Early printk baudrate initialization DTS description

[David VomLehn (dvomlehn)]

> -----Original Message-----
> From: devicetree-discuss [mailto:devicetree-discuss-
> bounces+dvomlehn=cisco.com at lists.ozlabs.org] On Behalf Of Michal Simek
> Sent: Tuesday, June 26, 2012 4:17 AM
> To: devicetree-discuss at lists.ozlabs.org
> Cc: John Williams
> Subject: Early printk baudrate initialization DTS description
> 
> Hi,
> 
> we are trying to solve one problem with early printk initialization
> on Microblaze uart16550. I have also checked arm bootup code and
> the situation is the same there (NOTE: This IP is possible to use on
> Zynq too).
> 
> The problem is about baudrate initialization. Current code just expect
> that early console is initialized by previous bootloader/firmware (in
> our case u-boot).
> But if there is not initialization and directly elf is loaded there is
> no initialization
> done and kernel bootlog is shown when regular serial driver is probed.
> 
> I have create a solution which add baudrate initialization to
> Microblaze early printk code.
> And also two options how to load baudrate setting from device-tree.
> 
> Code expects that early console is specified in chosen node.
> 
> chosen {
> 	linux,stdout-path = "/axi at 0/serial at 40400000";
> } ;
> 
> Code finds out which IP it is and based on that initialize early
printk
> driver.
> Baudrate setting is counted from clock-frequency and current-speed
> defined directly in uart node.

The terminology is not quite clear to me. Many people use the term baud
rate when they mean the more accurate term "data rate" and I'm pretty
sure this is what you mean, but is this different than "current-speed"?

> 
> RS232_Uart_1: serial at 40400000 {
> 	clock-frequency = <50000000>;
> 	compatible = "xlnx,axi-uart16550-1.01.a", "xlnx,xps-uart16550-
> 2.00.a", "ns16550a";
> 	current-speed = <115200>;
> 	device_type = "serial";
> 	reg = < 0x40400000 0x10000 >;
> 	reg-offset = <0x1000>;
> 	reg-shift = <2>;
> 	...irq, etc
> } ;
> 
> This was the first version which is working but using current-speed is
> probably not the best solution.
> 
> I have also create version where baudrate is directly specified in
> chosen node. Clock frequency is still
> loaded directly from the node.
> 
> chosen {
> 	linux,stdout-path = "/axi at 0/serial at 40400000,115200";
> } ;
> 
> What solution is more compatible with device-tree?

Since nobody else has responded for a couple of days, I'll throw out the
opinion that I prefer the first solution. The second solution requires
you to do additional parsing and data validation which the device tree
has already done for you in the first approach. It's also consistent
with the ePAPR example in
https://www.power.org/resources/downloads/Power_ePAPR_APPROVED_v1.0.pdf.

> Michal
--
David VL