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SubjectRe: [PATCH V4 2/2] PCI: vmd: Override ASPM on TGL/ADL VMD devices
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On Thu, 2021-12-16 at 11:26 -0600, Bjorn Helgaas wrote:
> [+cc Rajat for LTR max latency write]
>
> On Wed, Dec 15, 2021 at 09:56:00PM -0800, David E. Box wrote:
> > From: Michael Bottini <michael.a.bottini@linux.intel.com>
> >
> > On Tiger Lake and Alder Lake platforms, VMD controllers do not have ASPM
> > enabled nor LTR values set by BIOS. This leads high power consumption on
> > these platforms when VMD is enabled as reported in bugzilla [1].  Enable
> > these features in the VMD driver using pcie_aspm_policy_override() to set
> > the ASPM policy for the root ports.
>
> s/leads high/leads to high/
>
> Does this depend on "Tiger Lake and Alder Lake platforms"
> specifically, or does it depend on a BIOS design choice, i.e., "don't
> configure ASPM or LTR for devices below a VMD"?

It was a BIOS design choice to have this done by the OS driver for both Tiger Lake and Alder Lake.

>
> The subject says "override ASPM on VMD devices," but it looks like
> this affects the ASPM configuration of devices *below* the VMD, not of
> the VMD itself.

Yes.

>
> It looks like this only affects *NVMe* devices, since
> vmd_enable_aspm() checks for PCI_CLASS_STORAGE_EXPRESS.  Why is that?
> Is there something special about NVMe?  I'd think you would want to do
> this for *all* devices below a VMD.

We need this for all devices under the PCIe root ports below VMD. Those will only be NVMe device
AFAICS.

>
> Since it only affects PCI_CLASS_STORAGE_EXPRESS devices, I don't think
> it actually "sets ASPM policy for the root ports".  vmd_enable_aspm()
> calls pcie_aspm_policy_override() on endpoints.  It's true that the
> link ASPM state happens to be attached to the upstream end of the
> link, but that's an ASPM implementation detail.
>
> This all needs to be clear in the subject and commit log.

Ack

>
> > To do this, add an additional flag in VMD features to specify devices that
> > must have their respective policies overridden.
>
> I'm not clear on why you want this to apply to only certain VMDs and
> not others.  Do some BIOSes configure ASPM for devices below some
> VMDs?

Not currently. But the plan is for future devices to move back to having BIOS do the programming.

>
> > [1] https://bugzilla.kernel.org/show_bug.cgi?id=213717
> >
> > Signed-off-by: Michael Bottini <michael.a.bottini@linux.intel.com>
> > Signed-off-by: David E. Box <david.e.box@linux.intel.com>
> > Tested-by: Adhitya Mohan <me@adhityamohan.in>
> > ---
> > V4
> >  - Refactor vmd_enable_apsm() to exit early, making the lines shorter
> >    and more readable. Suggested by Christoph.
> > V3
> >  - No changes
> > V2
> >  - Use return status to print pci_info message if ASPM cannot be enabled.
> >  - Add missing static declaration, caught by lkp@intel.com
> >
> >  drivers/pci/controller/vmd.c | 43 +++++++++++++++++++++++++++++++++---
> >  1 file changed, 40 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/pci/controller/vmd.c b/drivers/pci/controller/vmd.c
> > index a45e8e59d3d4..880afd450a14 100644
> > --- a/drivers/pci/controller/vmd.c
> > +++ b/drivers/pci/controller/vmd.c
> > @@ -20,6 +20,8 @@
> >  
> >  #include <asm/irqdomain.h>
> >  
> > +#include "../pci.h"
> > +
> >  #define VMD_CFGBAR     0
> >  #define VMD_MEMBAR1    2
> >  #define VMD_MEMBAR2    4
> > @@ -67,6 +69,12 @@ enum vmd_features {
> >          * interrupt handling.
> >          */
> >         VMD_FEAT_CAN_BYPASS_MSI_REMAP           = (1 << 4),
> > +
> > +       /*
> > +        * Device must have ASPM policy overridden, as its default policy is
> > +        * incorrect.
> > +        */
> > +       VMD_FEAT_QUIRK_OVERRIDE_ASPM            = (1 << 5),
>
> I think you specifically want to *enable* some ASPM link states, not
> just "override the default policy."  "Override" tells us nothing about
> whether you are enabling or disabling ASPM.  Applies to subject line
> as well.

Ack

>
> >  };
> >  
> >  static DEFINE_IDA(vmd_instance_ida);
> > @@ -661,6 +669,30 @@ static int vmd_alloc_irqs(struct vmd_dev *vmd)
> >         return 0;
> >  }
> >  
> > +/*
> > + * Override the BIOS ASPM policy and set the LTR value for PCI storage
> > + * devices on the VMD bride.
>
> I don't think there's any BIOS "policy" here.  At this point BIOS is
> no longer involved at all, so all that's left is whatever ASPM config
> the BIOS did or did not do.
>
> Why only storage?

Only storage devices will be on these root ports.

>
> s/bride/bridge/
>
> > + */
> > +static int vmd_enable_aspm(struct pci_dev *pdev, void *userdata)
> > +{
> > +       int features = *(int *)userdata, pos;
> > +
> > +       if (!(features & VMD_FEAT_QUIRK_OVERRIDE_ASPM) ||
> > +           pdev->class != PCI_CLASS_STORAGE_EXPRESS)
> > +               return 0;
> > +
> > +       pos = pci_find_ext_capability(pdev, PCI_EXT_CAP_ID_LTR);
> > +       if (!pos)
> > +               return 0;
> > +
> > +       pci_write_config_word(pdev, pos + PCI_LTR_MAX_SNOOP_LAT, 0x1003);
> > +       pci_write_config_word(pdev, pos + PCI_LTR_MAX_NOSNOOP_LAT, 0x1003);
>
> 1) Where did this magic 0x1003 value come from?  Does that depend on
> the VMD device?  The endpoint?  The circuit design?  The path between
> endpoint and VMD?  What if there are switches in the path?

The number comes from the BIOS team. They are tied to the SoC. I don't believe there can be switches
in the path but Nirmal and Jonathan should know for sure. From what I've seen these root ports are
wired directly to M.2 slots on boards that are intended for storage devices.

>
> 2) There exist broken devices where WORD config accesses don't work:
> https://lore.kernel.org/all/20211208000948.487820-1-rajatja@google.com/
>
> We might need a way to quirk config accesses to those devices, but we
> don't have one yet.  So for now this needs to be a single DWORD write.

Ack

>
> > +       if (pcie_aspm_policy_override(pdev))
> > +               pci_info(pdev, "Unable of override ASPM policy\n");
>
> s/Unable of/Unable to/
>
> I think we might need a message about when we *do* override the
> policy.  A note in dmesg might be useful for debugging.  I'm worried
> about the LTR programming because I really don't understand how we
> should be doing that.

Sure.

>
> > +       return 0;
> > +}
> > +
> >  static int vmd_enable_domain(struct vmd_dev *vmd, unsigned long features)
> >  {
> >         struct pci_sysdata *sd = &vmd->sysdata;
> > @@ -807,6 +839,8 @@ static int vmd_enable_domain(struct vmd_dev *vmd, unsigned long features)
> >         pci_scan_child_bus(vmd->bus);
> >         pci_assign_unassigned_bus_resources(vmd->bus);
> >  
> > +       pci_walk_bus(vmd->bus, vmd_enable_aspm, &features);
>
> Do you support hotplug under VMD?  This will not happen for hot-added
> devices.

That I don't know. Nirmal, Jonathan?

David

>
> >         /*
> >          * VMD root buses are virtual and don't return true on pci_is_pcie()
> >          * and will fail pcie_bus_configure_settings() early. It can instead be
> > @@ -948,15 +982,18 @@ static const struct pci_device_id vmd_ids[] = {
> >         {PCI_DEVICE(PCI_VENDOR_ID_INTEL, 0x467f),
> >                 .driver_data = VMD_FEAT_HAS_MEMBAR_SHADOW_VSCAP |
> >                                 VMD_FEAT_HAS_BUS_RESTRICTIONS |
> > -                               VMD_FEAT_OFFSET_FIRST_VECTOR,},
> > +                               VMD_FEAT_OFFSET_FIRST_VECTOR |
> > +                               VMD_FEAT_QUIRK_OVERRIDE_ASPM,},
> >         {PCI_DEVICE(PCI_VENDOR_ID_INTEL, 0x4c3d),
> >                 .driver_data = VMD_FEAT_HAS_MEMBAR_SHADOW_VSCAP |
> >                                 VMD_FEAT_HAS_BUS_RESTRICTIONS |
> > -                               VMD_FEAT_OFFSET_FIRST_VECTOR,},
> > +                               VMD_FEAT_OFFSET_FIRST_VECTOR |
> > +                               VMD_FEAT_QUIRK_OVERRIDE_ASPM,},
> >         {PCI_DEVICE(PCI_VENDOR_ID_INTEL, PCI_DEVICE_ID_INTEL_VMD_9A0B),
> >                 .driver_data = VMD_FEAT_HAS_MEMBAR_SHADOW_VSCAP |
> >                                 VMD_FEAT_HAS_BUS_RESTRICTIONS |
> > -                               VMD_FEAT_OFFSET_FIRST_VECTOR,},
> > +                               VMD_FEAT_OFFSET_FIRST_VECTOR |
> > +                               VMD_FEAT_QUIRK_OVERRIDE_ASPM,},
> >         {0,}
> >  };
> >  MODULE_DEVICE_TABLE(pci, vmd_ids);
> > --
> > 2.25.1
> >


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