Re: [PATCH v6 4/6] serial: core: Add support for DEVNAME:0.0 style naming for kernel console

Andy Shevchenko
2024-02-13T15:53:35Z

On Tue, Feb 13, 2024 at 10:45:11AM +0200, Tony Lindgren wrote:
> We can now add hardware based addressing for serial ports. Starting with
> commit 84a9582fd203 ("serial: core: Start managing serial controllers to
> enable runtime PM"), and all the related fixes to this commit, the serial
> core now knows to which serial port controller the ports are connected.
>
> The serial ports can be addressed with DEVNAME:0.0 style naming. The names
> are something like 00:04:0.0 for a serial port on qemu, and something like
> 2800000.serial:0.0 on platform device using systems like ARM64 for example.
>
> The DEVNAME is the unique serial port hardware controller device name, AKA
> the name for port->dev. The 0.0 are the serial core controller id and port
> id.
>
> Typically 0.0 are used for each controller and port instance unless the
> serial port hardware controller has multiple controllers or ports.
>
> Using DEVNAME:0.0 style naming actually solves two long term issues for
> addressing the serial ports:
>
> 1. According to Andy Shevchenko, using DEVNAME:0.0 style naming fixes an
> issue where depending on the BIOS settings, the kernel serial port ttyS
> instance number may change if HSUART is enabled
>
> 2. Device tree using architectures no longer necessarily need to specify
> aliases to find a specific serial port, and we can just allocate the
> ttyS instance numbers dynamically in whatever probe order
>
> To do this, let's match the hardware addressing style console name to
> the character device name used, and add a preferred console using the
> character device name.
>
> Note that when using console=DEVNAME:0.0 style kernel command line, the
> 8250 serial console gets enabled later compared to using console=ttyS
> naming for ISA ports. This is because the serial port DEVNAME to character
> device mapping is not known until the serial driver probe time. If used
> together with earlycon, this issue is avoided.

...

> +int serial_base_add_preferred_console(struct uart_driver *drv,
> + struct uart_port *port)
> +{
> + const char *port_match __free(kfree);

= NULL

> + int ret;
> +
> + ret = serial_base_add_prefcon(drv->dev_name, port->line);
> + if (ret)

Otherwise here might be a problem.

> + return ret;

> + port_match = kasprintf(GFP_KERNEL, "%s:%i.%i", dev_name(port->dev),
> + port->ctrl_id, port->port_id);
> + if (!port_match)
> + return -ENOMEM;
> +
> + /* Translate a hardware addressing style console=DEVNAME:0.0 */
> + return serial_base_add_one_prefcon(port_match, drv->dev_name, port->line);
> +}

--
With Best Regards,
Andy Shevchenko