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

[H. Nikolaus Schaller]

Hi Rob,
many thanks for picking up this unsolved topic!

> Am 18.08.2016 um 03:14 schrieb Rob Herring <robh@xxxxxxxxxx>:
> 
> 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 like that a driver can simply call uart_dev_config(udev, 115200, 'n', 8, 0);  instead of our
     uart_register_rx_notification(data->uart, rx_notification, &termios); where we have to partially
     fill the termios structure.

* it appears to need more code than our proposal did:

> 
> Rob
> 
> 
> Rob Herring (3):
>  uart bus: Introduce new bus for UART slave devices
>  tty: serial_core: make tty_struct optional
>  tty: serial_core: add uart controller registration
> 
> drivers/Kconfig                  |   2 +
> drivers/Makefile                 |   1 +
> drivers/tty/serial/serial_core.c |  11 +-
> drivers/tty/tty_buffer.c         |   2 +
> drivers/uart/Kconfig             |  17 ++
> drivers/uart/Makefile            |   3 +
> drivers/uart/core.c              | 458 +++++++++++++++++++++++++++++++++++++++
> drivers/uart/loopback.c          |  72 ++++++
> include/linux/serial_core.h      |   3 +-
> include/linux/uart_device.h      | 163 ++++++++++++++
> 10 files changed, 730 insertions(+), 2 deletions(-)
> create mode 100644 drivers/uart/Kconfig
> create mode 100644 drivers/uart/Makefile
> create mode 100644 drivers/uart/core.c
> create mode 100644 drivers/uart/loopback.c
> create mode 100644 include/linux/uart_device.h

thereof 9 files, ~650 changes w/o loopback demo

vs.

> On 10/16/2015 11:08 AM, H. Nikolaus Schaller wrote:
>> H. Nikolaus Schaller (3):
>> tty: serial core: provide a method to search uart by phandle
>> tty: serial_core: add hooks for uart slave drivers
>> misc: Add w2sg0004 gps receiver driver
>> 
>> .../devicetree/bindings/misc/wi2wi,w2sg0004.txt    |  18 +
>> .../devicetree/bindings/serial/slaves.txt          |  16 +
>> .../devicetree/bindings/vendor-prefixes.txt        |   1 +
>> Documentation/serial/slaves.txt                    |  36 ++
>> drivers/misc/Kconfig                               |  18 +
>> drivers/misc/Makefile                              |   1 +
>> drivers/misc/w2sg0004.c                            | 443 +++++++++++++++++++++
>> drivers/tty/serial/serial_core.c                   | 214 +++++++++-
>> include/linux/serial_core.h                        |  25 +-
>> include/linux/w2sg0004.h                           |  27 ++
>> 10 files changed, 793 insertions(+), 6 deletions(-)
>> create mode 100644 Documentation/devicetree/bindings/misc/wi2wi,w2sg0004.txt
>> create mode 100644 Documentation/devicetree/bindings/serial/slaves.txt
>> create mode 100644 Documentation/serial/slaves.txt
>> create mode 100644 drivers/misc/w2sg0004.c
>> create mode 100644 include/linux/w2sg0004.h

Thereof 4 files, ~260 changes w/o gps demo and documentation/bindings.

BR and thanks,
Nikolaus