Re: LIS331DLH accelerometer driver, IIO or not?

[Jonathan Cameron <jic23@cam.ac.uk>]

On 05/27/2012 04:14 AM, Darren Hart wrote:
> 
> 
> On 05/26/2012 10:40 AM, Jonathan Cameron wrote:
>> On 05/26/2012 12:53 PM, Éric Piel wrote:
>>> On 25-05-12 07:10, Greg Kroah-Hartman wrote:
>>>> On Thu, May 24, 2012 at 09:29:53PM -0700, Darren Hart wrote:
>>>>> I'm working to enable the LIS331DLH accelerometer on the Fish River
>>>>> Island II embedded atom development kit.
>>>>>
>>>>> I am more interested in enabling people to do bizarre and interesting
>>>>> things with the device, so I'm leaning toward continuing with my IIO
>>>>> implementation.
>>>>
>>>> Make it an IIO driver and then we can delete the misc driver, which
>>>> shouldn't have snuck in there in the first place :)
>>>>
>>>
>>> To be more fair to the misc driver, I wouldn't say it snucked in there,
>>> but more "it ended up there as the least worse place" ;-) Actually, the
>>> main problem is that there seemed to be no maintainer interested in
>>> taking care of accelerometer devices. Now that the IIO subsystem is out
>>> of staging, it might be a right place. That said, I don't know much
>>> about the user interface to IIO. I know that I liked the idea of having
>>> an joystick device created for an accelerometer because that allows to
>>> get many programs to access the device almost without any modifications.
>> I agree that this sort of device should have an input interface. Not
>> sure if a joystick is the right option, but that's more one for Dmitry
>> to comment on.
>>>
>>> I'd happy to help merge the lis3lv02d driver into IIO. IMHO, the main
>>> steps are:
>>>  * make sure all the various buses are supported (e.g., I²C, SPI, and
>>> also "ACPI-HP")
>>>  * ensure the various versions of the accelerometer are supported (there
>>> are 3 supported currently)
>>>  * check that the driver is automatically loaded on HP laptops (via ACPI
>>> entry)
>>>  * for each of the current interfaces decide if they should be ported or
>>> dropped (/dev/js*, /dev/freefall, sysfs...)
>>>
>>> What do you think Jonathan?
>> You've laid it out extremely clearly.  Thanks, I agree with these steps,
>> though they may occur from a slightly different angle given Darren is
>> interested in a part that is not (I believe) currently supported by
>> your existing driver.   Hence he may initially want to do a separate
>> driver with that (keeping in mind the aim of mergining in the existing
>> driver).  I have an ancient driver for the lis3l02dq alone (in
>> iio from the start) that will also get eaten up by Daren's new driver
>> (and the ability to test on that part on spi).
>> staging/iio/accel/lis3l02dq*.c
> 
> I think I should start with getting the lis331dlh support completed, if
> for no other reason than to keep the scope manageable as I write my
> first real driver. From that I would like to merge in Jonathan's IIO
> lis3l02dq driver to get the multi-chip support part right. Then we
> should look at expanding the scope of the interface and finally merging
> with misc/lis3102dq. I believe that should meet with everyone's suggestions.
> 
> One thing I would like to understand better is what sort of interface
> does userspace current expect. Phone Gap, for example, provides a very
> high level interface to applications in m/s^2 for each axis. Is there
> some interface we should ensure all accelerometer driver's implement?
> 
> I suspect a /dev/accel interface that reads out xyz values in ms/s^2
> would make sense. We would need to ensure that allows for polled as well
> as event driven. Thoughts?

Immediate comment is don't call it /dev/accel.  That's just taken out
using it for all the combined gyro/ accel units out there.
Also doing scaling in kernel is expensive, fiddly and inefficient
(no floating point - and lots of users don't actually care about
absolute scale).  Then we get the question of what this gains us over
existing options (input or /dev/iio/iio:device0) beyond a memorable
location?  Maybe we leave this question for now...
> 
>