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

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Message-ID: <1274274023.6930.10047.camel@macbook.infradead.org>
Date:	Wed, 19 May 2010 14:00:23 +0100
From:	David Woodhouse <dwmw2@...radead.org>
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

On Tue, 2010-05-11 at 16:54 -0400, Mike Frysinger wrote:
> 
> > 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 ? 

Not necessarily. If you have cache-coherent DMA, then there's no need to
align things. You only need cache-alignment when you have
cache-incoherent DMA and have to manage the cache in software. And in
that case, the architecture must set ARCH_KMALLOC_MINALIGN to an
appropriate value so that kmalloc() meets the guarantee.

However, your problem is kind of unrelated to that -- your problem is
actually that you're putting both a DMA buffer _and_ other stuff into
the same kmalloc'd buffer. Don't do that. Instead, allocate your DMA
buffer separately and put a _pointer_ to it into the structure.

Or if you really _must_ include it in your structure, then align to
ARCH_KMALLOC_MINALIGN bytes both before and after the DMA buffer by
adding __attribute__((__aligned__(ARCH_KMALLOC_MINALIGN))) to both the
DMA buffer _and_ the element which follows it in your struct (if any).

Using ____cacheline_aligned wastes a lot of space on the architectures
that don't need it.

Arguably, this __dma_aligned definition should go somewhere generic...

diff --git a/drivers/input/touchscreen/ad7877.c b/drivers/input/touchscreen/ad7877.c
index 0d2d7e5..ae5d56e 100644
--- a/drivers/input/touchscreen/ad7877.c
+++ b/drivers/input/touchscreen/ad7877.c
@@ -151,6 +151,11 @@ enum {
 /*
  * Non-touchscreen sensors only use single-ended conversions.
  */
+#ifdef ARCH_KMALLOC_MINALIGN
+#define __dma_aligned __attribute__((__aligned__(ARCH_KMALLOC_MINALIGN)))
+#else
+#define __dma_aligned
+#endif
 
 struct ser_req {
 	u16			reset;
@@ -163,7 +168,7 @@ struct ser_req {
 	 * DMA (thus cache coherency maintenance) requires the
 	 * transfer buffers to live in their own cache lines.
 	 */
-	u16 sample ____cacheline_aligned;
+	u16 sample __dma_aligned;
 };
 
 struct ad7877 {
@@ -203,7 +208,7 @@ struct ad7877 {
 	 * DMA (thus cache coherency maintenance) requires the
 	 * transfer buffers to live in their own cache lines.
 	 */
-	u16 conversion_data[AD7877_NR_SENSE] ____cacheline_aligned;
+	u16 conversion_data[AD7877_NR_SENSE] __dma_aligned;
 };
 
 static int gpio3;


-- 
David Woodhouse                            Open Source Technology Centre
David.Woodhouse@...el.com                              Intel Corporation

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