Message-ID: <1424294020.17007.21.camel@misato.fc.hp.com>
Subject: Re: [PATCH v2 6/7] x86, mm: Support huge I/O mappings on x86
From: Toshi Kani <toshi.kani@hp.com>
To: Ingo Molnar <mingo@kernel.org>
Cc: akpm@linux-foundation.org, hpa@zytor.com, tglx@linutronix.de,
        mingo@redhat.com, arnd@arndb.de, linux-mm@kvack.org, x86@kernel.org,
        linux-kernel@vger.kernel.org, Elliott@hp.com
Date: Wed, 18 Feb 2015 14:13:40 -0700
In-Reply-To: <20150218204414.GA20943@gmail.com>
References: <1423521935-17454-1-git-send-email-toshi.kani@hp.com>
	 <1423521935-17454-7-git-send-email-toshi.kani@hp.com>
	 <20150218204414.GA20943@gmail.com>
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On Wed, 2015-02-18 at 21:44 +0100, Ingo Molnar wrote:
> * Toshi Kani <toshi.kani@hp.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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