Re: generic_buffer and 24 bits samples

[Lars-Peter Clausen <lars@metafoo.de>]

On 04/15/2016 11:56 AM, Crestez Dan Leonard wrote:
> 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?

It does not 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.

Yes, sounds reasonable. Do you want to send a patch?

Thanks,
- Lars