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

[Bjorn Helgaas <helgaas@kernel.org>]

[+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@canonical.com>
> ---
>  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?

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?

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.

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.

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
>