Hello,
On 23 June 2014 15:10, Arnd Bergmann arnd@arndb.de wrote:
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?
For CPPC, its a field for version number, number of entries and then followed by a bunch of PCC entries that have the following structure:
51 struct pcc_register_resource { 52 u8 descriptor; 53 u16 length; 54 u8 space_id; 55 u8 bit_width; 56 u8 bit_offset; 57 u8 access_size; 58 u64 address; 59 } __attribute__ ((packed));
These essentially describe the PCC register space to be used by the respective protocol. e.g. CPPC uses these to exchange CPU performance metrics between the OS and the firmware. I believe MPST and RASF also follow the same format.
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.
Right, a generic lookup method would be useful. I think we should probably revisit this option when/if there are ACPI cases which use anything other than the PCC mailbox controller.
But treating PCC special probably does make most sense here, at least the lookup path.
Agreed.
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.
Cheers, Ashwin