linux-kernel - Re: [PATCH v4 3/8] PCI: Don't block runtime PM for Thunderbolt host hotplug ports

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Message-ID: <20170112014707.GA24136@wunner.de>
Date:   Thu, 12 Jan 2017 02:47:07 +0100
From:   Lukas Wunner <lukas@...ner.de>
To:     Mika Westerberg <mika.westerberg@...ux.intel.com>
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,
        Chen Yu <yu.c.chen@...el.com>,
        Tomas Winkler <tomas.winkler@...el.com>,
        Amir Levy <amir.jer.levy@...el.com>,
        Bjorn Helgaas <helgaas@...nel.org>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
Subject: Re: [PATCH v4 3/8] PCI: Don't block runtime PM for Thunderbolt host
 hotplug ports

On Wed, Jan 11, 2017 at 12:02:10PM +0200, Mika Westerberg wrote:
> On Sun, Jan 08, 2017 at 09:41:45AM +0100, Lukas Wunner wrote:
> > Hotplug ports generally block their parents from suspending to D3hot as
> > otherwise their interrupts couldn't be delivered.
> > 
> > An exception are Thunderbolt host controllers:  They have a separate
> > GPIO pin to side-band signal plug events even if the controller is
> > powered down or its parent ports are suspended to D3.  They can be told
> > apart from Thunderbolt controllers in attached devices by checking if
> > they're situated below a non-Thunderbolt device (typically a root port,
> > or the downstream port of a PCIe switch in the case of the MacPro6,1).
> > 
> > To enable runtime PM for Thunderbolt on the Mac, the downstream bridges
> > of a host controller must not block runtime PM on the upstream bridge as
> > power to the chip is only cut once the upstream bridge has suspended.
> > Amend the condition in pci_dev_check_d3cold() accordingly.
> > 
> > Cc: Mika Westerberg <mika.westerberg@...ux.intel.com>
> > 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>
> > Signed-off-by: Lukas Wunner <lukas@...ner.de>
> > ---
> >  drivers/pci/pci.c | 13 ++++++++++++-
> >  1 file changed, 12 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> > index 8ed098d..0b03fe7 100644
> > --- a/drivers/pci/pci.c
> > +++ b/drivers/pci/pci.c
> > @@ -2271,6 +2271,7 @@ bool pci_bridge_d3_possible(struct pci_dev *bridge)
> >  
> >  static int pci_dev_check_d3cold(struct pci_dev *dev, void *data)
> >  {
> > +	struct pci_dev *parent, *grandparent;
> >  	bool *d3cold_ok = data;
> >  
> >  	if (/* The device needs to be allowed to go D3cold ... */
> > @@ -2284,7 +2285,17 @@ static int pci_dev_check_d3cold(struct pci_dev *dev, void *data)
> >  	    !pci_power_manageable(dev) ||
> >  
> >  	    /* Hotplug interrupts cannot be delivered if the link is down. */
> > -	    dev->is_hotplug_bridge)
> > +	    (dev->is_hotplug_bridge &&
> > +
> > +		/*
> > +		 * Exception:  Thunderbolt host controllers have a pin to
> > +		 * side-band signal plug events.  Their hotplug ports are
> > +		 * recognizable by having a non-Thunderbolt device as
> > +		 * grandparent.
> > +		 */
> > +		!(dev->is_thunderbolt && (parent = pci_upstream_bridge(dev)) &&
> > +				 (grandparent = pci_upstream_bridge(parent)) &&
> > +						!grandparent->is_thunderbolt)))
> 
> Can you move this to its own helper function?

I can certainly do that.

Could one of you guys confirm that the code above is safe on non-Macs?

Specifically, the very first Thunderbolt chips (Light Ridge, Eagle Ridge)
had no POC, i.e. they were unable to power themselves off.  Apple put an
ARM Cortex (NXP LPC1112) on the logic board which snoops on the connector
lines for hotplug detection while the Thunderbolt controller is powered
down.  The power rails to the controller are brought up and down with
separate load switches.  This functionality became integrated into the
controller starting with Cactus Ridge in 2012.

So I know the above code is safe on Macs.  However on non-Macs these
extra chips for power management may not exist, i.e. the controller
stays on all the time and then I shouldn't suspend the upstream bridge
to D3.  I assume that such machines do not exist as Apple was pretty
much the only vendor with Thunderbolt gear in the 2010-2012 time frame.
The only other one I know of was the Sony Vaio Z21 which used the
optical version of Thunderbolt to attach a docking station, but these
are rare.

If you know of non-Macs which might be broken by the above code snippet,
I could dmi-quirk this to Macs plus constrain to CONFIG_THUNDERBOLT being
enabled.

Thanks,

Lukas

> 
> >  
> >  		*d3cold_ok = false;
> >  
> > -- 
> > 2.11.0