[alsa-devel] [PATCH 1/2] ASoC: Remove byte swap in 4x12 SPI write

Mark Brown broonie at opensource.wolfsonmicro.com
Wed May 11 19:48:37 CEST 2011

On Wed, May 11, 2011 at 10:35:37AM -0700, Lars-Peter Clausen wrote:

> I've probably not properly explained the problem in my previous patch.
> endian.

This is totally unrelated to your previous patch (as is the last one),
I'm doing some different work on the physical I/O code and frankly
hadn't thought you had any patches you hadn't resent and got applied

> The problem is due to how 16-bit SPI transfers work on linux and how
> snd_soc_cache stores the data which is going to be send.

We shouldn't be using 16 bit transfers at all, that's insane.  If the
board has gone and done that that's silly, and if drivers are relying
on the board having done that then it's at best incredibly fragile.
There certainly aren't any which check the word size.

> ASoC though uses a u8 array. So when this array is interpreted as a u16 array
> instead the values will be in big-endian. As a result using 16-bit spi
> transfers on a little-endian machine will send out the data byte-swapped.
> Your patch does not change since.

Right, if the SPI controllers are being set up with anything other than
8 bit word sizes in transfers this is going to fall over.  However,
since we can't rely on SPI controllers supporting anything other than 8
bit transfers and we don't currently have a sensible way of negotiating
the word size with the controller driver anyway I'd really expect us to
only ever use 8 bit words without a bunch more infrastructure work.

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