Re: [PATCH 2/4] x86/amd_nb: add support for newer PCI topologies

[Bjorn Helgaas <helgaas@kernel.org>]

On Wed, Nov 07, 2018 at 11:15:37AM -0800, Srinivas Pandruvada wrote:
> On Tue, 2018-11-06 at 17:20 -0600, Bjorn Helgaas wrote:
> > [+cc Sumeet, Srinivas for INT3401 questions below]
> > [Beginning of thread:
> > 
> https://lore.kernel.org/linux-pci/20181102181055.130531-1-brian.woods@amd.com/
> > ]
> > 
> > On Tue, Nov 06, 2018 at 11:00:59PM +0100, Borislav Petkov wrote:
> > > On Tue, Nov 06, 2018 at 03:42:56PM -0600, Bjorn Helgaas wrote:
> > > > This isn't some complicated new device where the programming
> > > > model changed on the new CPU.  This is a thermometer that was
> > > > already supported.  ACPI provides plenty of functionality that
> > > > could be used to support this generically, e.g., see
> > > > drivers/acpi/thermal.c,
> > > > drivers/thermal/int340x_thermal/processor_thermal_device.c,
> > > > etc.
> > > 
> > > Ok, you say ACPI but how do you envision practically doing that?
> > > I mean, this is used by old boxes too - ever since K8. So how do
> > > we go and add ACPI functionality to old boxes?
> > > 
> > > Or do you mean it should simply be converted to do
> > > pci_register_driver() with a struct pci_driver pointer which has
> > > all those PCI device IDs in a table? I'm looking at the last
> > > example
> > > drivers/thermal/int340x_thermal/processor_thermal_device.c you
> > > gave above.
> > 
> > No, there would be no need to change anything for boxes already in
> > the field.  But for *new* systems, you could make devices or
> > thermal zones in the ACPI namespace (they might even already be
> > there for use by Windows).
> > 
> > drivers/thermal/int340x_thermal/processor_thermal_device.c claims
> > either INT3401 ACPI devices or listed PCI devices. 
> 
> To enumerate a driver to get processor temperature and get power
> properties, we have two methods:
> -  The older Atom processors valleyview and Baytrail had no PCI device
> for the processor thermal management. There was INT3401 ACPI device to
> handle this.
> 
> - The newer processors for core and Atom, has a dedicate PCI device and
> there is no INT3401 ACPI device anymore.

This is really interesting because it's completely the reverse of what
I would have expected.

You use INT3401 on the *older* processors, where int3401_add() is
called for any platform devices with INT3401 PNP ID:

  int3401_add(plat_dev)                   # platform/ACPI .probe
    proc_thermal_add(dev)
      adev = ACPI_COMPANION(dev)
      int340x_thermal_zone_add(adev)
	thermal_zone_device_register()

The sensors are read in this path, where thermal_zone_get_temp() is
the generic thermal entry point:

  thermal_zone_get_temp()
    tz->ops->get_temp()
      int340x_thermal_get_zone_temp()    # ops.get_temp
	acpi_evaluate_integer(..., "_TMP", ...)

The above works for any platform that supplies the INT3401 device
because the _TMP method that actually reads the sensor is supplied by
the platform firmware.

On *newer* processors, you apparently use this path:

  proc_thermal_pci_probe(pci_dev)        # PCI .probe
    pci_enable_device(pci_dev)
    proc_thermal_add(dev)
      adev = ACPI_COMPANION(dev)
      int340x_thermal_zone_add(adev)
	thermal_zone_device_register()

Except for enabling the PCI device and a BSW_THERMAL hack, this is
exactly the *SAME*: you add a thermal zone for the ACPI device and
read the sensor using ACPI _TMP methods.

But now you have to add new PCI IDs (Skylake, Broxton, CannonLake,
CoffeeLake, GeminiLake, etc) for every new platform.  This seems like
a regression, not an improvement.  What am I missing?

> Since OEM systems will have different power properties and thermal
> trips, there is a companion ACPI device, which provides PPCC and
> thermal trips and optionally output from another temperature sensor
> from the default on processor cores.

Bjorn