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

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Date:   Sat, 28 Jan 2017 16:09:14 -0600
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Lukas Wunner <lukas@...ner.de>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org,
        Andreas Noever <andreas.noever@...il.com>,
        linux-pci@...r.kernel.org, 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 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.

> If the BIOS cut-off date is ever lowered to 2010, the Thunderbolt
> special case can be removed.
> 
> Cc: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> Cc: Andreas Noever <andreas.noever@...il.com>
> Cc: Tomas Winkler <tomas.winkler@...el.com>
> Cc: Amir Levy <amir.jer.levy@...el.com>
> Reviewed-by: Mika Westerberg <mika.westerberg@...ux.intel.com>
> Signed-off-by: Lukas Wunner <lukas@...ner.de>
> ---
>  drivers/pci/pci.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index a881c0d3d2e8..8ed098db82e1 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -2224,7 +2224,7 @@ void pci_config_pm_runtime_put(struct pci_dev *pdev)
>   * @bridge: Bridge to check
>   *
>   * This function checks if it is possible to move the bridge to D3.
> - * Currently we only allow D3 for recent enough PCIe ports.
> + * Currently we only allow D3 for recent enough PCIe ports and Thunderbolt.
>   */
>  bool pci_bridge_d3_possible(struct pci_dev *bridge)
>  {
> @@ -2258,6 +2258,11 @@ bool pci_bridge_d3_possible(struct pci_dev *bridge)
>  		    year >= 2015) {
>  			return true;
>  		}
> +
> +		/* Even the oldest 2010 Thunderbolt controller supports D3. */
> +		if (bridge->is_thunderbolt)
> +			return true;
> +
>  		break;
>  	}
>  
> -- 
> 2.11.0
>