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

[Mika Westerberg]

On Thu, Jan 12, 2017 at 02:47:07AM +0100, Lukas Wunner wrote:
> 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@xxxxxxxxxxxxxxx>
> > > Cc: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> > > Cc: Andreas Noever <andreas.noever@xxxxxxxxx>
> > > Cc: Tomas Winkler <tomas.winkler@xxxxxxxxx>
> > > Cc: Amir Levy <amir.jer.levy@xxxxxxxxx>
> > > Signed-off-by: Lukas Wunner <lukas@xxxxxxxxx>
> > > ---
> > >  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.

The thunderbolt chips I have seen all include the side-band hotplug
detection GPIO. In addition the whole PCIe hierarchy is powered down
when there is nothing connected. So in that sense, I don't see how this
could break them.

Constraining this to CONFIG_THUNDERBOLT does not limit anything because
distros will have it enabled anyway ;-) Having DMI quirk might be good
idea, just in case.