Re: [PATCH] PCI/ASPM: Enable ASPM for links under VMD domain

[Derrick, Jonathan]

Hi Bjorn

On Wed, 2020-09-09 at 20:55 -0500, Bjorn Helgaas wrote:
> [+cc Saheed]
> 
> On Fri, Aug 21, 2020 at 08:32:20PM +0800, Kai-Heng Feng wrote:
> > New Intel laptops with VMD cannot reach deeper power saving state,
> > renders very short battery time.
> > 
> > As BIOS may not be able to program the config space for devices under
> > VMD domain, ASPM needs to be programmed manually by software. This is
> > also the case under Windows.
> > 
> > The VMD controller itself is a root complex integrated endpoint that
> > doesn't have ASPM capability, so we can't propagate the ASPM settings to
> > devices under it. Hence, simply apply ASPM_STATE_ALL to the links under
> > VMD domain, unsupported states will be cleared out anyway.
> > 
> > Signed-off-by: Kai-Heng Feng <kai.heng.feng@xxxxxxxxxxxxx>
> > ---
> >  drivers/pci/pcie/aspm.c |  3 ++-
> >  drivers/pci/quirks.c    | 11 +++++++++++
> >  include/linux/pci.h     |  2 ++
> >  3 files changed, 15 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/pci/pcie/aspm.c b/drivers/pci/pcie/aspm.c
> > index 253c30cc1967..dcc002dbca19 100644
> > --- a/drivers/pci/pcie/aspm.c
> > +++ b/drivers/pci/pcie/aspm.c
> > @@ -624,7 +624,8 @@ static void pcie_aspm_cap_init(struct pcie_link_state *link, int blacklist)
> >  		aspm_calc_l1ss_info(link, &upreg, &dwreg);
> >  
> >  	/* Save default state */
> > -	link->aspm_default = link->aspm_enabled;
> > +	link->aspm_default = parent->dev_flags & PCI_DEV_FLAGS_ENABLE_ASPM ?
> > +			     ASPM_STATE_ALL : link->aspm_enabled;
> 
> This function is ridiculously complicated already, and I really don't
> want to make it worse.
> 
> What exactly is the PCIe topology here?  Apparently the VMD controller
> is a Root Complex Integrated Endpoint, so it's a Type 0 (non-bridge)
> device.  And it has no Link, hence no Link Capabilities or Control and
> hence no ASPM-related bits.  Right?
That's correct. VMD is the Type 0 device providing config/mmio
apertures to another segment and MSI/X remapping. No link and no ASPM
related bits.

Hierarchy is usually something like:

Segment 0           | VMD segment
Root Complex -> VMD | Type 0 (RP/Bridge; physical slot) - Type 1
                    | Type 0 (RP/Bridge; physical slot) - Type 1

> 
> And the devices under the VMD controller?  I guess they are regular
> PCIe Endpoints, Switch Ports, etc?  Obviously there's a Link involved
> somewhere.  Does the VMD controller have some magic, non-architected
> Port on the downstream side?
Correct: Type 0 and Type 1 devices, and any number of Switch ports as
it's usually pinned out to physical slot.


> 
> Does this patch enable ASPM on this magic Link between VMD and the
> next device?  Configuring ASPM correctly requires knowledge and knobs
> from both ends of the Link, and apparently we don't have those for the
> VMD end.
VMD itself doesn't have the link to it's domain. It's really just the
config/mmio aperture and MSI/X remapper. The PCIe link is between the
Type 0 and Type 1 devices on the VMD domain. So fortunately the VMD
itself is not the upstream part of the link.

> 
> Or is it for Links deeper in the hierarchy?  I assume those should
> just work already, although there might be issues with latency
> computation, etc., because we may not be able to account for the part
> of the path above VMD.
That's correct. This is for the links within the domain itself, such as
between a type 0 and NVMe device.

> 
> I want aspm.c to eventually get out of the business of managing struct
> pcie_link_state.  I think it should manage *device* state for each end
> of the link.  Maybe that's a path forward, e.g., if we cache the Link
> Capabilities during enumeration, quirks could modify that directly,
> and aspm.c could just consume that cached information.  I think Saheed
> (cc'd) is already working on patches in this direction.
> 
> I'm still not sure how this works if VMD is the upstream end of a
> Link, though.
> 
> >  	/* Setup initial capable state. Will be updated later */
> >  	link->aspm_capable = link->aspm_support;
> > diff --git a/drivers/pci/quirks.c b/drivers/pci/quirks.c
> > index bdf9b52567e0..2e2f525bd892 100644
> > --- a/drivers/pci/quirks.c
> > +++ b/drivers/pci/quirks.c
> > @@ -5632,3 +5632,14 @@ static void apex_pci_fixup_class(struct pci_dev *pdev)
> >  }
> >  DECLARE_PCI_FIXUP_CLASS_HEADER(0x1ac1, 0x089a,
> >  			       PCI_CLASS_NOT_DEFINED, 8, apex_pci_fixup_class);
> > +
> > +/*
> > + * Device [8086:9a09]
> > + * BIOS may not be able to access config space of devices under VMD domain, so
> > + * it relies on software to enable ASPM for links under VMD.
> > + */
> > +static void pci_fixup_enable_aspm(struct pci_dev *pdev)
> > +{
> > +	pdev->dev_flags |= PCI_DEV_FLAGS_ENABLE_ASPM;
> > +}
> > +DECLARE_PCI_FIXUP_HEADER(PCI_VENDOR_ID_INTEL, 0x9a09, pci_fixup_enable_aspm);
> > diff --git a/include/linux/pci.h b/include/linux/pci.h
> > index 835530605c0d..66a45916c7c6 100644
> > --- a/include/linux/pci.h
> > +++ b/include/linux/pci.h
> > @@ -227,6 +227,8 @@ enum pci_dev_flags {
> >  	PCI_DEV_FLAGS_NO_FLR_RESET = (__force pci_dev_flags_t) (1 << 10),
> >  	/* Don't use Relaxed Ordering for TLPs directed at this device */
> >  	PCI_DEV_FLAGS_NO_RELAXED_ORDERING = (__force pci_dev_flags_t) (1 << 11),
> > +	/* Enable ASPM regardless of how LnkCtl is programmed */
> > +	PCI_DEV_FLAGS_ENABLE_ASPM = (__force pci_dev_flags_t) (1 << 12),
> >  };
> >  
> >  enum pci_irq_reroute_variant {
> > -- 
> > 2.17.1
> >