request for snap interface that expose userspace device APIs

[Jian Luo]

Embedded systems (which are likely called IoT devices these days :) sometime need access to peripherals via user-space device APIs, namely i2cdev[1], spidev[2] and uio[3]. Snaps of type core and gadget should be able to expose these interfaces to other privileged snaps.

Accesses to these character special devices should be granted through this interface:
- /dev/spidevB.C rw,ioctl
- /dev/i2c-%d rw,ioctl
- /dev/uio%d rw,mmap,poll,select

Optionally read-only accesses to sysfs entries can be granted to get device information:
- /sys/class/i2c-dev/** r
- /sys/class/uio/** r

Tests can be done with tools in [4] [5] [6] accordingly.

This interface is IMHO similar to bool_file, gpio and serial-port in regards to security concerns.

[1] https://www.kernel.org/doc/Documentation/i2c/dev-interface
[2] https://www.kernel.org/doc/Documentation/spi/spidev
[3] https://www.kernel.org/doc/htmldocs/uio-howto/userspace_driver.html
[4] https://launchpad.net/ubuntu/+source/i2c-tools
[5] https://github.com/torvalds/linux/blob/master/tools/spi/spidev_test.c
[6] http://www.osadl.org/projects/downloads/UIO/user/lsuio-0.2.0.tar.gz

[Lorn Potter (lorn-potter)]

There is already an i2c interface,
https://github.com/snapcore/snapd/pull/2113

an iio interface in the works based on the i2c interface.

spidev and uio could also be written based on what i2c is, with obvious changes.

[Didier Roche-Tolomelli (didrocks)]

The thing is that they are not exposed in gadget snaps for various devices if I'm correct (like the GPIO one). I did open a bug last week for this: https://bugs.launchpad.net/snappy/+bug/1641752

[Zygmunt Krynicki (zyga)]

This bug is rather old so I'd like to summarize where we are now:

- there are i2c, iio and spi interfaces in snapd
- there is no uio interface yet

Interfaces must be defined by the gadget but we now have support for hot-plug detection that could be used instead of static declaration.

I'm not familiar with uio devices, can you share some information about development boards that have them? I found updated reference to the kernel documentation here https://www.kernel.org/doc/html/v4.17/driver-api/uio-howto.html but none of the reference platforms for snapd seems to have those.

[Zygmunt Krynicki (zyga)]

I've started working on the new interface. Scheduled it for 2.43 but may slip to 2.43.1 depending on release timings.

[Zygmunt Krynicki (zyga)]

I've implemented and tested the interface and proposed it for merging into snapd https://github.com/snapcore/snapd/pull/7863

[Zygmunt Krynicki (zyga)]

The new interface was shipped with snapd 2.43 which was since released to the stable channel. Marking as released.