On Thu, Aug 18, 2016 at 01:01:24PM +0200, Marcel Holtmann wrote:

Hi Greg,

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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).

Some quick comments (can't do any real life tests in the next weeks) from my (biased) view:

* tieing the solution into uart_port is the same as we had done. The difference seems to
 me that you completely bypass serial_core (and tty) while we want to integrate it with standard tty operation.

 We have tapped the tty layer only because it can not be 100% avoided if we use serial_core.

* one feedback I had received was that there may be uart device drivers not using serial_core. I am not sure if your approach addresses that.

* what I don't see is how we can implement our GPS device power control driver:
- the device should still present itself as a tty device (so that cat /dev/ttyO1 reports NMEA records) and should
  not be completely hidden from user space or represented by a new interface type invented just for this device
  (while the majority of other GPS receivers are still simple tty devices).
- how we can detect that the device is sending data to the UART while no user space process has the uart port open
  i.e. when does the driver know when to start/stop the UART.

I am actually not convinced that GPS should be represented as
/dev/ttyS0 or similar TTY. It think they deserve their own driver
exposing them as simple character devices. That way we can have a
proper DEVTYPE and userspace can find them correctly. We can also
annotate them if needed for special settings.

I would _love_ to see that happen, but what about the GPS line
discipline that we have today?  How would that match up with a char
device driver?

we have a GPS line discipline? What is that one doing? As far as I
know all GPS implementations are fully userspace.

Hm, for some reason I thought that was what n_gsm.c was being used for,
but I could be wrong, I've never seen the hardware that uses that
code...

greg k-h