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

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Message-ID: <20160818202900.hyvm4hfxedifuefn@earth>
Date:   Thu, 18 Aug 2016 22:29:00 +0200
From:   Sebastian Reichel <sre@...nel.org>
To:     Rob Herring <robh@...nel.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Marcel Holtmann <marcel@...tmann.org>,
        Jiri Slaby <jslaby@...e.com>, Pavel Machek <pavel@....cz>,
        Peter Hurley <peter@...leysoftware.com>,
        NeilBrown <neil@...wn.name>,
        "Dr . H . Nikolaus Schaller" <hns@...delico.com>,
        Arnd Bergmann <arnd@...db.de>,
        Linus Walleij <linus.walleij@...aro.org>,
        linux-bluetooth@...r.kernel.org, linux-serial@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 0/3] UART slave device bus

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

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