linux-kernel - Re: [PATCH v5 2/8] PCI: Allow runtime PM on Thunderbolt ports

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Message-ID: <20170210175716.GE29169@bhelgaas-glaptop.roam.corp.google.com>
Date:   Fri, 10 Feb 2017 11:57:16 -0600
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     Lukas Wunner <lukas@...ner.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andreas Noever <andreas.noever@...il.com>,
        Linux PCI <linux-pci@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>
Subject: Re: [PATCH v5 2/8] PCI: Allow runtime PM on Thunderbolt ports

On Mon, Jan 30, 2017 at 08:15:40AM +0100, Rafael J. Wysocki wrote:
> On Sat, Jan 28, 2017 at 11:09 PM, Bjorn Helgaas <helgaas@...nel.org> wrote:
> > On Sun, Jan 15, 2017 at 09:03:45PM +0100, Lukas Wunner wrote:
> >> Currently PCIe ports are only allowed to go to D3 if the BIOS is dated
> >> 2015 or newer to avoid potential issues with old chipsets.  However for
> >> Thunderbolt we know that even the oldest controller, Light Ridge (2010),
> >> is able to suspend its ports to D3 just fine.
> >>
> >> We're about to add runtime PM for Thunderbolt on the Mac.  Apple has
> >> released two EFI security updates in 2015 which encompass all machines
> >> with Thunderbolt, but the achieved power saving should be made available
> >> to users even if they haven't updated their BIOS.  To this end,
> >> special-case Thunderbolt in pci_bridge_d3_possible().
> >
> > I think this whole paragraph is unnecessary detail.  I first thought
> > you had some connection with a firmware security issue, but now I see
> > the only point is that if you have pre-2015 firmware, you could update
> > it since newer firmware is available.
> >
> >> This allows the Thunderbolt controller to power down but the root port
> >> to which the Thunderbolt controller is attached remains in D0 unless
> >> the EFI update is installed.  Users can pass pcie_port_pm=force on the
> >> kernel command line if they cannot install the EFI update but still want
> >> to benefit from the additional power saving of putting the root port
> >> into D3.  In practice, root ports can be suspended to D3 without issues
> >> at least on 2012 Ivy Bridge machines.
> >
> > I'm not sure I like advertising pcie_port_pm=force.  That just means a
> > few leet folks will use this parameter and run in a subtly different
> > configuration than everybody else, and possibly trip over subtly
> > different issues.  The audience (users who read kernel change logs and
> > are willing to use special boot parameters, but who can't install an
> > EFI update) seems small.
> 
> That basically is for somebody who has a product and knows that the
> feature works there, but doesn't want or simply can't patch the kernel
> (which is shipped by a distro or similar).

Yes, OK.  This isn't adding a new parameter, and I'm OK with the actual
change here, so

Acked-by: Bjorn Helgaas <bhelgaas@...gle.com>