Hi Rob,

Thanks for going forward and implementing this. I also started,
but was far from a functional state.

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.
- Split out the controller for uart_ports into separate driver. Do we see
  a need for controller drivers that are not standard serial drivers?
- 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)?
- Test with other UART drivers
- Convert a real driver/line discipline over to UART bus.

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).

I had a look at the uart_dev API:

int uart_dev_config(struct uart_device *udev, int baud, int parity, int bits, int flow);
int uart_dev_connect(struct uart_device *udev);

  The flow control configuration should be done separately. e.g.:
  uart_dev_flow_control(struct uart_device *udev, bool enable);

int uart_dev_tx(struct uart_device *udev, u8 *buf, size_t count);
int uart_dev_rx(struct uart_device *udev, u8 *buf, size_t count);

  UART communication does not have to be host-initiated, so this
  API requires polling. Either some function similar to poll in
  userspace is needed, or it should be implemented as callback.
  
-- Sebastian