Re: [RFC PATCH 0/3] UART slave device bus

One Thousand Gnomes
2016-08-22T17:11:09Z

> Would it make sense then to define a DT binding that can cover these
> four cases independent of the Linux usage:
>
> a) an existing tty line discipline matched to a tty port
> b) a serio device using the N_MOUSE line discipline (which
> happens to cover non-mouse devices these days)

These two are the same basic thing

port x expects ldisc y {with properties ...}

> c) a uart_port slave attached directly to the uart (like in your
> current code)

c) needs to be a tty_port slave attached directly to a tty_port.
Important detail, but as uart_port is just a subset of tty_port it's a
trivial detail to the DT.

> d) the same slave drivers using a new tty line discipline

and this is also just a/b again.

What use cases and connectivity do you need to describe. Looking at the
ACPI platforms we have

- the expected serial port configuration
- the properties of the port (FIFO etc)
- the power management for the port

- the children of the port
- the power management of the children (at a very simplistic abstracted
level)

So we want to be able to describe something like

ttyS0 {
baud: 1152008N1
protocol: bluetooth hci
fixed: yes
powermanagement: { ... }
}

and if I look at the usermode crapfest on a lot of Android systems it
looks similar but with the notion of things like being able to describe

- Use GPIO mode sleeping and assume first char is X to save power

- Power up, wait n ms, write, read, wait n ms, power down (which
has to be driven at the ldisc/user level as only the ldisc
understands transactions, or via ioctls (right now Android user
space tends to do hardcoded writes to /sys.. gpio to drive power

- And a few variants thereof (power up on write, off on a timer
etc)

So I can imagine wanting to describe something like

- The bluetooth HCI hardware is managed by gpio 11 (or UART DTR,
or PMIC n etc)
The uart can switch into GPIO mode and is gpio 15

or

- Raise gpio 4 when writing, drop it after 50mS with no read/write

Then the ldisc needs to make port->ops. calls for enabling/disabling low
power mode and expected char, and the uarts that can do it need to
implement the gpio/uart switching and any timers.

Alan