Re: [PATCH] ARM: add serial.h and set BASE_BAUD to 0

[Russell King - ARM Linux <linux@armlinux.org.uk>]

On Thu, Sep 20, 2018 at 10:13:57PM +0200, Rafał Miłecki wrote:
> From: Rafał Miłecki <rafal@milecki.pl>
> 
> For years arm has been using serial.h from asm-generic which sets
> BASE_BAUD value to the (1843200 / 16). This is incorrect as:
> 1) This value obviously isn't correct for all devices
> 2) There are no device specific serial.h with CONFIG_ARCH_MULTIPLATFORM

This goes back a long way.  The BASE_BAUD definition is the base baud
for 8250 compatible UARTs _only_, no others.

However, as of 182ead3e418a ("earlycon: Remove hardcoded port->uartclk
initialization in of_setup_earlycon"), port->uartclk is no longer
initialised using BASE_BAUD.  As acknowledged in 814453adea7d
("earlycon: Initialize port->uartclk based on clock-frequency property")
the initialisation using BASE_BAUD was bogus, and there is now the
clock-frequency DT property which should be present to set this up.

Now, setting BASE_BAUD to zero as per your patch will break the
8250 serial driver - this relies on BASE_BAUD being set to the
current value, and yes, ARM hardware that uses this will break.  So
this is not a solution.

The only solution is that BASE_BAUD must not be abused - it must not
be used by non-8250 hardware, and thankfully there are already
solutions in the kernel (such as clock-frequency) to allow the clock
rate to be specified.

-- 
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