Re: [Qemu-arm] [RFC v3] qapi: command category to stimulate high-level machine devices

[Stefan Hajnoczi]

On Sun, Jun 03, 2018 at 04:34:12PM -0400, Steffen Görtz wrote:
> Changes in v2:
>   - Remove stray 1
> 
> Changes in v1:
>   - Fix coding style issues
> 
> Add a new category "stimulate" to host commands that
> act upon high-level devices associated with machines/boards.
> 
> This is patch is part of an effort to add support for BBC:microbit
> educational computer to QEMU.
> 
> The microbit board, as many other microcontroller boards,
> contains typical trivial (digital) input and output options such as buttons 
> and LEDs,
> but also more sophiscated possibilities such as analog inputs and
> complex dedicated sensor ICs that are connected to the microbit machine
> by means of inter-ic bus (i2c).
> These devices interact with peripheral devices of the microcontroller
> in use, for example with the GPIO peripheral of the used NRF51.
> 
> One aim of our efforts is to create a user interface that models the
> look and feel of the physical microbit board and lets the user interact
> with its inputs and provide feedback on its outsputs.
> 
> For this it is necessary to transmit user inputs such as the pressing of a
> button to the machine.
> In return, when the state of an output is changed on the machine, this
> change needs to be reflected in the user interface.
> 
> At the moment, there are only a few QMP commands that provide user input to 
> the
> machine, eg. send-keys, input-button. These commands require very

QMP command names (from qapi/ui.json): send-key and input-send-event.

> high-level concepts, which are not really suitable for applications that
> microcontrollers are typically used in in.

I agree that existing commands aren't suitable.  They are tied to
QemuConsole, which makes sense for graphics + keyboard/mouse input
devices but doesn't exist for microcontrollers.

I have CCed Gerd Hoffmann, the ui/ maintainer, to see if he agrees.

> This RFC is meant to start a discussion on how to provide stimuli to
> low-complex inputs and outputs typically found in embedded
> microcontroller applications. To the best of my knowledge, there are
> currently no examples of how to provide such stimuli to either SoC
> device peripheral or machine level concepts like pushbuttons and LEDs.
> 
> To my understanding, the following concepts/apis are needed:
> - QMP commands to send stimuli to machine level concepts like
> pushbuttons
> - Machine level devices that receive these stimuli and act as an adapter
> to manipulate the associated SoC peripheral devices

The QEMU Object Model offers "interfaces".  Many object-oriented
programming languages have similar concepts (interfaces, virtual base
classes, etc).

Interfaces could be used to connect board-specific devices with the
monitor command.  But this is an implementation detail, we first need to
agree on the API design.

> - For outputs, machine level devices are needed that observe changes in
> SoC peripherals and emit QMP events that clients can receive
> 
> This patch adds a new qmp command "buttons-set-state" to update the
> push-down state of arbitrarily identified buttons (string identifier).

Can you list the types of devices that can consume events?

 * Pushbutton
 * GPIO input pin
 * Analog-to-digital converter
 * Temperature sensor
 * Accelerometer
 * Magnetometer
 * LED voltage decay light sensing
 * ?

And list the types of devices that can produce events?

 * LED states
 * GPIO output pin
 * Digital-to-analog converter
 * ?

Many of these inputs/outputs can be tied to an external UI.  A degree of
timing precision is required so that the UI is responsive, although
cycle-accurate timing is not what I'd expect from QMP.

Stefan

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