Re: [PATCH 0/4] xen/arm: Unbreak ACPI

[Julien Grall <julien@xxxxxxx>]

Hi,

On 12/10/2020 20:02, Stefano Stabellini wrote:
> On Sat, 10 Oct 2020, Julien Grall wrote:
> > On 28/09/2020 07:47, Masami Hiramatsu wrote:
> > > Hello,
> > Hi Masami,
> >
> > > This made progress with my Xen boot on DeveloperBox (
> > > https://www.96boards.org/product/developerbox/ ) with ACPI.
> > I have reviewed the patch attached and I have a couple of remarks about it.
> >
> > The STAO table was originally created to allow an hypervisor to hide devices
> > from a controller domain (such as Dom0). If this table is not present, then it
> > means the OS/hypervisor can use any device listed in the ACPI table.
> >
> > Additionally, the STAO table should never be present in the host ACPI table.
> >
> > Therefore, I think the code should not try to find the STAO. Instead, it
> > should check whether the SPCR table is present.
> Yes, that makes sense, but that brings me to the next question.
>
> SPCR seems to be required by SBBR, however, Masami wrote that he could
> boot on a system without SPCR, which gets me very confused for two
> reasons:
>
> 1) Why there is no SPCR? Isn't it supposed to be mandatory? Is it
> because there no UART on Masami's system?
I can't comment specically on Masami's system, but I can make two broad 
comments:
   1) Not all the systems have to be compliant with the SBBR. But in 
theory, only systems passing the SBBR test can be claimed to be 
complained. This brings to the second point...
   2) "Mandatory" is what the specs aim to enforce. In my experience, 
some of the vendors may bend the rule and still claim they are compliant.
Even Linux documentation says that the SPCR is mandatory. But in the 
reality, the implementation will just ignore it.
From my understanding of the code, Linux will only use it to discover 
the preferred console and enable earlycon. Without it, Linux will 
require the user to specify console=<...> or earlycon=<...>.
The UART will then be discovered via the DSDT.

> 2) If there is no SPCR, how did Masami manage to boot Xen?
> I take without any serial output? Just with the framebuffer?
After his patch, Xen can boot without the SPCR. You will just see no 
logs. But I belive, he enabled earlyprintk for his platform.
Cheers,

--
Julien Grall