[Letux-kernel] RFC: iio input bridge

H. Nikolaus Schaller hns at goldelico.com
Thu Dec 22 19:08:21 CET 2016

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> Am 16.12.2016 um 14:59 schrieb H. Nikolaus Schaller <hns at goldelico.com>:
> Hi,
> we still had some trouble on the GTA04 with the accelerometer
> input driver (evtest /dev/input/accel). The reason is that it
> is some misc driver for the BMA150 and we have mis-used it for
> the bma180 although we didn't recognise that the bma180 has
> different registers. Recently it was even proposed to remove the
> (wrong) bma180 support completely.
> Additionally, for the Pyra and GTA04A5 with BMC150 accelerometer
> we even have no input compatible driver.
> Writing new input accelerometer drivers isn't a good strategy
> since this feature is considered old by kernel maintainers because
> there are already good iio drivers around and they suggest to
> use those.
> This is easy for new designs where user-space is also easily
> modifiable.
> But the problem is that some (older) user spaces (e.g. Replicant)
> assumes that device position is reported through /dev/input.
> And Linus say: "we never break user space...". Therefore
> simply dropping the bma180 support and using iio only is no
> working solution.
> A new idea was to add an iio-input bridge driver into the
> iio core framework which simply looks for iio accelerometers
> and takes the first 3 channels as X, Y and Z. It then registers
> a new /dev/input device, reads the iio channels every 1/10th
> second, scales the values and sends input events.
> An important design choice was to implement it in a way that
> we don't need any DT updates and that it is self-configuring,
> at least for boards with a single 3-axis accelerometer. And
> that the extension is completely disabled if not configured into
> the kernel.
> The result of hacking a little around is that we now have the
> /dev/input/accel for all devices (GTA04 with BMA180, GTA04A5
> and Pyra with BMC150 - if installed - and BNO055 should also
> work if configured for accelerometer).
> Code isn't clean but works:
> 	http://git.goldelico.com/?p=gta04-kernel.git;a=shortlog;h=refs/heads/work/hns/iio-input-bridge
> Comments are welcome.
> BR,
> Nikolaus
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