On 06/30/2012 03:41 AM, David VomLehn (dvomlehn) wrote:
-----Original Message-----
From: devicetree-discuss [mailto:devicetree-discuss-
[email protected]] On Behalf Of Michal Simek
Sent: Tuesday, June 26, 2012 4:17 AM
To: [email protected]
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@0/serial@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"?
ok. Sorry not serial expert but please correct me.
I need to read required baudrate setting (9600,...,115200, etc) which is used
for communication(it should be numeric bits per second) and clock frequency.
And based on the code setups baud_divisor for the baud rate generator counter.
And based on ePAPR.. Specifies the current speed of a serial device in bits per
second.
A boot program should set this property if it has initialized the serial device.
RS232_Uart_1: serial@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@0/serial@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.
This is done in early phase which means that I don't believe that there is
any parsing done for the first approach too. It is simple value reading
with no checking.
And yes, current-speed should be used exactly for this purpose.
Thanks,
Michal
--
Michal Simek, Ing. (M.Eng)
w: www.monstr.eu p: +42-0-721842854
Maintainer of Linux kernel 2.6 Microblaze Linux - http://www.monstr.eu/fdt/
Microblaze U-BOOT custodian
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