linux-kernel - Re: [PATCH v3] ad7877: keep dma rx buffers in seperate cache lines

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Message-ID: <AANLkTimzxrZyQAfocTwIt5zCVyXP7bReKOvP4K7m8epO@mail.gmail.com>
Date:	Tue, 11 May 2010 16:54:41 -0400
From:	Mike Frysinger <vapier.adi@...il.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Pekka Enberg <penberg@...helsinki.fi>,
	Matt Mackall <mpm@...enic.com>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Oskar Schirmer <os@...ix.com>,
	Michael Hennerich <Michael.Hennerich@...log.com>,
	linux-input@...r.kernel.org, linux-kernel@...r.kernel.org,
	Daniel Glöckner <dg@...ix.com>,
	Oliver Schneidewind <osw@...ix.com>,
	Johannes Weiner <jw@...ix.com>, Nick Piggin <npiggin@...e.de>,
	David Rientjes <rientjes@...gle.com>,
	David Brownell <dbrownell@...rs.sourceforge.net>,
	Grant Likely <grant.likely@...retlab.ca>
Subject: Re: [PATCH v3] ad7877: keep dma rx buffers in seperate cache lines

On Tue, May 11, 2010 at 16:46, Christoph Lameter wrote:
> On Tue, 11 May 2010, Mike Frysinger wrote:
>> > DMA. If the arch can only DMA into cacheline aligned objects then the
>> > correct method is to force kmalloc alignment to cacheline size.
>>
>> these are SPI drivers and are usable on any arch that supports a SPI
>> bus (which is pretty much every arch).  forget about "embedded"
>> arches.
>>
>> the issue here is simple: a SPI driver (AD7877) needs to do a receive
>> SPI transfer into a DMA safe buffer.  what is the exact API to
>> dynamically allocate memory for the structure with this buffer
>> embedded in it such that the start of the structure is cached aligned
>> ?  creating a dedicated kmem cache may work, but it isnt a scalable
>> solution if every SPI driver needs to create its own cache.
>
> kmalloc returns a pointer to a DMA safe buffer. There is no requirement on
> the x86 hardware that the DMA buffers have to be cache aligned. Cachelines
> will be invalidated as needed.

so this guarantee is made by the kmalloc() API ?  and for arches where
the cacheline invalidation is handled in software rather than
hardware, they must declare a min alignment value for kmalloc to be at
least as big as their cache alignment ?

does the phrase "DMA safe buffer" imply cache alignment ?
-mike
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