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

[Rafael J. Wysocki]

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?

> Currently we achieve that via quirk_apple_wait_for_thunderbolt() in
> drivers/pci/quirks.c. It would be more elegant if we could make this
> relationship explicit with "device links" and let the core handle it.
> 
> Or am I mistaken and this particular use case is not what "device links"
> are intended for?

I'm not sure yet.

Thanks,
Rafael