linux-kernel - Re: [PATCH] staging iio: Replace kmalloc with local variable

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Message-ID: <20110607160206.6cb3168d@endymion.delvare>
Date:	Tue, 7 Jun 2011 16:02:06 +0200
From:	Jean Delvare <khali@...ux-fr.org>
To:	Ben Dooks <ben-i2c@...ff.org>
Cc:	Jonathan Cameron <jic23@....ac.uk>,
	anish <anish198519851985@...il.com>, gregkh@...e.de,
	manuel.stahl@....fraunhofer.de, lucas.demarchi@...fusion.mobi,
	devel@...verdev.osuosl.org, linux-kernel@...r.kernel.org,
	linux-iio@...r.kernel.org, Linux I2C <linux-i2c@...r.kernel.org>
Subject: Re: [PATCH] staging iio: Replace kmalloc with local variable

On Tue, 7 Jun 2011 14:41:35 +0100, Ben Dooks wrote:
> On Tue, Jun 07, 2011 at 02:00:23PM +0100, Jonathan Cameron wrote:
> > On 06/07/11 13:39, anish wrote:
> > > From: anish kumar <anish198519851985@...il.com>
> > > 
> > > replaced kmalloc with local variable as I2C(in this case) doesn't require
> > > kmalloc memory it can do with stack memory.
> > I've cc'd linux-i2c just to check I'm right about the whole i2c doesn't need
> > dma safe buffers bit...
> 
> No, it is down to the i2c driver, and from recollection dma from stack is
> not recommended, due to things like cache line alignment. Please do not
> do this.

To be clearer, it is down to the i2c BUS (adapter) driver, NOT the i2c
device driver. So the patch is correct.

Keep in mind that not all I2C/SMBus controllers care about DMA. In fact,
most don't (at least in the set I am maintaining - might be different
an embedded.) So calling kmalloc for every transfer in every I2C device
driver "just in case" is very much counterproductive.

And, if a controller does DMA and needs well-aligned, dynamically
allocated buffer, its driver would hopefully allocate the buffer ONCE
and keep it around, rather than reallocating it for every transfer.

-- 
Jean Delvare
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