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

[arnd@arndb.de (Arnd Bergmann)]

On Friday 20 June 2014 15:29:18 Ashwin Chaugule wrote:
> 
> On 20 June 2014 15:08, Arnd Bergmann <arnd@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.

It's mostly a matter of consistency: We can have multiple interrupt
controllers, pin controllers, clock controllers, dma engines, etc,
and in the DT case we use references to the nodes wherever we have
other devices referring to a mailbox name.

I believe Intel's embedded chips are moving in the same direction
with their ACPI support. If the ACPI spec gains support for mailbox
devices, locking them into having only a single device may be
a problem later for them.

Note that "device" here doesn't have to mean a platform device that
is instantiated from DSDT, it can be any mailbox provider that is
registered in an arbitrary way, as long as you have a method to map
back from the (consumer-device, name-string) tuple back to the
(provider, channel) tuple. I have read your patch again now and noticed
that you actually tried to do this, but unfortunately you got it
wrong by requiring the consumer to fill out the name of the provider
in the request. You can't do that, because it's not generic enough
to support devices that can be reused, and it means that drivers
using the API are never portable between DT and ACPI. You have to
get rid of the "ctrl_name" field in the mbox_client structure and
change the lookup to be based only on cd->dev and cl->chan_name,
using whatever tables you have available in ACPI.

	Arnd