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

Greg Kroah-Hartman
2016-08-18T10:22:52Z

On Wed, Aug 17, 2016 at 08:14:42PM -0500, Rob Herring wrote:
> Currently, devices attached via a UART are not well supported in the
> kernel. The problem is the device support is done in tty line disciplines,
> various platform drivers to handle some sideband, and in userspace with
> utilities such as hciattach.
>
> There have been several attempts to improve support, but they suffer from
> still being tied into the tty layer and/or abusing the platform bus. This
> is a prototype to show creating a proper UART bus for UART devices. It is
> tied into the serial core (really struct uart_port) below the tty layer
> in order to use existing serial drivers.
>
> This is functional with minimal testing using the loopback driver and
> pl011 (w/o DMA) UART under QEMU (modified to add a DT node for the slave
> device). It still needs lots of work and polish.
>
> TODOs:
> - Figure out the port locking. mutex plus spinlock plus refcounting? I'm
> hoping all that complexity is from the tty layer and not needed here.

It should be.

> - Split out the controller for uart_ports into separate driver. Do we see
> a need for controller drivers that are not standard serial drivers?

What do you mean by "controller" drivers here? I didn't understand them
in the code.

> - Implement/test the removal paths
> - Fix the receive callbacks for more than character at a time (i.e. DMA)
> - Need better receive buffering than just a simple circular buffer or
> perhaps a different receive interface (e.g. direct to client buffer)?

Why? Is the code as-is slow?

> - Test with other UART drivers
> - Convert a real driver/line discipline over to UART bus.

That's going to be the real test, I recommend trying that as soon as
possible as it will show where the real pain points are :)

> Before I spend more time on this, I'm looking mainly for feedback on the
> general direction and structure (the interface with the existing serial
> drivers in particular).

Yes, I like the idea (minor nit, you still have SPMI in a lot of places
instead of UART), so I recommend keeping going with it.

> drivers/uart/Kconfig | 17 ++
> drivers/uart/Makefile | 3 +
> drivers/uart/core.c | 458 +++++++++++++++++++++++++++++++++++++++
> drivers/uart/loopback.c | 72 ++++++

Why not just put this in drivers/tty/uart/ ?

thanks,

greg k-h