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Subject: Re: generic_buffer and 24 bits samples
To: Gregor Boirie <gregor.boirie@parrot.com>,
	"linux-iio@vger.kernel.org" <linux-iio@vger.kernel.org>
References: <570FB712.5070903@parrot.com>
From: Lars-Peter Clausen <lars@metafoo.de>
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Date: Thu, 14 Apr 2016 18:00:07 +0200
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On 04/14/2016 05:28 PM, Gregor Boirie wrote:
> Greetings,
> 
> I'm trying to use an st_pressure based sensor to sample pressure
> data using generic_buffer tool. However it seems that it does not
> support data packed onto 24 bits samples.
> st_pressure driver defines scan_type for my device as:
> <code>
> /* ... */
>     .scan_type = {
>         .sign = 'u',
>         .realbits = 24,
>         .storagebits = 24,
>         .endianness = IIO_LE,
>     },
> </code>
> 
> What is the proper way to make this work ? Using 32 bits storagebits
> field ? Enhance generic_buffer to support 24 bits samples ? Anything
> else ??
> 
> It seems iio_compute_scan_bytes consider sample data as a simple byte
> stream. So I'm wondering what are the alignment constraints for sample
> start address ? Should they be aligned onto their natural "word"
> boundaries, i.e. 16 bits for u16, 32 bits for u32, etc... ? And for
> 24 bits samples ?

IIO really only supports powers of two >= 8 for the storagebits size.
Everything else is undefined.

Samples are aligned to their storage size.