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Re: [Xen-devel] [PATCH] arm/acpi: Add Server Base System Architecture UART support

Hello Shanker,

On 27/05/16 15:01, Shanker Donthineni wrote:
On 05/27/2016 08:04 AM, Julien Grall wrote:
On 27/05/16 01:28, Shanker Donthineni wrote:
The ARM Server Base System Architecture describes a generic UART
interface. It doesn't support clock control registers to set
baudrate. So, extend the driver probe() to handle SBSA interface
types and set the baudrate to 115200 for SBSA interfaces.

I cannot find any mention of the baudrate in the SBSA. Where does it come from?

Yes, no where mentioned about the baudrate in SBSA document. I used 115200 
based on the  the Linux  PL011 driver.

Looking at the Linux code, it is a default when there is no valid configuration (which may not be suitable for any platform?).

Whilst Linux userspace cares about the baudrate, Xen only use it to configure the hardware. Given that the UART should have been configured by the hardware-specific software, the proper value should be BAUD_AUTO.

If you look at the code, pl011_init_preirq will read the baud rate when uart->baud == BAUD_AUTO but will never be used after.

Also the driver is using registers which should not be touch for the SBSA UART 
(see appendix B in the SBSA ARM-DEN-0029 v3.0). So this need to be address to 
get a proper support.

Yes, I agree with you, Which registers it is touching other than baudrate 
control registers?

Well, I only gave a quick look and noticed the difference between the SBSA and the pl011 (e.g CR, IFLS,...) . I am sure you can find out the rest of the registers by looking at the code (pl011-uart.h) and the spec.


Julien Grall

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