Re: [PATCH v6 17/17] Documentation: ACPI for ARM64

[Stefano Stabellini]

On Wed, 21 Jan 2015, Hanjun Guo wrote:
> Hi Christoffer,
> 
> Sorry for the late reply, I got no answer yet but
> with one question below.
> 
> On 2015å01æ20æ 04:33, Christoffer Dall wrote:
> > Hi,
> > 
> > On Sun, Jan 04, 2015 at 06:55:18PM +0800, Hanjun Guo wrote:
> > > From: Graeme Gregory <graeme.gregory@xxxxxxxxxx>
> > > 
> > > Add documentation for the guidelines of how to use ACPI
> > > on ARM64.
> > > 
> > > Reviewed-by: Suravee Suthikulpanit <Suravee.Suthikulpanit@xxxxxxx>
> > > Signed-off-by: Graeme Gregory <graeme.gregory@xxxxxxxxxx>
> > > Signed-off-by: Al Stone <al.stone@xxxxxxxxxx>
> > > Signed-off-by: Hanjun Guo <hanjun.guo@xxxxxxxxxx>
> > > ---
> > >   Documentation/arm64/arm-acpi.txt | 327
> > > +++++++++++++++++++++++++++++++++++++++
> > >   1 file changed, 327 insertions(+)
> > >   create mode 100644 Documentation/arm64/arm-acpi.txt
> > > 
> > > diff --git a/Documentation/arm64/arm-acpi.txt
> > > b/Documentation/arm64/arm-acpi.txt
> > > new file mode 100644
> > > index 0000000..21e7020
> > > --- /dev/null
> > > +++ b/Documentation/arm64/arm-acpi.txt
> > > @@ -0,0 +1,327 @@
> > > +ACPI on ARMv8 Servers
> > > +---------------------
> > > +ACPI can be used for ARMv8 general purpose servers designed to follow
> > > +the ARM SBSA (Server Base System Architecture) and SBBR (Server Base
> > > +Boot Requirements) specifications, currently available to those with
> > > +an ARM login at http://silver.arm.com.
> > > +
> > > +The ARMv8 kernel implements the reduced hardware model of ACPI version
> > > +5.1 and later.  Links to the specification and all external documents
> > > +it refers to are managed by the UEFI Forum.  The specification is
> > > +available at http://www.uefi.org/specifications and external documents
> > > +can be found via http://www.uefi.org/acpi.
> > > +
> > > +If an ARMv8 system does not meet the requirements of the SBSA, or cannot
> > > +be described using the mechanisms defined in the required ACPI
> > > specifications,
> > > +then it is likely that Device Tree (DT) is more suitable than ACPI for
> > > the
> > > +hardware.
> > > +
> > > +
> > > +Relationship with Device Tree
> > > +-----------------------------
> > > +ACPI support in drivers and subsystems for ARMv8 should never be mutually
> > > +exclusive with DT support at compile time.
> > > +
> > > +At boot time the kernel will only use one description method depending on
> > > +parameters passed from the bootloader (including kernel bootargs).
> > > +
> > > +Regardless of whether DT or ACPI is used, the kernel must always be
> > > capable
> > > +of booting with either scheme (in kernels with both schemes enabled at
> > > compile
> > > +time).
> > > +
> > > +
> > > +Booting using ACPI tables
> > > +-------------------------
> > > +The only defined method for passing ACPI tables to the kernel on ARMv8
> > > +is via the UEFI system configuration table.
> > > +
> > 
> > This is a bit concerning for the approach we are currently taking to
> > support ACPI on Xen [1].
> > 
> > Background: Xen is a tiny hypervisor which cannot parse the DSDT or any
> > other non-static table. Xen relies on Linux in Dom0 to manage most
> > (basically everything except the GIC, serial port, SMMU, and timers)
> > hardware resources and relies on Dom0 to parse the ACPI tables.
> > 
> > While Xen itself is typically booted by UEFI and finds RSDP through the
> > UEFI system table, Xen does NOT run another UEFI instance to boot Dom0
> > nor does it pass through the presence of UEFI to Dom0 in any way.
> > Instead, it just created a DT (with modifications concerning the
> > hardware mentioned above) and boots Linux in Dom0 directly (on x86 any
> > required UEFI call is performed through paravirtualized hypercalls).
> > 
> > In the case of ACPI, Xen adds a property to the chosen node and
> > populates it with the RDSP in Dom0's address space.  Patches have not
> 
> Sorry I'm not familiar with hypervisor, I have question here about
> x86 on xen, how ACPI works on XEN for x86?
 
The whole thing works differently on x86: on native x86 the OS finds the
RDSP by scanning a couple of memory regions looking for the string "RSD
PTR".

Xen and Dom0 both do that independently (Dom0 can map that memory
region).