On Monday 23 June 2014 14:25:26 Ashwin Chaugule wrote:

Hi Arnd,

On 21 June 2014 05:34, Arnd Bergmann [off-list ref] wrote:

quoted

quoted

linux/mailbox_client.h

 18  * struct mbox_client - User of a mailbox
 19  * @dev:        The client device
 20  * @chan_name:      The "controller:channel" this client wants

Please correct me if I'm mistaken, but I think this comment in the
header is confusing.

Yes, definitely. Thanks for pointing that out.

It gives the impression that the user is expected
to fill in this field as "controller name: channel id". But, looking
at an example of a DT based mbox client [1] , that doesnt seem to be
the case. And "chan_name" is compared with "mbox-names", which seems
to contain a list of Channel names. The mailbox is then identified by
a separate DT binding : "mbox", which has the mailbox name and the
channel id. So shouldnt this comment not say anything about the
"controller" and the DT binding should be changed to "channel-names",
instead of "mbox-names" to keep things consistent?

The comment should be changed, but the property name is good the
way it is. We follow the exact same pattern we have for registers,
interrupts, dma-channels, etc.

quoted

However, it seems you still make the same mistake here: The name that
gets passed as chan_name in the mailbox API is a local identifier
that is supposed to be interpreted for the client device and used
to look up a pointer to the mailbox device and channel. If you require
drivers to put global data (e.g. the mbox->name, or the channel
number) in there, it's impossible to write a driver that works on
both DT and ACPI. If you want to use the mbox_request_channel()
interface from a driver, you need some form of lookup table in
the ACPI data to do the conversion.

Fair point. The more I think about this, it seems that if we want to
use the mailbox framework for ACPI kernels, we should have a PCC
specific bypass, something like the one you suggested below. The ACPI
spec defines PCC as the only "mailbox" like mechanism. There are 3 PCC
clients defined as well; CPPC, MPST and RASF. Each of these have their
own ACPI tables and so they dont require special DSDT entries.

Ok, I see. Can you describe what data is in these tables?

Moreover, these PCC client drivers will be very ACPI specific anyway.
So, trying to emulate DT like mbox controller-client matching in ACPI
at this point is rather pointless. It will require creating dummy DSDT
entries for the PCC mailbox controller and PCC mailbox clients which
have their own well defined ACPI tables (and so dont belong in the OS
agnostic DSDT) and then coming up with customized Device Specific
Methods (DSMs) for mbox clients to refer to mbox controllers.

Actually you wouldn't necessarily need DSDT entries, the ACPI core could
just call platform_device_create() to instantiate the devices based on
the PCC tables.

The other alternative is to skip the mailbox framework altogether. One
thing to note is that the PCC driver and its clients should work on
X86, ARMv8 and any other platform that has ACPI support. Currently the
Mailbox framework looks platform agnostic but is tied to DT, so it may
not work well for everyone. But like I mentioned early on, the
framework provides for async notification and queuing which is useful
for PCC, so I'd prefer the PCC specific bypass option.

The mailbox API should still work fine without DT, it would be easy
enough to add a lookup mechanism for architectures that create their
own platform devices from hardcoded kernel structures, or from ACPI
tables that are meant to emulate the DT bindings on embedded x86.

But treating PCC special probably does make most sense here, at
least the lookup path.

quoted

The alternative would be not to use mbox_request_channel() at all
for now, but to add a new interface that can only be used PCC and
that matches by ID but is independent of the use of ACPI or DT,
something like:

struct mbox_chan *pcc_mbox_get_channel(struct mbox_client *cl,
                        char *name, unsigned chan_id,
                        struct mbox_chan **chan)
{
        struct mbox_controller *mbox;
        mbox = mbox_find_pcc_controller(name, ...);

        *chan = &mbox->chans[chan_id];
        return init_channel(*chan, cl);
}

This would mean that we'd have to special-case "pcc" users, which is
not very nice, but at least it would work on both DT and ACPI,
and a future ACPI version could still add support for the mailbox
API later.

I'll play around with this idea a bit and see how it looks.

Ok, thanks for looking into this.

	Arnd