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

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Date:	Tue, 11 May 2010 23:37:42 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Mike Frysinger <vapier.adi@...il.com>
Cc:	Christoph Lameter <cl@...ux.com>,
	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

> 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.

If you are embedding structures then one solution is to cheat a bit and
make the structure, compiler and existing kernel compile abuse do the
work. Something like

struct {
	void *except_where_prohibited;
	long boat;
	unsigned photograph;

	u8 pad[0] __cacheline_aligned;
}..

ought to do the trick providing you align the start of the object - which
should happen naturally with kmalloc or dma_* apis


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