Intel PMC driver on Broadwell system to gather C-State statistics

[Paul Menzel <pmenzel@molgen.mpg.de>]

Dear Linux folks,


The Intel Broadwell-U laptop Dell Latitude E7250 (BIOS A19 01/23/2018), 
according to PowerTOP, only reaches package C-State C7 and not C8, C9, 
C10, while the four CPUs itself do reach C-State C10 and CE.

I was asked to look at:

1.  `/sys/kernel/debug/pmc_core/package_cstate_show`
2.  `/sys/kernel/debug/pmc_core/ltr`

Trying to gather statistics, after the Debian Linux kernel 5.9.1 is now 
built with `INTEL_PMC_CORE=y`, `/sys/kernel/debug/pmc_core/` is still 
not created despite `sudo modprobe intel_pmc_core` being successful. 
(It’s not loaded automatically.)

     [ 1063.644680] calling  pmc_core_driver_init+0x0/0x1000 
[intel_pmc_core] @ 4252
     [ 1063.644721] initcall pmc_core_driver_init+0x0/0x1000 
[intel_pmc_core] returned 0 after 36 usecs

The ACPI device `INT33A1` is there.

>     Scope (_SB)
>     {
>         Device (PEPD)         
>         {
>             Name (_HID, "INT33A1" /* Intel Power Engine */)  // _HID: Hardware ID
>             Name (_CID, EisaId ("PNP0D80") /* Windows-compatible System Power Management Controller */)  // _CID: Compatible ID
>             Name (_UID, One)  // _UID: Unique ID
>             Name (PEPP, Zero)
>             Name (DEVS, Package (0x03)
>             {        
>                 0x02, 
>                 Package (0x01)
>                 {
>                     "\\_SB.PCI0.GFX0"
>                 },    
>             
>                 Package (0x01)        
>                 {                           
>                     "\\_SB.PCI0.SAT0.PRT1"
>                 }
>             })
>             Name (DEVX, Package (0x08)

The table `intel_pmc_core_ids` does not contain the Broadwell-U ID 
though, so I guess it’s not supported.

> $ lspci -nn
> 00:00.0 Host bridge [0600]: Intel Corporation Broadwell-U Host Bridge -OPI [8086:1604] (rev 09)
> 00:02.0 VGA compatible controller [0300]: Intel Corporation HD Graphics 5500 [8086:1616] (rev 09)
> 00:03.0 Audio device [0403]: Intel Corporation Broadwell-U Audio Controller [8086:160c] (rev 09)
> 00:04.0 Signal processing controller [1180]: Intel Corporation Broadwell-U Processor Thermal Subsystem [8086:1603] (rev 09)
> 00:14.0 USB controller [0c03]: Intel Corporation Wildcat Point-LP USB xHCI Controller [8086:9cb1] (rev 03)
> 00:16.0 Communication controller [0780]: Intel Corporation Wildcat Point-LP MEI Controller #1 [8086:9cba] (rev 03)
> 00:19.0 Ethernet controller [0200]: Intel Corporation Ethernet Connection (3) I218-LM [8086:15a2] (rev 03)
> 00:1b.0 Audio device [0403]: Intel Corporation Wildcat Point-LP High Definition Audio Controller [8086:9ca0] (rev 03)
> 00:1c.0 PCI bridge [0604]: Intel Corporation Wildcat Point-LP PCI Express Root Port #1 [8086:9c90] (rev e3)
> 00:1c.3 PCI bridge [0604]: Intel Corporation Wildcat Point-LP PCI Express Root Port #4 [8086:9c96] (rev e3)
> 00:1d.0 USB controller [0c03]: Intel Corporation Wildcat Point-LP USB EHCI Controller [8086:9ca6] (rev 03)
> 00:1f.0 ISA bridge [0601]: Intel Corporation Wildcat Point-LP LPC Controller [8086:9cc3] (rev 03)
> 00:1f.2 SATA controller [0106]: Intel Corporation Wildcat Point-LP SATA Controller [AHCI Mode] [8086:9c83] (rev 03)
> 00:1f.3 SMBus [0c05]: Intel Corporation Wildcat Point-LP SMBus Controller [8086:9ca2] (rev 03)
> 01:00.0 SD Host controller [0805]: O2 Micro, Inc. SD/MMC Card Reader Controller [1217:8520] (rev 01)
> 02:00.0 Network controller [0280]: Intel Corporation Wireless 7265 [8086:095a] (rev 59)

Any idea, why the probe function `pmc_core_probe()` succeeds, despite 
the code below?

         cpu_id = x86_match_cpu(intel_pmc_core_ids);
         if (!cpu_id)
                 return -ENODEV;

The watchdog driver iTCO_wdt seems to load the module `pmc_core_bxt` 
despite I am unable to find the ACPI device `INT34D2` in the dissembled 
AML/ASL files.


Kind regards,

Paul