On 2014-7-28 17:07, Arnd Bergmann wrote:

On Saturday 26 July 2014 19:34:48 Olof Johansson wrote:

quoted

On Thu, Jul 24, 2014 at 6:00 AM, Hanjun Guo [off-list ref] wrote:

quoted

+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.

Possibly overriden by kernel bootargs. And as debated for quite a
while earlier this year, acpi should still default to off -- if a DT
and ACPI are both passed in, DT should at this time be given priority.

I think this would be harder to do with the way that ACPI is passed in
to the kernel. IIRC, you always have a minimal DT information based on
the ARM64 boot protocol, but in the case of ACPI, this contains pointers
to the ACPI tables, which are then used for populating the Linux platform
devices (unless acpi=disabled is set), while the other contents of the
DTB may be present but we skip the of_platform_populate state.

If this is correct, then replacing the firmware-generated dtb with a
user-provided on would implicitly remove the ACPI tables from visibility,
which is exactly what we want.

It's possible that I'm misremembering it though, and it should be
documented better.

quoted

quoted

+Regardless of whether DT or ACPI is used, the kernel must always be capable
+of booting with either scheme.

It should always be possible to compile out ACPI. There will be plenty
of platforms that will not implement it, so disabling CONFIG_ACPI
needs to be possible.

Right.

Actually, if platforms don't implement ACPI, acpi_disabled will always be set to
1 at early boot stage which before the device tree is unflattened, so device
tree will work as expected even if CONFIG_ACPI=y on such platforms.

Thanks
Hanjun