Re: [Xen-devel] Design doc of adding ACPI support for arm64 on Xen - version 2

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Wed, 12 Aug 2015, Ian Campbell wrote:
> On Wed, 2015-08-12 at 11:04 +0800, Shannon Zhao wrote:
> > Hi Julien,
> > 
> > On 2015/8/12 0:19, Julien Grall wrote:
> > > Hi Shannon,
> > > 
> > > On 07/08/15 03:11, Shannon Zhao wrote:
> > > > 2. Create minimal DT to pass required information to Dom0
> > > > ----------------------------------------------------------
> > > > The minimal DT mainly passes Dom0 bootargs, address and size of 
> > > > initrd
> > > > (if available), address and size of uefi system table, address and 
> > > > size
> > > > of uefi memory table, uefi-mmap-desc-size and uefi-mmap-desc-ver.
> > > > 
> > > > An example of the minimal DT:
> > > > / {
> > > >     #address-cells = <2>;
> > > >     #size-cells = <1>;
> > > >     chosen {
> > > >         bootargs = "kernel=Image console=hvc0 
> > > > earlycon=pl011,0x1c090000
> > > > root=/dev/vda2 rw rootfstype=ext4 init=/bin/sh acpi=force";
> > > >         linux,initrd-start = <0xXXXXXXXX>;
> > > >         linux,initrd-end = <0xXXXXXXXX>;
> > > >         linux,uefi-system-table = <0xXXXXXXXX>;
> > > >         linux,uefi-mmap-start = <0xXXXXXXXX>;
> > > >         linux,uefi-mmap-size = <0xXXXXXXXX>;
> > > >         linux,uefi-mmap-desc-size = <0xXXXXXXXX>;
> > > >         linux,uefi-mmap-desc-ver = <0xXXXXXXXX>;
> > > >     };
> > > > };
> > > > 
> > > > For details loook at
> > > > https://github.com/torvalds/linux/blob/master/Documentation/arm/uefi.
> > > > txt
> > > 
> > > AFAICT, the device tree properties in this documentation are only used
> > > in order to communicate between the UEFI stub and Linux.
> > > 
> > > This means that those properties are not standardize and can change at
> > > any time by Linux folks. They don't even live in 
> > > Documentation/devicetree/
> > > 
> > > I would also expect to see the same needs for FreeBSD running as DOM0
> > > with ACPI.
> > > 
> > I'm not very clear about how FreeBSD communicates with UEFI. And when
> > booting with DT, how does FreeBSD communicate with UEFI? Not through
> > these properties?
> 
> These properties are in effect a Linux internal interface defined between
> the "Linux UEFI stub" and the "Linux kernel proper". The stub and the
> kernel are notionally separate entities, although they are in the same tree
> etc there is a well defined transition/entry point between the two. Since
> they are in the same tree even though they are in theory "separate" I
> expect they will tend to co-evolve.
> 
> IIRC we discussed with some of the maintainers (at Connect?) making this a
> more formal interface, i.e. exposing the entry point to "Linux kernel
> proper" which understands these properties to other than just the "Linux
> UEFI stub" specifically to external entities such as Xen.
> 
> Probably part of this work needs to formalise that, such as by moving this
> binding into the proper external bindings dir.
> 
> At which point BSD can (hopefully!) choose to support the same interface.

CCing Ard, who originally proposed this.

Yes, I think it would be good to submit a patch to Linux to formalize
the bindings and clear out that the interface has become external.

As a reference, this was the original thread:

http://marc.info/?l=linux-kernel&m=142362266626403&w=2

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