Re: [RFC 0/1] Add new sysfs ABI for chromebook devices

[Jonathan Cameron <jic23@jic23.retrosnub.co.uk>]

On Fri,  3 May 2019 12:54:45 +0200
Fabien Lahoudere <fabien.lahoudere@collabora.com> wrote:

> Chromebooks embedded controller (EC) provides new field in the 3rd version of 
> its protocol. Those values need to be exposed as sysfs entries.
> 
> Minimum and maximum frequencies may probably be a standard iio ABI.
> There is an existing ABI (sampling_frequency_available) but only for discrete 
> set of values.
> We have different possible solution and I want to ask interested people for the 
> best one:
> 1. we keep the ABI proposed in this RFC (in the chromebook specific code)
> 2. we move min and max freq as 2 different standard ABI
> 3. we add a new standard iio ABI to set a range (sampling_frequency_range)
> 4. we change the 'sampling_frequency_available' ABI to return discrete values 
> like '2 4 6 8' or a range like '[2 - 8]'
> 5. the solution I didn't think about
Gah, it's been on my todo list for far too long to put out proper docs for
it but we do have a way of doing this in IIO.

Best bet is too look at the code in
industrialio-core.c for iio_read_channel_info_avail, and specifically
iio_format_avail_range

That produces strings of the format [MIN STEP MAX] so more or less your
option 4, with the [] syntax indicating to userspace that it's a range
version.

We did this a while back to provide a single interface for consumer drivers
and for userspace.  If you poke around in the period around when that
patch was merged you'll find a discussion which was pretty much the
options you have above.

Note if anyone else wants to have a go at docs for this feature I would
definitely welcome it!

Jonathan

> 
> Thanks
> 
> Gwendal Grignou (1):
>   iio: common: cros_ec_sensors: add extra sensor API
> 
>  .../ABI/testing/sysfs-bus-iio-cros-ec         |  24 ++++
>  .../cros_ec_sensors/cros_ec_sensors_core.c    | 126 +++++++++++++++++-
>  .../linux/iio/common/cros_ec_sensors_core.h   |   7 +
>  include/linux/mfd/cros_ec_commands.h          |  21 +++
>  4 files changed, 177 insertions(+), 1 deletion(-)
>