Re: Accelerometer events

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

On 07/16/2014 04:41 PM, Martin Fuzzey wrote:
> Hi all,
>
> I am in the process of adding events support to the mma8452 driver (patches
> will be forthcoming when cooked a little more).
>
> However I have a few questions.
>
> The hardware can generate an interrupt when the absolute value of the high
> pass filtered acceleration exceeds a configurable threshold.
> This is called "transient detection" in the datasheet.
> There is only a single threshold but the interrupt can be enabled or
> disabled for each axis.
>
> So I define this as:
> static const struct iio_event_spec mma8452_transient_event[] = {
>      {
>          .type = IIO_EV_TYPE_MAG,
>          .dir = IIO_EV_DIR_RISING,
>          .mask_separate = BIT(IIO_EV_INFO_ENABLE),
>          .mask_shared_by_type = BIT(IIO_EV_INFO_VALUE),
>      },
> };
>
> That causes the following sysfs entries to be created:
> # ls -l /sys/bus/iio/devices/iio:device0/events
> -rw-r--r-- 1 root root 4096 Jul 16 13:25 in_accel_mag_rising_value
> -rw-r--r-- 1 root root 4096 Jul 16 13:25 in_accel_x_mag_rising_en
> -rw-r--r-- 1 root root 4096 Jul 16 13:25 in_accel_y_mag_rising_en
> -rw-r--r-- 1 root root 4096 Jul 16 13:26 in_accel_z_mag_rising_en
>
> IE one enable switch per axis and one threshold (as expected).
>
> However Documentation/ABI/testing/sysfs-bus-iio seems to indicate that the
> threshold values
> should have raw or input variants.  [ /sys/.../events/in_accel_raw_mag_value]
>
> This part is handled by the core, not the driver. Is there a problem here?
>
> When raw values are used for the threshold, how is userspace to know the
> conversion factor?
> In the case of the mma8452 it cannot use the current scale factor available in
>      /sys/bus/iio/devices/iio\:device0/in_accel_scale
> because the transient detection is always configured using 8G/127 units,
> regardless of the full
> scale value (2G, 4G, 8G) selected for measurements.
>
> Does this mean there should be another in_accel_scale in the events sysfs
> subdirectory?
>

So far the issue hasn't come up, so there is no support for this yet. But it 
should be possible to add support for it.

> My understanding of "rising"/"falling" is the crossing over / under the
> threshold.
> However this hardware can only generate interrupts for exceeding the
> threshold (for magnitude)
> It can however determine the sign of the acceleration (+ve or -ve) when this
> occurs - how should
> this information be sent as an event?
> I could use the "direction" attribute supplied to IIO_UNMOD_EVENT_CODE bit
> this seems to be an abuse.

Since there are also falling magnitude events using the direction bit 
doesn't really work since you couldn't distinguish between the two types of 
events. But adding a new field to the event that contains the sign of the 
magnitude should be possible.

>
> All this transient detection part works with high pass filtered data (and
> the filter frequency is
> configurable). There is no mention of high pass filters in the ABI, only low
> pass. So how should this
> be handled?

Add a attribute that can configure the high pass filter frequency.

>
> Finally I have been using the example userspace code in drivers/staging/Doc/
> iio_event_monitor.c
> But that does
>      #include <linux/iio/events.h>
>
> Which is not exported by the kernel in include/uapi - is this a problem?
> (Easy enough to hack around for testing
> but if user space code is supposed to be using these interfaces...)

Yep, that is a issue that needs to be fixed. We should clean these files up 
and remove things that shouldn't be exposed to userspace before moving the 
files to include/uapi.

- Lars