On 2018-09-21 15:59, Russell King - ARM Linux wrote:
On Thu, Sep 20, 2018 at 10:13:57PM +0200, Rafał Miłecki wrote:
From: Rafał Miłecki <rafal@xxxxxxxxxx>
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.
Thanks a lot for looking at that. I was not aware of the commit
182ead3e418a ("earlycon: Remove hardcoded port->uartclk initialization
in of_setup_earlycon"), too bad it wasn't marked for stable.
I simply assumed that
port->uartclk = BASE_BAUD * 16;
is correct and that has poisoned all my further reasoning.
I've just confirmed that backporting 182ead3e418a to the 4.14 fixes the
problem for me (it results in the same 8250_early.c behavior as setting
BASE_BAUD to 0).
I'm planning to backport both:
182ead3e418a ("earlycon: Remove hardcoded port->uartclk initialization
in of_setup_earlycon")
814453adea7d ("earlycon: Initialize port->uartclk based on
clock-frequency property")
to 4.14+ as they fix regression triggered (not to say "caused") by the
commit 31cb9a8575ca0 ("earlycon: initialise baud field of earlycon
device structure").
Please DROP my serial.h patch and thanks again.
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.
Somehow my serial console kept working with BASE_BAUD set to 0. I'm not
sure why.
