linux-kernel - Re: [RFC] ARM DMA mapping TODO, v1

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Message-ID: <BANLkTimJoc_DR3bjxehDFcTM13CavAX_=A@mail.gmail.com>
Date:	Fri, 29 Apr 2011 17:42:59 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	"linaro-mm-sig@...ts.linaro.org" <linaro-mm-sig@...ts.linaro.org>,
	Catalin Marinas <catalin.marinas@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [RFC] ARM DMA mapping TODO, v1

On Friday, 29 April 2011, Arnd Bergmann <arnd@...db.de> wrote:
> On Wednesday 27 April 2011, Catalin Marinas wrote:
>> > >
>> > > It's not broken since we moved to using Normal non-cacheable memory
>> > > for the coherent DMA buffers (as long as you flush the cacheable alias
>> > > before using the buffer, as we already do). The ARM ARM currently says
>> > > unpredictable for such situations but this is being clarified in
>> > > future updates and the Normal non-cacheable vs cacheable aliases can
>> > > be used (given correct cache maintenance before using the buffer).
>> >
>> > Thanks for that information, I believe a number of people in the
>> > previous discussions were relying on the information from the
>> > documentation. Are you sure that this is not only correct for the
>> > cores made by ARM ltd but also for the other implementations that
>> > may have relied on documentation?
>>
>> It is a clarification in the ARM ARM so it covers all the cores made by
>> architecture licensees, not just ARM Ltd. It basically makes the
>> "unpredictable" part more predictable to allow certain types of aliases
>> (e.g. Strongly Ordered vs Normal memory would still be disallowed).
>>
>> All the current implementations are safe with Normal memory aliases
>> (cacheable vs non-cacheable) but of course, there may be some
>> performance benefits in not having any alias.
>
> A lot of the discussions we are about to have in Budapest will be
> around solving the problem of having only valid combinations of
> mappings, so we really need to have a clear statement in specification
> form about what is actually valid.
>
> Would it be possible to have an updated version of the relevant
> section of the ARM ARM by next week so we can use that as the
> base for our discussions?

I'll ask the architecture people here in ARM and get back to you
(there is holiday until Tuesday next week in the UK).

-- 
Catalin
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