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

[Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>]

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.
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.

Thanks,
Srinivas



>  It looks like it
> tries the platform (INT3401) devices first, and if it finds any, it
> ignores any matching PCI devices.  This *could* be so it uses INT3401
> devices on new platforms and falls back to the PCI devices otherwise,
> although there *are* recent updates that add PCI IDs.
> 
> It looks like INT3401 is Intel-specific since it uses a PPCC method
> which isn't defined by the ACPI spec.  But AMD could define a similar
> PNP ID and have new platforms expose ACPI devices with _TMP methods
> that know how to read the sensor on that platform.
> 
> Or maybe even drivers/acpi/thermal.c, which claims every Thermal Zone
> (ACPI 6.2, sec 11), would be sufficient.  I don't know what the
> relationship between hwmon and other thermal stuff, e.g.,
> Documentation/thermal/sysfs-api.txt is.  acpi/thermal.c looks tied
> into the drivers/thermal stuff (it registers "thermal_zone" devices),
> but not to hwmon.
> 
> > > But maybe there's some real value in the nitty-gritty device-
> > > specific
> > > code in amd_nb.c.  If so, I guess you're stuck with updates like
> > > this
> > > and negotiating with the distros to do backports and new
> > > releases.
> > 
> > Well, even if it is converted to a different registration scheme,
> > you
> > still need to add new PCI device IDs to the table, no? So *some*
> > sort of
> > enablement still needs to happen.
> 
> As long as we have a driver that knows how to claim a known PNP ID,
> and every new platform exposes a device compatible with that ID, the
> driver should just work.
> 
> Bjorn