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

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Message-ID: <CAJZ5v0h71xxvzex2D6s9W8Qvq3+BqOQnTJRKy32gMu=tuQLxDA@mail.gmail.com>
Date:	Fri, 17 Jun 2016 14:54:56 +0200
From:	"Rafael J. Wysocki" <rafael@...nel.org>
To:	Lukas Wunner <lukas@...ner.de>
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>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	Andreas Noever <andreas.noever@...il.com>
Subject: Re: [PATCH v2 02/10] driver core: Functional dependencies tracking support

On Fri, Jun 17, 2016 at 12:36 PM, Lukas Wunner <lukas@...ner.de> wrote:
> Hi Marek,
>
> On Fri, Jun 17, 2016 at 08:26:52AM +0200, Marek Szyprowski wrote:
>> From: "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
>>
>> Currently, there is a problem with handling cases where functional
>> dependencies between devices are involved.
>>
>> What I mean by a "functional dependency" is when the driver of device
>> B needs both device A and its driver to be present and functional to
>> be able to work.  This implies that the driver of A needs to be
>> working for B to be probed successfully and it cannot be unbound from
>> the device before the B's driver.  This also has certain consequences
>> for power management of these devices (suspend/resume and runtime PM
>> ordering).
>>
>> Add support for representing those functional dependencies between
>> devices to allow the driver core to track them and act on them in
>> certain cases where they matter.
>
> Rafael has indicated that he intends to respin this series:
> https://lkml.org/lkml/2016/6/8/1061

That's OK.

> 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?

Thanks,
Rafael