Subject: [Linaro-acpi] [RFC] ACPI on arm64 TODO List

From: [email-masked] (Al Stone)
Date: Tue, 16 Dec 2014 17:37:22 -0700

On 12/16/2014 08:48 AM, Mark Rutland wrote:
>>> 4. Set clear expectations for those providing ACPI for use with Linux
>>>    * Problem:
>>>      * Hardware/Firmware vendors can and will create ACPI tables that
>>>        cannot be used by Linux without some guidance
>>>      * Kernel developers cannot determine whether the kernel or firmware
>>>        is broken without knowing what the firmware should do
>>>    * Solution: document the expectations, and iterate as needed.
>>>      Enforce when we must.
>>>    * Status: initial kernel text available; AMD has offered to make
>>>      their guidance document generic; firmware summit planned for
>>>      deeper discussions.
>>
>> After seeing the AMD patches, I would add to this point that we need to
>> find a way to come up with shared bindings for common hardware such as
>> the ARM pl011/pl022/pl061/pl330 IP blocks or the designware
>> i2c/spi/usb3/ahci blocks.
>>
>> What I remember from this item on your list is actually a different
>> problem: We need to define more clearly what kinds of machines we
>> would expect ACPI support for and which machines we would not.
>>
>> Fine-grained clock support is one such example: if anybody needs
>> to expose that through a clock driver in the kernel, we need to be
>> very clear that we will not support that kind of machine through
>> ACPI, so we don't get developers building BIOS images that will
>> never be supported. Other examples would be non-compliant PCI
>> hosts or machines that are not covered by SBSA.
> 
> [...]
> 
>>> 6. How does the kernel handle_DSD usage?
>>>    * Problem:
>>>      * _DSD defines key-value properties in the DT style. How do we
>>>        ensure _DSD bindings are well defined?
>>>      * How do we ensure DT and _DSD bindings remain consistent with
>>>        each other?
>>>    * Solution: public documentation for all bindings, and a process
>>>      for defining them
>>>    * Status: proposal to require patch authors to point at public
>>>      binding documentation; kernel Documentation/devicetree/bindings
>>>      remains the default if no other location exists; UEFI forum has
>>>      set up a binding repository.
>>
>> I think we also need to make a decision here on whether we want to use
>> PRP0001 devices on ARM64 servers, and to what degree. I would prefer
>> if we could either make them required for any devices that already have
>> a DT binding and that are not part of the official ACPI spec, or we
>> decide to not use them at all and make any PRP0001 usage a testcase
>> failure.
> 
> I am rather concerned about the relationship between items described
> with _DSD and ACPI's existing device model. Describing the relationship
> between devices and their input clocks, regulators, and so on defeats
> much of the benefit ACPI is marketed as providing w.r.t. abstraction of
> the underlying platform (and as Arnd mentioned above, that's not the
> kind of platform we want to support with ACPI).

My belief is that all those things should be set up into a known good
state by UEFI on initial boot.  If they need to change, say as the
result of going into a deeper sleep state or something, that's what
the ACPI power management objects are for; Linux would execute one of
the ACPI methods already defined by the spec to control transition to
the desired state, and that method would have within it the ability
to change whatever clocks or regulators it deems necessary.  The
kernel should not have to track these things.

If someone is describing all those relationships in _DSD, I agree that
is not the kind of ARM platform I think we want to deal with.  This is
touched on, iirc, in arm-acpi.txt, but apparently too briefly.

> I have not seen good guidelines on the usage of _DSD in that respect,
> and I'm worried we'll end up with clock controllers half-owned by AML
> and half-owned by the kernel's clock framework, and this separation
> varying from board to board (and FW revision to FW revision). I think
> that needs to be clarified at the ACPI spec level in addition to
> anything we have in the kernel documentation.

Hrm.  The spec (section 6.2.5) basically says that there exists a
thing called _DSD and that when invoked, it returns a UUID followed
by a data structure.  A separate document (on http://www.uefi.org/acpi)
for each UUID defines the data structure it corresponds to.  The only
one defined so far is for device properties:

http://www.uefi.org/sites/default/files/resources/_DSD-implementation-guide-toplevel.htm

I think you are suggesting good guidelines for both, yes?  The usage
of new UUIDs and data structures, along with the usage of device
properties for the UUID we have, right?  I've been trying to write
such a thing but all I've accomplished so far is throwing away a
couple of drafts that got ugly.  I'll keep at it, though.

> I'm still of the opinion that conflating _DSD and DT is a bad idea.

Could you explain your usage of "conflating" here?  I think I understand
what you mean, but I'd rather be sure.

-- 
ciao,
al
-----------------------------------
Al Stone
Software Engineer
Linaro Enterprise Group
al.stone at linaro.org
-----------------------------------