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

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Message-ID: <CAL_Jsq+OfT1AXaEKOgZ4oVfpVZhu=55dYoHTG7D_prpnTqpeOg@mail.gmail.com>
Date:   Thu, 18 Aug 2016 18:08:24 -0500
From:   Rob Herring <robh@...nel.org>
To:     Sebastian Reichel <sre@...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

On Thu, Aug 18, 2016 at 3:29 PM, Sebastian Reichel <sre@...nel.org> wrote:
> 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);

No objection, but out of curiosity, why?

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

Rob