Re: Xen on ARM DT based boot protocol

[Andre Przywara <andre.przywara@calxeda.com>]

On 05/23/2013 11:04 PM, Stefano Stabellini wrote:

Hi,

> On Thu, 23 May 2013, David Vrabel wrote:
>> On 23/05/13 13:39, Stefano Stabellini wrote:
>>> Hi Ian,
>>> Andre was pointing out to me today that the current boot protocol has a
>>> problem:
>>>
>>>
>>> chosen {
>>>                bootargs = "dom0_mem=128M console=com1,vga";
>>>                xen,dom0-bootargs = "earlyprintk=xenboot console=ttyAMA1 root=/dev/mmcblk0 debug rw init=/bin/bash";
>>> };
>>>
>>>
>>> because we are using bootargs for Xen arguments, the bootloader cannot
>>> just pass the same two lines to Xen on ARM and to Linux on ARM.
>>
>> I think it is unlikely the command line arguments for Linux are the same
>> for bare metal and dom0.  Even in this example here you have a
>> Xen-specific option (earlyprintk=xenboot).
>
> Actually that is not true, in fact in the example above the only
> difference is earlyprintk=xenboot that is just a debug option.

Interestingly Linux/ARM does not seem to expect a parameter to 
earlyprintk, the actual device to use is hardcoded in 
arch/arm/include/debug/<mach>.S and earlyprintk is just a switch. So 
earlyprintk=xen is broken anyway ;-)

But using bootargs for Dom0 could just be a fallback, so what about:

if xen,xen-bootargs is provided: use it for Xen, else use "";
if xen,dom0-bootargs is provided: use it for Dom0, else
	if bootargs is provided: use that for Dom0
write Dom0 cmdline into chosen/bootargs in Dom0's dtb

And is there a need for the "xen," prefix? I thought that this syntax is 
only needed for the compatible property, no?

So eventually there could be one dtb to rule them all:
chosen {
	xen,xen-bootargs = "...";
	xen,dom0-bootargs = "...";
	bootargs = "...";
}
My concern is not so much about the bootloader's understanding of the 
dtb, but more about the vendor provided one, which one would expect to 
be in flash ROM, for instance. We can provide only one here.

Will send a patch tomorrow.

Regards,
Andre.