On Monday 23 June 2014 15:46:08 Ashwin Chaugule wrote:

Hello,

On 23 June 2014 15:10, Arnd Bergmann [off-list ref] wrote:

quoted

quoted

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.

Interesting. So I guess it's one entry per client of the PCC? How
exactly does the client know which index to use in this table?

	Arnd