linux-kernel - Re: [PATCH v3] PCI/ASPM: Enable ASPM for bridge-to-bridge link

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Date:   Thu, 7 May 2020 14:50:34 +0300
From:   Mika Westerberg <mika.westerberg@...ux.intel.com>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     Kai-Heng Feng <kai.heng.feng@...onical.com>, bhelgaas@...gle.com,
        Heiner Kallweit <hkallweit1@...il.com>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Keith Busch <keith.busch@...el.com>,
        Chris Packham <chris.packham@...iedtelesis.co.nz>,
        Yicong Yang <yangyicong@...ilicon.com>,
        Krzysztof Wilczynski <kw@...ux.com>,
        "open list:PCI SUBSYSTEM" <linux-pci@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] PCI/ASPM: Enable ASPM for bridge-to-bridge link

On Wed, May 06, 2020 at 04:29:47PM -0500, Bjorn Helgaas wrote:
> On Wed, May 06, 2020 at 09:14:38AM +0300, Mika Westerberg wrote:
> > On Wed, May 06, 2020 at 01:34:21AM +0800, Kai-Heng Feng wrote:
> > > The TI PCIe-to-PCI bridge prevents the Intel SoC from entering power
> > > state deeper than PC3 due to disabled ASPM, consumes lots of unnecessary
> > > power. On Windows ASPM L1 is enabled on the device and its upstream
> > > bridge, so it can make the Intel SoC reach PC8 or PC10 to save lots of
> > > power.
> > > 
> > > In short, ASPM always gets disabled on bridge-to-bridge link.
> > 
> > Excelent finding :) I've heard several reports complaining that we can't
> > enter PC10 when TBT is enabled and I guess this explains it.
> 
> I'm curious about this.  I first read this patch as affecting
> garden-variety Links between a Root Port or Downstream Port and the
> Upstream Port of a switch.  But the case we're talking about is
> specifically when the downstream device is PCI_EXP_TYPE_PCI_BRIDGE,
> i.e., a PCIe to PCI/PCI-X bridge, not a switch.
> 
> AFAICT, a Link to a PCI bridge is still a normal Link and ASPM should
> still work.  I'm sort of surprised that you'd find such a PCIe to
> PCI/PCI-X bridge in a Thunderbolt topology, but maybe that's a common
> thing?

It actually is not common and now that you mention I'm wondering how
this can help at all. I also thought this applies to all ports which
would explain the issue we have but if it only applies to PCIe to
PCI/PCI-X bridge it should not make any difference in TBT systems.

> I guess "PC8" and "PC10" are some sort of Intel-specific power states?

Package C-state 8 and Package C-state 10. These are power states the
whole (Intel) CPU package can enter when individual CPU cores are in
correct low power states.