Re: [PATCH v2 02/10] driver core: Functional dependencies tracking support

[Rafael J. Wysocki]

On Monday, July 25, 2016 12:48:32 AM Lukas Wunner wrote:
> On Thu, Jul 21, 2016 at 02:25:15AM +0200, Rafael J. Wysocki wrote:
> > On Thursday, July 21, 2016 01:25:53 AM Lukas Wunner wrote:
> > > On Thu, Jul 21, 2016 at 12:51:31AM +0200, Rafael J. Wysocki wrote:
> > > > On Wednesday, July 20, 2016 05:23:40 PM Lukas Wunner wrote:
> > > > > On Wed, Jul 20, 2016 at 02:52:42PM +0200, Rafael J. Wysocki wrote:
> > > > > > On Wednesday, July 20, 2016 08:24:50 AM Lukas Wunner wrote:
> > > > > > > On Wed, Jul 20, 2016 at 02:33:18AM +0200, Rafael J. Wysocki wrote:
> > > > > > > > On Friday, June 17, 2016 04:07:38 PM Lukas Wunner wrote:
> > > > > > > > > On Fri, Jun 17, 2016 at 02:54:56PM +0200, Rafael J. Wysocki wrote:
> > > > > > > > > > On Fri, Jun 17, 2016 at 12:36 PM, Lukas Wunner <lukas@xxxxxxxxx> wrote:
> > > > > > > > > > > On Fri, Jun 17, 2016 at 08:26:52AM +0200, Marek Szyprowski wrote:
> > > > > > > > > > > > From: "Rafael J. Wysocki" <rafael.j.wysocki@xxxxxxxxx>
> > > > > > > > > > > We also have such a functional dependency for Thunderbolt on Macs:
> > > > > > > > > > > On resume from system sleep, the PCIe hotplug ports may not resume
> > > > > > > > > > > before the thunderbolt driver has reestablished the PCI tunnels.
> > > > > > > > > > > Currently this is enforced by quirk_apple_wait_for_thunderbolt()
> > > > > > > > > > > in drivers/pci/quirks.c. It would be good if we could represent
> > > > > > > > > > > this dependency using something like Rafael's approach instead of
> > > > > > > > > > > open coding it, however one detail in Rafael's patches is problematic:
> > > > > > > > > > >
> > > > > > > > > > > > New links are added by calling device_link_add() which may happen
> > > > > > > > > > > > either before the consumer device is probed or when probing it, in
> > > > > > > > > > > > which case the caller needs to ensure that the driver of the
> > > > > > > > > > > > supplier device is present and functional and the DEVICE_LINK_PROBE_TIME
> > > > > > > > > > > > flag should be passed to device_link_add() to reflect that.
> > > > > > > > > > >
> > > > > > > > > > > The thunderbolt driver cannot call device_link_add() before the
> > > > > > > > > > > PCIe hotplug ports are bound to a driver unless we amend portdrv
> > > > > > > > > > > to return -EPROBE_DEFER for Thunderbolt hotplug ports on Macs
> > > > > > > > > > > if the thunderbolt driver isn't loaded.
> > > > > > > > > > >
> > > > > > > > > > > It would therefore be beneficial if device_link_add() can be
> > > > > > > > > > > called even *after* the consumer is bound.
> > > > > > > > > > 
> > > > > > > > > > I don't quite follow.
> > > > > > > > > > 
> > > > > > > > > > Who's the provider and who's the consumer here?
> > > > > > > > > 
> > > > > > > > > thunderbolt.ko is the supplier.
> > > > > > > > 
> > > > > > > > But it binds to the children of the ports that are supposed to be its
> > > > > > > > consumers?
> > > > > > > > 
> > > > > > > > Why is that even expected to work?
> > > > > > > 
> > > > > > > No, the consumers are aunts (or uncles) of the supplier, if you will. :-)
> > > > > > > 
> > > > > > > The consumers are the hotplug ports (named "Downstream Bridge 1 / 2" in
> > > > > > > the drawing below). The supplier is the NHI:
> > > > > > > 
> > > > > > >       (Root Port) ---- Upstream Bridge --+-- Downstream Bridge 0 ---- NHI
> > > > > > >                                          +-- Downstream Bridge 1 --
> > > > > > >                                          +-- Downstream Bridge 2 --
> > > > > > >                                          ...
> > > > > > > 
> > > > > > > We're calling pci_power_up() and pci_restore_state() from
> > > > > > > pci_pm_resume_noirq(). And that will fail for devices below
> > > > > > > the hotplug ports if the PCI tunnels haven't been re-established
> > > > > > > yet by the NHI.
> > > > > > 
> > > > > > So the NHI is a PCIe device, right?
> > > > > > 
> > > > > > Does the Thunderbolt driver bind to that device?
> > > > > 
> > > > > The NHI is a PCI device but not a bridge. It has class 0x88000.
> > > > > Yes, thunderbolt.ko binds to the NHI.
> > > > > 
> > > > > And portdrv binds to the upstream bridge and downstream bridges.
> > > > > Those have class 0x60400.
> > > > 
> > > > OK, so why would there be a problem with creating links from the NHI
> > > > (producer) to the ports (consumers) before binding portdrv to them?
> > > 
> > > Because the ordering in which drivers bind isn't guaranteed. At least
> > > on my machine (Debian), portdrv always binds before thunderbolt.
> > 
> > But what drivers have to do with that really?  Do you need drivers to
> > know that the dependency is there?
> > 
> > Just add likns *before* even probing for drivers (yes, you can do that)
> > and the core will handle that for you.
> 
> Forgive me for being dense: How do you suggest to add links before
> probing drivers? Only way I could think of is with a PCI quirk.
> 
> Which is what we're already doing right now (see drivers/pci/quirk.c:
> quirk_apple_wait_for_thunderbolt()). And it ain't pretty.

Well, maybe not, but doing it once during enumeration would be better than
on every resume.

Plus there is runtime PM to cover.

> > > I guess I could amend portdrv to return -EPROBE_DEFER on Macs if
> > > no driver is bound to the NHI. Doesn't feel pretty to me though.
> > > 
> > > Ultimately this seems to be the same issue as with calling
> > > dev_pm_domain_set() for a bound device. Perhaps device_link_add()
> > > can likewise be allowed if a runtime PM ref is held for the devices
> > > and the call happens under lock_system_sleep()?
> > 
> > No, the whole synchronization scheme in the links code would have had to be
> > changed for that to really work.
> > 
> > And it really is about what is needed (at least in principle) to run your
> > device.  If you think you need device X with a driver to handle device Y
> > correctly, then either you need it all the time, from probe to remove, or
> > you just don't really need it at all.
> 
> Real life isn't as simple as that.
> 
> In this case, we have consumers (hotplug ports) which are doing fine
> if the driver for the supplier (NHI) is not loaded. But once it loads,
> the links must be in place.

Hmm.

What if it is not loaded and the system suspends.  Will everything work
as expected after the subsequent resume?

Thanks,
Rafael