[Linaro-acpi] [RFC v2 1/3] Mailbox: Add support for ACPI

[Ashwin Chaugule]

Hi Arnd,

On 20 June 2014 15:08, Arnd Bergmann <arnd at arndb.de> wrote:
> On Friday 20 June 2014 14:55:16 Ashwin Chaugule wrote:
>> So, in order to get an mbox->dev for ACPI platforms, we'd need an
>> entry in the DSDT table. That seems rather pointless, since the DSDT
>> is reserved for devices and is supposed to be OS agnostic. Since the
>> mailbox controller itself is not really a "device" with a resource
>> descriptor, I dont see the point in adding a dummy DSDT entry for the
>> sake of getting this `struct device`. Also, I'm told adding new
>> entries to this table requires registering a unique 4 character
>> identifier and approval from some committees. If there are other ways
>> to get this structure I'd like to hear about it.
>>
>> The other alternative would be to piggy back on the ACPI CPU detection
>> code, which looks for the ACPI0007 device node in the DSDT and use
>> that as the mbox controller device. This node is already registered
>> and is an established method to detect CPUs. But I'm not sure what
>> happens when CPUs are hotplugged off, we surely dont want mailbox
>> clients such as PCC to break.
>
> The main question here is whether you expect having to support multiple
> mailbox devices in an ACPI system. If you think there is never more than
> one, you wouldn't need a DSDT entry, but if you can end up in a situation
> where another device needs to specify which mailbox it is using, then
> you need that entry anyway.

At this point, I dont see the need for multiple mailbox devices. But
I'm not seeing why we'd need a DSDT entry only if there are more than
one mailbox devices? I'd obviously prefer not having a DSDT entry for
this, and the patch I posted is the only way I could see to keep DT
and ACPI mbox supported at runtime without DSDT involved. Please let
me know if there are better ways.

Cheers,
Ashwin