Re: Kernel wishlist item: Better IIO API

[Bastien Nocera <hadess@hadess.net>]

On Tue, 2014-11-04 at 10:37 +0200, Daniel Baluta wrote:
> On Tue, Nov 4, 2014 at 10:27 AM, Bastien Nocera <hadess@hadess.net> wrote:
> > On Tue, 2014-11-04 at 10:18 +0200, Daniel Baluta wrote:
> >> On Tue, Nov 4, 2014 at 1:01 AM, Bastien Nocera <hadess@hadess.net> wrote:
> >> > On Wed, 2014-10-29 at 19:30 +0100, Lars-Peter Clausen wrote:
> >> >> On 10/29/2014 06:47 PM, Bastien Nocera wrote:
> >> >> > On Wed, 2014-10-29 at 18:39 +0100, Lars-Peter Clausen wrote:
> >> >> >> On 10/29/2014 06:33 PM, Bastien Nocera wrote:
> >> >> >>> On Wed, 2014-10-29 at 18:21 +0100, Lars-Peter Clausen wrote:
> >> >> >>>> On 10/29/2014 03:30 PM, Bastien Nocera wrote:
> >> >> >>>>> Hey,
> >> >> >>>>>
> >> >> >>>>> I've posted this a couple of days ago:
> >> >> >>>>> http://www.hadess.net/2014/10/a-gnome-kernel-wishlist.html
> >> >> >>>>> along with a mail to LKML:
> >> >> >>>>> http://thread.gmane.org/gmane.linux.kernel/1810083
> >> >> >>>>>
> >> >> >>>>> I've recently added to my list an item about IIO:
> >> >> >>>>> https://wiki.gnome.org/BastienNocera/KernelWishlist
> >> >> >>>>>
> >> >> >>>>> Are there any plans for a better API for the IIO subsystem? The API
> >> >> >>>>> might be good enough to drive from shell scripts, or helpers that only
> >> >> >>>>> need to work with one variant of a device, but my attempts at trying to
> >> >> >>>>> use the IIO subsystem to provide an accelerometer to do automatic
> >> >> >>>>> display rotation[1] showed that the API is really cumbersome.
> >> >> >>>>>
> >> >> >>>>> The code I wrote spends most of its time creating sysfs paths, reading
> >> >> >>>>> values in different formats, and mangling filenames[2].
> >> >> >>>>>
> >> >> >>>>> Is an ioctl-based API planned? Something where I could get/set
> >> >> >>>>> structures to gather metadata about the device, and set it up easily, so
> >> >> >>>>> reading data from it is easier?
> >> >> >>>>
> >> >> >>>> No, unfortunately not and I'm not sure if such a ABI would be accepted if
> >> >> >>>> proposed.
> >> >> >>>
> >> >> >>> Why not?
> >> >> >>
> >> >> >> Because it means there will be ambiguity in the API on how to do things.
> >> >> >> Which is typically not a desired property.
> >> >> >>
> >> >> >>>
> >> >> >>>> But checkout libiio[1][2], it hides the details of the sysfs file manipulation.
> >> >> >>>
> >> >> >>> I'm not sure that's any better unfortunately. I've certainly tried to do
> >> >> >>> that already in my code, but that doesn't change that the user-space API
> >> >> >>> is barely usable.
> >> >> >>
> >> >> >> It's not completely unusable ;)
> >> >> >
> >> >> > In the end, you prefer the "self-documenting" of using sysfs files,
> >> >> > rather than an API which you can document in a header file?
> >> >>
> >> >> If it was for me we'd be using a state-full IOCTL ABI rather than a
> >> >> stateless sysfs ABI. I'm definitely not happy with the current interface,
> >> >> but it's the interface we have. But the problem with userspace ABI (in
> >> >> comparison to in-kernel API) is that we can just change things at random,
> >> >> but we have to stick with the existing interface.
> >> >>
> >> >> The sysfs ABI is not meant to be self documenting and it is not
> >> >> undocumented. The documentation for the different attributes can be found in
> >> >> Documentation/ABI/testing/sysfs-bus-iio[1].
> >> >
> >> > That's useful if a bit terse. Thanks though.
> >> >
> >> >> > I don't understand that. My questions on this very mailing-list, and
> >> >> > comments that were made to users of my code[1] clearly show that the
> >> >> > existing API is anything but "not ambiguous".
> >> >>
> >> >> That bug report sounds like bugs in the driver.
> >> >
> >> > Not really. Some drivers need the "in_accel_hysteresis" set, some don't
> >> > have that sysfs file, for example.
> >> >
> >> >> > I've used the Bluetooth, input, rfkill, and inotify APIs as provided
> >> >> > directly by the Linux kernel (not through a layer) and they're of better
> >> >> > quality than the IIO one.
> >> >> >
> >> >> > I just don't see how one could support a class of IIO sensors with the
> >> >> > existing API.
> >> >>
> >> >> I can understand your frustration. A API that is not usable in a generic way
> >> >> is not really useful. So we should try to fix that, but we are bound by the
> >> >> framework itself and can't just throw everything away.
> >> >>
> >> >> So lets start by trying to identify what is missing. Which information do
> >> >> you think could be provided by using a IOCTL interface which you need or
> >> >> want which is not provided by the current sysfs interface or can not be
> >> >> provided by the current sysfs interface.
> >> >
> >> > (pseudo code ahead)
> >> >
> >> > First, being able to cut down on the string manipulation would be great.
> >> > So instead of doing:
> >> > accel_x_path = build_filename (sysfs_path, "in_accel_x_raw");
> >> > accel_y_path = build_filename (sysfs_path, "in_accel_y_raw");
> >> > accel_z_path = build_filename (sysfs_path, "in_accel_z_raw");
> >> > if (exists(accel_x_path) && exists(accel_y_path) && exists(accel_z_path)) {
> >> >   // We have an accelerometer
> >> >   do_something();
> >> > }
> >> > free(..x);
> >> > free(..y);
> >> > free(..z);
> >> > free(sysfs_path);
> >> >
> >> > I could query for the device's capabilities:
> >> > fd = open ("/dev/iio0");
> >> > ioctl(fd, IIO_GET_CAPS, &caps);
> >> > if (caps.channels & (IIO_CAP_ACCEL_X | IIO_CAP_ACCEL_Y | IIO_CAP_ACCEL_Z)) {
> >> >   // We have an accelerometer
> >> >   do_something();
> >> > }
> >> >
> >> > Note that, from the data given, I don't know how to make out whether
> >> > something is an accelerometer, or a quartenion sensor, or which one we
> >> > should prefer on specific machines.
> >> >
> >> > This would also be stateful, so that 1) enabling the various channels,
> >> > 2) changing the hysteresis would be reset when the fd is closed. That
> >> > would cut down on the power consumption when unused, or when the service
> >> > that uses that data crashes.
> >> >
> >> > Finally, there's some documentation, and it's not quite finished, or
> >> > there's something fishy on this device:
> >> > $ ls /sys//devices/platform/80860F41:04/i2c-12/i2c-SMO8500:00/iio:device0/events
> >> > in_accel_x_thresh_thresh_en      in_accel_x_thresh_thresh_value
> >> > in_accel_y_thresh_thresh_period  in_accel_z_thresh_thresh_en
> >> > in_accel_z_thresh_thresh_value
> >> > in_accel_x_thresh_thresh_period  in_accel_y_thresh_thresh_en
> >> > in_accel_y_thresh_thresh_value   in_accel_z_thresh_thresh_period
> >>
> >> Could you paste here the initialization part of iio_chan_spec + iio_event_spec?
> >
> > How would I do that?
> 
> Just look into the code and search for iio_chan_spec and
> iio_event_spec declaration.

Oh, the code. Well, you've already sent a patch for it, so you should
know ;)

I've sent the patch adding support for the accelerometer in my device
yesterday:
http://thread.gmane.org/gmane.linux.kernel.iio/14487

I'll test your patch in a little while once I've had the chance to play
with the Bluetooth support code.

Cheers