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

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Date:   Fri, 19 Aug 2016 07:21:26 +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>,
        "open list:BLUETOOTH DRIVERS" <linux-bluetooth@...r.kernel.org>,
        "linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 0/3] UART slave device bus

Hi,

On Thu, Aug 18, 2016 at 06:08:24PM -0500, Rob Herring wrote:
> On Thu, Aug 18, 2016 at 3:29 PM, Sebastian Reichel <sre@...nel.org> wrote:
> > 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);
> 
> No objection, but out of curiosity, why?

Nokia's bluetooth uart protocol disables flow control during speed
changes.

> > 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.
> 
> What's the userspace need?

I meant "Either some function similar to userspace's poll() is
needed, ...". Something like uart_dev_wait_for_rx()

Alternatively the rx function could be a callback, that
is called when there is new data.

> I'm assuming the only immediate consumers are in-kernel.

Yes, but the driver should be notified about incoming data.

-- Sebastian

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