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

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Date:	Wed, 20 Jul 2016 08:24:50 +0200
From:	Lukas Wunner <lukas@...ner.de>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:	Marek Szyprowski <m.szyprowski@...sung.com>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"open list:AMD IOMMU (AMD-VI)" <iommu@...ts.linux-foundation.org>,
	linux-samsung-soc@...r.kernel.org,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Joerg Roedel <joro@...tes.org>,
	Inki Dae <inki.dae@...sung.com>, Kukjin Kim <kgene@...nel.org>,
	Krzysztof Kozlowski <k.kozlowski@...sung.com>,
	Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
	Ulf Hansson <ulf.hansson@...aro.org>,
	Mark Brown <broonie@...nel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Andreas Noever <andreas.noever@...il.com>,
	linux-pci@...r.kernel.org
Subject: Re: [PATCH v2 02/10] driver core: Functional dependencies tracking
 support

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@...ner.de> wrote:
> > > > On Fri, Jun 17, 2016 at 08:26:52AM +0200, Marek Szyprowski wrote:
> > > > > From: "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
> > > > 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.

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?

Thanks,

Lukas