linux-kernel - Re: [PATCH 3/8] regmap: Add SPI bus support

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Date:	Tue, 21 Jun 2011 02:00:07 +0200
From:	Lars-Peter Clausen <lars@...afoo.de>
To:	Mark Brown <broonie@...nsource.wolfsonmicro.com>
CC:	linux-kernel@...r.kernel.org,
	Dimitris Papastamos <dp@...nsource.wolfsonmicro.com>,
	Liam Girdwood <lrg@...com>,
	Samuel Oritz <sameo@...ux.intel.com>,
	Graeme Gregory <gg@...mlogic.co.uk>
Subject: Re: [PATCH 3/8] regmap: Add SPI bus support

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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