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

From: Bjorn Helgaas <helgaas@kernel.org>
To: "Rafael J. Wysocki" <rafael@kernel.org>
Cc: Lukas Wunner <lukas@wunner.de>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Andreas Noever <andreas.noever@gmail.com>,
	Linux PCI <linux-pci@vger.kernel.org>,
	Linux PM <linux-pm@vger.kernel.org>,
	"Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
	Mika Westerberg <mika.westerberg@linux.intel.com>
Subject: Re: [PATCH v5 2/8] PCI: Allow runtime PM on Thunderbolt ports
Date: Fri, 10 Feb 2017 11:57:16 -0600	[thread overview]
Message-ID: <20170210175716.GE29169@bhelgaas-glaptop.roam.corp.google.com> (raw)
In-Reply-To: <CAJZ5v0gZgp0RVo_2toASw+-GF=FV8=TdEmauP+7+-czU5kshjg@mail.gmail.com>

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@kernel.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@google.com>