Re: [Xen-devel] Design doc of adding ACPI support for arm64 on Xen

[Julien Grall <julien.grall@xxxxxxxxxx>]

On 05/08/15 17:52, Stefano Stabellini wrote:
> On Wed, 5 Aug 2015, Stefano Stabellini wrote:
>> On Wed, 5 Aug 2015, Julien Grall wrote:
>>> On 05/08/15 14:03, Shannon Zhao wrote:
>>>>
>>>>
>>>> On 2015/8/5 20:48, Julien Grall wrote:
>>>>> On 05/08/15 12:49, Shannon Zhao wrote:
>>>>>>>>> That's great!
>>>>>>>>> Keep in mind that many ARM platforms have non-PCI busses, so I think
>>>>>>>>> we'll need an amba and a platform bus_notifier too, in addition to
>>>>>>> the
>>>>>>>>> existing pci bus notifier.
>>>>>>>>>
>>>>>>>
>>>>>>> Thanks for your reminding. I thought about amba. Since ACPI of current
>>>>>>> linux kernel doesn't support probe amba bus devices, so this
>>>>>>> bus_notifier will not be used at the moment. But there are some voice
>>>>>>> that we need to make ACPI support amba on the linux arm kernel mail
>>>>>>> list. And to me it doesn't matter to add the amba bus_notifier.
>>>>> This comment raised one question. What happen if the hardware has MMIO
>>>>> region not described in the ACPI?
>>>>>
>>>> This sounds weird. If a device is described in ACPI table, it will not
>>>> describe the MMIO region which the driver will use? Does this situation
>>>> exist?
>>>
>>> Buggy ACPI tables, not possible to describe the ACPI ... There is plenty
>>> of reason.
>>>
>>> I don't know if there is current problem on ACPI (I don't have much work
>>> with it). But it presents on the device tree. We may a lots of specific
>>> platform mapping in Xen (see specific_mapping) because of buggy DT.
>>>
>>> We can't rule out and needs to provide a way to cope with this. If we
>>> don't do it, it will fall on us sooner or later.
>>>
>>>> If the hardware has mmio region not described in the ACPI, how does the
>>>> driver know the region and use it?
>>>
>>> Hardcoded.
>>
>> I wouldn't worry about buggy drivers or tables for now.
>>
>> I think that the Linux maintainers should take care of that by refusing
>> to merge any drivers that do such a thing anyway.
> 
> If worse comes to worst, we can do the mapping from the platform file in
> Xen, which is bad, but would be appropriate to work around buggy
> firmware.

True. Although, I seem to remembered that it wasn't possible to get the
name of the platform (or something similar) in Xen with ACPI.

Anyway, let's wait until someone provides a such platform. I just wanted
to mention in case we find a better place to call the mapping hypercall.

Regards,

-- 
Julien Grall

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