/O mappings on x86

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Date:	Wed, 18 Feb 2015 14:13:40 -0700
From:	Toshi Kani <toshi.kani@...com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	akpm@...ux-foundation.org, hpa@...or.com, tglx@...utronix.de,
	mingo@...hat.com, arnd@...db.de, linux-mm@...ck.org,
	x86@...nel.org, linux-kernel@...r.kernel.org, Elliott@...com
Subject: Re: [PATCH v2 6/7] x86, mm: Support huge I/O mappings on x86

On Wed, 2015-02-18 at 21:44 +0100, Ingo Molnar wrote:
> * Toshi Kani <toshi.kani@...com> wrote:
> 
> > This patch implements huge I/O mapping capability interfaces on x86.
> 
> > +#ifdef CONFIG_HUGE_IOMAP
> > +#ifdef CONFIG_X86_64
> > +#define IOREMAP_MAX_ORDER       (PUD_SHIFT)
> > +#else
> > +#define IOREMAP_MAX_ORDER       (PMD_SHIFT)
> > +#endif
> > +#endif  /* CONFIG_HUGE_IOMAP */
> 
> > +#ifdef CONFIG_HUGE_IOMAP
> 
> Hm, so why is there a Kconfig option for this? It just 
> complicates things.
> 
> For example the kernel already defaults to mapping itself 
> with as large mappings as possible, without a Kconfig entry 
> for it. There's no reason to make this configurable - and 
> quite a bit of complexity in the patches comes from this 
> configurability.

This Kconfig option was added to disable this feature in case there is
an issue.  That said, since the patchset also added a new nohugeiomap
boot option for the same purpose, I agree that this Kconfig option can
be removed.  So, I will remove it in the next version.

An example of such case is with multiple MTRRs described in patch 0/7.
However, I believe it is very unlikely to have such platform/use-case,
and it can also be avoided by a driver creating a separate mapping for
each MTRR range. 

Thanks,
-Toshi

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