Re: [PATCH V2 01/10] tty: Fallback to use dynamic major number

[Arnd Bergmann <arnd@arndb.de>]

On Thursday 27 November 2014 13:50:01 Grant Likely wrote:
> 
> Should be sufficient. Basically, if the hardware doesn't exist, the
> driver shouldn't be trying to grab the minor numbers.
> 
> Also, on hardware where both exists, there should be some sane
> fallback so that all UARTs get assigned numbers. On DT systems we can
> also use /aliases to ensure consistent assignment of numbers.

>From what I can see, this is really the ISA compatibility code
in the 8250 driver, and we should be able to make that optional
or even move it into a separate glue driver.

Basically the serial8250_init function tries to do a lot of things
at once (skipping error handling):

        serial8250_isa_init_ports();
        ret = sunserial_register_minors(&serial8250_reg, UART_NR);
	serial8250_reg.nr = UART_NR;
        ret = uart_register_driver(&serial8250_reg);
        ret = serial8250_pnp_init();
        serial8250_isa_devs = platform_device_alloc("serial8250",
                                                   PLAT8250_DEV_LEGACY);
        ret = platform_device_add(serial8250_isa_devs);
        serial8250_register_ports(&serial8250_reg, &serial8250_isa_devs->dev);
        ret = platform_driver_register(&serial8250_isa_driver);

The only thing we want from this is the uart_register_driver() call,
everything else is only needed together with the ISA support. The way
that uart_register_driver() works unfortunately implies that you know
the maximum number of ports at driver init time, which would need
to be changed if you want to share the numbers.

One idea I had a few years ago when we discussed this was to treat
the major 4 allocation differently so you could share it between all
sorts of drivers as an opt-in, but could have all unmodified continue
using their own device names and numbers.

	Arnd