Re: generic_buffer and 24 bits samples

[Crestez Dan Leonard <leonard.crestez@intel.com>]

On 04/14/2016 07:00 PM, Lars-Peter Clausen wrote:
> 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.

How does 24-bit alignment work?

Maybe the IIO core should check that storagebits is one of 8/16/32/64 
and otherwise fail at register time? A quick grep finds only 2 drivers 
using storagebits = 24 and they could easily be modified to 32.

--
Regards,
Leonard