Re: [RFC][PATCH v2]: PCI: PCIe links may not get configured for ASPMunder POWERSAVE mode

[Kenji Kaneshige]

(2011/03/18 23:52), Chumbalkar, Nagananda wrote:
> > -----Original Message-----
> > From: Kenji Kaneshige [mailto:kaneshige.kenji@xxxxxxxxxxxxxx]
> > Sent: Friday, March 18, 2011 12:29 AM
> > To: Chumbalkar, Nagananda
> > Cc: jbarnes@xxxxxxxxxxxxxxxx; rjw@xxxxxxx; mjg59@xxxxxxxxxxxxx; linux-
> > kernel@xxxxxxxxxxxxxxx; linux-pci@xxxxxxxxxxxxxxx
> > Subject: Re: [RFC][PATCH v2]: PCI: PCIe links may not get configured for
> > ASPM under POWERSAVE mode
> >
> > (2011/03/18 13:21), Naga Chumbalkar wrote:
> > > v2 ->   v1:
> > > 	. Kept the logic in pci_raw_set_power_state
> > > 	. Changed the ASPM enabling logic
> > > 	. Modified the text that describes the problem
> > >
> > > v1	: http://marc.info/?l=linux-pci&m=130013164703283&w=2
> > >
> > > The assumption made in commit 41cd766b065970ff6f6c89dd1cf55fa706c84a3d
> > > (PCI: Don't enable aspm before drivers have had a chance to veto it)
> > that
> > > pci_enable_device() will result in re-configuring ASPM when
> > aspm_policy is
> > > POWERSAVE is no longer valid.  This is due to commit
> > > 97c145f7c87453cec90e91238fba5fe2c1561b32 (PCI: read current power
> > state
> > > at enable time) which resets dev->current_state to D0. This makes the
> > > equality check (below) become true, so pcie_aspm_pm_state_change()
> > never
> > > gets called.
> > > ./drivers/pci/pci.c: pci_raw_set_pci_power_state()
> > > 546         /* Check if we're already there */
> > > 547         if (dev->current_state == state)
> > > 548                 return 0;
> > >
> > > So OSPM doesnn't configure the PCIe links for ASPM.
> > >
> > > The patch below does the following:
> > > At the end of each Root Bridge scan make a call to configure ASPM when
> > the
> > > ASPM policy is set to "powersave" mode. Note that if a previous pass
> > had
> > > completed the configuration for all devices under that Bridge then the
> > > configuration will not take place again because
> > pcie_config_aspm_link()
> > > checks to see if the link is already in the requested state.
> > > We won't reconfigure ASPM when _OSC control is not granted.
> >
> > Which _OSC controls do we need for configuring ASPM?
>
> There is no _OSC Control bit for ASPM. However, we expect the BIOS to
> support _OSC for a Root Bridge that originates a PCIe hierarchy. If this
> is not the case - would it be okay to disable ASPM also?
>
> Commit 852972acff8f10f3a15679be2059bb94916cba5d (ACPI: Disable ASPM if the
> Platform won't provide _OSC control for PCIe) describes the above scenario.
>
> To quote from there:
> The PCI SIG documentation for the _OSC OS/firmware handshaking interface
> states:
>
> "If the _OSC control method is absent from the scope of a host bridge
> device, then the operating system must not enable or attempt to use any
> features defined in this section for the hierarchy originated by the host
> bridge."
>
> The obvious interpretation of this is that the OS should not attempt to use
> PCIe hotplug, PME or AER - however, the specification also notes that an
> _OSC method is *required* for PCIe hierarchies, and experimental validation
> with An Alternative OS indicates that it doesn't use any PCIe functionality
> if the _OSC method is missing. That arguably means we shouldn't be using
> MSI or extended config space, but right now our problems seem to be limited
> to vendors being surprised when ASPM gets enabled on machines when other
> OSs refuse to do so. So, for now, let's just disable ASPM if the _OSC
> method doesn't exist or refuses to hand over PCIe capability control.

I understood. Thank you for clarification.

Regards,
Kenji Kaneshige

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