Date:	Fri, 8 Jul 2016 22:23:59 -0700
From:	Greg KH <gregkh@...uxfoundation.org>
To:	Andreas Noever <andreas.noever@...il.com>
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 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,

greg k-h

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