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Date: Tue, 11 May 2010 15:46:59 -0500 (CDT)
From: Christoph Lameter <cl@linux.com>
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To: Mike Frysinger <vapier.adi@gmail.com>
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       Grant Likely <grant.likely@secretlab.ca>
Subject: Re: [PATCH v3] ad7877: keep dma rx buffers in seperate cache lines
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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.