Re: Consider changing CONFIG_ACPI default on ARM?

[Elliott Mitchell <ehem+xen@xxxxxxx>]

On Sat, Aug 23, 2025 at 08:59:13AM +0100, Julien Grall wrote:
> 
> On 22/08/2025 21:09, Elliott Mitchell wrote:
> > There may be many more using it.  Perhaps this
> > should even be done on the 4.20 branch given how long this has been
> > working?
> 
> I am guessing you mean 4.21 which will be released in a couple of months
> time, correct?
> 
> We have been discussing among the committers on whether we are ok to enable
> ACPI despite the fact it is still not feature complete (see above). The
> discussion is not fully finalized but if we were to enable CONFIG_ACPI=y by
> default then I think we would need the following:
> 
>  * Select device-tree by default rather than ACPI

This appears to be the case.  `arm_smmu_device_probe()` only calls
`arm_smmu_device_acpi_probe()` if the SMMUv3 is absent from the
device-tree.  The file appears laced with assumptions about device-tree
support being enabled so I think this is the present state.

>  * Go through SUPPORT.MD and check what features we marked as SUPPORTED.md
> but doesn't work on ACPI.
> 
> Maybe you can help with that?

The only things which stood out to me was SMMUv1/SMMUv2/SMMUv3.  Since
SMMUv3 is marked experimental anyway, this doesn't seem to be a problem.

Note, I haven't been looking too closely at things so my look isn't all
that much better than than an Ouija board.

Mostly it seems multiple people have found rebuilding Debian's packages
and enabling ACPI was the best way of getting Debian/Xen on a RP4.  The
lack of bugs seems to suggest ACPI works quite well.  The downsides are
limits in what ACPI supports and needing to rebuild.


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