Message-ID: <4DFFDF07.2010200@metafoo.de>
Date: Tue, 21 Jun 2011 02:00:07 +0200
From: Lars-Peter Clausen <lars@metafoo.de>
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To: Mark Brown <broonie@opensource.wolfsonmicro.com>
CC: linux-kernel@vger.kernel.org,
        Dimitris Papastamos <dp@opensource.wolfsonmicro.com>,
        Liam Girdwood <lrg@ti.com>, Samuel Oritz <sameo@linux.intel.com>,
        Graeme Gregory <gg@slimlogic.co.uk>
Subject: Re: [PATCH 3/8] regmap: Add SPI bus support
References: <20110620124608.GB31140@opensource.wolfsonmicro.com> <1308574489-31322-1-git-send-email-broonie@opensource.wolfsonmicro.com> <1308574489-31322-3-git-send-email-broonie@opensource.wolfsonmicro.com> <4DFFD738.4090206@metafoo.de> <20110620234513.GC1905@opensource.wolfsonmicro.com>
In-Reply-To: <20110620234513.GC1905@opensource.wolfsonmicro.com>
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On 06/21/2011 01:45 AM, Mark Brown wrote:
> On Tue, Jun 21, 2011 at 01:26:48AM +0200, Lars-Peter Clausen wrote:
>> On 06/20/2011 02:54 PM, Mark Brown wrote:
> 
>>> +static int regmap_spi_read(struct device *dev,
>>> +			   const void *reg, size_t reg_size,
>>> +			   void *val, size_t val_size)
>>> +{
>>> +	struct spi_device *spi = to_spi_device(dev);
> 
>>> +	return spi_write_then_read(spi, reg, reg_size, val, val_size);
> 
>> spi_write_then_read will use a bounce buffer internally, since we already have
>> our own bounce buffer it is probably better to use the low-level spi interface
>> directly in this case.
> 
> I've got this horrible feeling that if we try that we'll discover that
> the reason the SPI API does this internally is just as valid here - if I
> remember correctly it's doing this due to restrictions on DMA from the
> stack and I'd strongly expect val to end up on the stack for registers.
> Or to look at it from the other point of view if we don't need the
> bounce buffers then why does spi_write_then_read() need them?

hm, right, I overlooked that val could be on the stack. I though we were always
using some kind of bounce buffer internally.
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