On Thu, Mar 1, 2018 at 11:47 AM Andy Shevchenko [off-list ref]
wrote:

On Thu, Mar 1, 2018 at 8:43 PM, Daniel Kurtz [off-list ref] wrote:

Please, hold on with new versions.
I'm not satisfied (yet?) by the approach.

Copying over your comment on v1:

It needs to be discussed.

Sure.

First of all, if you are going to do this you need to add a parse of
human readable formats (IIRC kernel has helpers), i.e. "48M", "38.4M"
and so on.

Next, I was under impression that purpose of earlycon (in difference
to earlyprintk) is to re-use existing drivers as fully as possible.

So, what exactly happens in your case? Are your driver lacks of
properly set clock? Or earlycon does simple not utilizing this
information?

"earlycon simply does not utilize the information".

earlycon parses iotype, mapbase and baud (from options).  However, it is
hard-coded to assume that the clock used to generate the UART bitclock is
always "BASE_BAUD * 16" (1843200).  While this may be true for many UARTs,
it isn't true for AMD's CZ/ST which has a 8250_dw and uses a fixed 48 MHz
clock.  The main 8250_dw driver uses devm_clk_get to get the "baudclk" and
uses its rate to initialize uartclk.  For AMD CZ/ST, this "baudclk" is
actually a set up in acpi_apd.c when there is an acpi match for "AMD0020",
with a rate read from the .fixed_clk_rate param of the corresponding
apd_device_desc.

This patch attempts to add a way to inform earlycon about this clock.  As
noted above, the information is actually already in the kernel and used by
8250_dw - I would happy be to hear recommendations for wiring this data
into earlycon that doesn't require adding another command line arg.

I see that support was also added recently to earlycon to let it use ACPI
SPCR to choose a console and configure its parameters... but AFAICT, this
path also doesn't allow specifying the uart clock.

-Dan
--
To unsubscribe from this list: send the line "unsubscribe linux-doc" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html