Re: [PATCH v2 4/6] ACPI: ARM: exclude DMI calls

[Al Stone <al.stone@linaro.org>]

On 11/25/2013 11:01 AM, Al Stone wrote:
> On 11/25/2013 10:45 AM, Matthew Garrett wrote:
>> On Mon, Nov 25, 2013 at 10:43:27AM -0700, Al Stone wrote:
>>> On 11/25/2013 08:30 AM, Matthew Garrett wrote:
>>>> Is ACPI_REDUCED_HARDWARE supposed to indicate support for the reduced
>>>> hardware profile, or that the platform *only* implements the reduced
>>>> hardware profile?
>>>
>>>  From what I can see in ACPICA, ACPI_REDUCED_HARDWARE indicates the
>>> platform *only* implements the reduced hardware profile.  This *seems*
>>> to be consistent with the specification -- see 3.11.1, second bullet,
>>> for example:
>>
>> Ok, so a kernel built without ACPI_REDUCED_HARDWARE would still support
>> the reduced hardware profile?
>
> Let me check on that.  The reduced hardware profile is a pretty
> strict subset, and I think I can see a way where I could force
> selecting the reduced HW profile on boot if the kernel has been
> built *without* ACPI_REDUCED_HARDWARE.  What I am not convinced
> of is that all of the proper guards are in place in ACPICA -- I
> trust that they have been, but I would like to double check.
>
> If I can get that to work properly, I'll add it to this patch set.

I thought I could get this to work, but I am going to have to defer
to the ACPICA upstream folks.  For the time being, I think that all
of the architectures that want to use ACPI in either legacy mode or
in stripped down reduced HW mode will have to use different kernels,
one for each mode.  I thought there might be enough safety checks to
allow the kernel to boot in legacy mode and switch into reduced HW at
boot, but there are not, in my opinion.  I don't see a way to make the
switch *and* maintain conformance with the spec without significant
change to ACPICA itself.

For example, enforcing that various functions are not allowed while
in reduced HW mode could be done by a check of the reduced HW flag on
entry.  If it is set, return the value the function would have returned
had it been stubbed out for reduced HW mode.  This is simple enough but
to do so would require modifying at least 29 functions in ACPICA, by my
count, not something upstream is particularly keen on -- nor am I.  I'd
rather step back and work with ACPICA over the longer term and see if
there's some way to get this functionality implemented properly instead
of trying to bolt it on somehow.

>>> ...if by "not supported" one takes that to mean "does not exist when
>>> compiled."  I can look at the ACPICA code again, just the same; perhaps
>>> there is some reasonable way to at least select one or the other at boot
>>> as the first step, and then allow switching between modes as a later
>>> step.
>>
>> I don't think you'd ever want to switch after init time. There's a flag
>> in the FADT that indicates whether a system is implementing the reduced
>> hardware profile or not.
>>
>
> Agreed.  I could see it as something to use when experimenting perhaps,
> but I think just allowing the switch at boot would cover the majority
> of the use cases.
>


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