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

[Sebastian Reichel]

Hi,

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.
> 
> Signed-off-by: Tony Lindgren <tony@xxxxxxxxxxx>
> ---

I would expect an update to the console= section in [0] somewhere in
this series.

[0] Documentation/admin-guide/kernel-parameters.txt

Greetings,

-- Sebastian