Message-ID: <CAMxnaaVjMV3+VkvaE3EsRu9e7X=92Ya4Hq0TTu1T=Ribwx3qnQ@mail.gmail.com>
Date:	Tue, 12 Jul 2016 23:46:16 +0200
From:	Andreas Noever <andreas.noever@...il.com>
To:	Greg KH <gregkh@...uxfoundation.org>
Cc:	Bjorn Helgaas <helgaas@...nel.org>, Lukas Wunner <lukas@...ner.de>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	Linux PM list <linux-pm@...r.kernel.org>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	Alan Stern <stern@...land.harvard.edu>,
	Huang Ying <ying.huang@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 00/13] Runtime PM for Thunderbolt on Macs

On Sat, Jul 9, 2016 at 7:23 AM, Greg KH <gregkh@...uxfoundation.org> wrote:
> On Thu, Jul 07, 2016 at 07:39:12PM +0200, Andreas Noever wrote:
>> On Tue, Jun 14, 2016 at 10:22 PM, Bjorn Helgaas <helgaas@...nel.org> wrote:
>> > On Tue, Jun 14, 2016 at 09:14:27PM +0200, Andreas Noever wrote:
>> >> On Tue, Jun 14, 2016 at 6:37 PM, Bjorn Helgaas <helgaas@...nel.org> wrote:
>> >> > [+cc linux-kernel]
>> >> >
>> >> > On Sat, May 21, 2016 at 11:48:42AM +0200, Andreas Noever wrote:
>> >> >>
>> >> >> Signed-off-by: Andreas Noever <andreas.noever@...il.com>
>> >> >>
>> >> >> Tested on MacBookPro10,1
>> >> >>
>> >> >> On Fri, May 13, 2016 at 1:15 PM, Lukas Wunner <lukas@...ner.de> wrote:
>> >> >> > This series powers Thunderbolt controllers on Macs down when nothing is
>> >> >> > plugged in, saving 1.7 W on machines with a Light Ridge controller and
>> >> >> > reportedly 4 W on Cactus Ridge 4C and Falcon Ridge 4C.
>> >> >> >
>> >> >> > Briefly, a custom ACPI method provided by Apple is used to cut power to
>> >> >> > the controller.  A GPE is enabled while the controller is powered down
>> >> >> > which side-band signals a plug event, whereupon power is reinstated using
>> >> >> > the ACPI method.  Note that even though this mechanism is ACPI-based,
>> >> >> > it does not use _PSx methods and is thus entirely nonstandard.
>> >> >
>> >> > I think the current arrangement was that Andreas would ack Thunderbolt
>> >> > patches and I would merge them via the PCI tree.  That makes some sense
>> >> > because Thunderbolt and PCIe are related, but the more I think about
>> >> > it, the less I'm happy with it.
>> >> >
>> >> > This series is a good example.  I'm sure it's good work and
>> >> > worthwhile.  But I can't really say anything about the content of it
>> >> > because most of it is Thunderbolt-specific and there's no public spec.
>> >> > It seems like this is basically a collection of reverse-engineered
>> >> > quirks that happen to work with the current state of Linux PM on
>> >> > certain Macs.  We don't know what might change on future Macs.  We
>> >> > don't know what might break when we make changes to Linux PM.
>> >> >
>> >> > I can't test this series, nor do I want to.  I can't test most of the
>> >> > patches I merge, but I can at least read the spec and see whether the
>> >> > patches make sense.  What I would *like* is to have public Thunderbolt
>> >> > specs and a kernel developer's guide so we know what to expect from
>> >> > the hardware and the firmware and we can write code that should work
>> >> > not just on current Macs, but also on non-Macs and future Macs.
>> >> >
>> >> > I don't think the current situation is really maintainable, and I'm
>> >> > not comfortable merging code that I can't maintain.
>> >> Most of the code is contained within the thunderbolt driver. I think
>> >> there is quite some precedence for reverse engineered drivers without
>> >> specs being part of the kernel. My understanding was that, since I am
>> >> listed in MAINTAINERS, I am responsible for the driver. Now our
>> >> changes often need improvements to the pci core, which is why I think
>> >> merging through your tree is a good idea (without transferring
>> >> responsibility). The changes to the drivers/pci should be supported by
>> >> the PCI-spec and make sense without knowing about thunderbolt (but it
>> >> might be the case that thunderbolt is the only user of these
>> >> features).
>> >>
>> >> Specifically for this series we want to:
>> >>  - whitelist thunderbolt bridges for PM. Detecting those bridges is
>> >> non-standard but I think this is acceptable, since this
>> >> blacklist/whitelist is basically a quirk.
>> >>  - Load our portdrv on tb bridges. PCI just sees another portdriver
>> >> and all the reverse engineered magic lives inside the driver.
>> >>  - Forward more PM callbacks to portdrivers (not tb specific)
>> >>  - hotplug D3cold fixes: resume around board_added/remove_board,
>> >> ignore interrupts in d3cold (not tb specific and probably a general
>> >> bugfix)
>> >>  - Make pci not fail if bridges have been put into D3cold by some
>> >> external mechanism.
>> >>
>> >> So maybe you could review the pci changes as a solution to the problem
>> >> "we want to load a custom portdriver which can put bridges into d3cold
>> >> in a device specific way". We certainly to not expect you to take
>> >> responsibility for the thunderbolt driver.
>> >
>> > That's a fine solution as far as I'm personally concerned.  I think
>> > it's poor for Linux overall, because I think it's fragile, and it's
>> > disappointing that a technology as important as Thunderbolt is so
>> > poorly supported by the promulgators.  But if you're willing to work
>> > in that environment, that's great.
>> >
>> > You maintain the thunderbolt code and merge changes, and I'll review
>> > the pieces that touch drivers/pci.  I do have a couple comments on
>> > those pieces, but I don't think they'll be major.
>> >
>> > I just want to get out of the business of merging drivers/thunderbolt
>> > code that I can't maintain.
>>
>> [+ Greg]
>>
>> Hi Greg,
>>
>> do you mind if we revert to the old scheme and merge TB changes
>> through your tree?
>
> I will be glad to take them, feel free to send them on to me.

Thanks!
Andreas

> thanks,
>
> greg k-h

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