Re: [RFC PATCH 0/7] Add Linux Motion Control subsystem

[David Lechner <dlechner@baylibre.com>]

On 2/27/25 10:28 AM, David Jander wrote:
> Request for comments on: adding the Linux Motion Control subsystem to the
> kernel.
> 
> The Linux Motion Control subsystem (LMC) is a new kernel subsystem and
> associated device drivers for hardware devices that control mechanical
> motion. Most often these are different types of motors, but can also be
> linear actuators for example.

This is something that I played around with when I first got into Linux
kernel hacking as a hobbyist. It's something I've always wanted to see get
upstreamed, so feel free to cc me on any future revisions of this series.
I'm very interested. :-)

We made drivers for basic DC motors driven by an H-bridge both with and without
position feedback and also a driver for hobby-type servo motors. For those
interested, there is code [1] and docs [2]. One thing we would do different
if doing it over again is use a character device instead of sysfs attributes
as the interface for starting/stopping/adjusting actuation.

[1]: https://github.com/ev3dev/lego-linux-drivers/tree/ev3dev-stretch/motors
[2]: http://docs.ev3dev.org/projects/lego-linux-drivers/en/ev3dev-stretch/motors.html

> 
> This subsystem defines a new UAPI for motion devices on the user-space
> side, as well as common functionality for hardware device drivers on the
> driver side.
> 
> The UAPI is based on a ioctl() interface on character devices representing
> a specific hardware device. The hardware device can control one or more
> actuators (motors), which are identified as channels in the UAPI. It is
> possible to execute motions on individual channels, or combined
> affecting several selected (or all) channels simutaneously. Examples of
> coordinated movements of several channels could be the individual axes
> of a 3D printer or CNC machine for example.
> 
> On the hardware side, this initial set of patches also includes two drivers
> for two different kinds of motors. One is a stepper motor controller
> device that containes a ramp generator capable of autonomously executing
> controlled motions following a multi-point acceleration profile
> (TMC5240), as well as a simple DC motor controller driver that can control
> DC motors via a half-bridge or full H-bridge driver such as the TI DRV8873
> for example.
> 
> Towards the IIO subsystem, LMC supports generating iio trigger events that
> fire at certain motion events, such as passing a pre-programmed position or
> when reaching the motion target position, depending on the capabilities of
> the hardware device. This enables for example triggering an ADC measurement
> at a certain position during a movement.

I would expect to be using the counter subsystem for position, at least in
cases where there is something like a quadrature encoder involved.

> 
> In the future, making use of PREEMPT_RT, even dumb STEP/DIR type stepper
> motor controller drivers may be implemented entirely in the kernel,
> depending on some characteristics of the hardware (latency jittter,
> interrupt latency and CPU speed mainly).
> 
> The existence of this subsystem may affect other projects, such as
> Linux-CNC and Klipper for example.
> 
> This code is already in use controlling machines with up to 16 stepper
> motors and up to 4 DC motors simutaneously. Up to this point the UAPI
> has shown to be adequate and sufficient. Careful thought has gone into
> the UAPI design to make sure it coveres as many use-cases as possible,
> while being versioned and extensible in the future, with backwards
> compatibility in mind.
> 
> David Jander (7):
>   drivers: Add motion control subsystem

Would it be too broad to call this an actuation subsystem instead where motion
is just one kind of actuation?

>   motion: Add ADI/Trinamic TMC5240 stepper motor controller
>   motion: Add simple-pwm.c PWM based DC motor controller driver
>   Documentation: Add Linux Motion Control documentation
>   dt-bindings: motion: Add common motion device properties
>   dt-bindings: motion: Add adi,tmc5240 bindings
>   dt-bindings: motion: Add motion-simple-pwm bindings
>