Re: [PATCH v2 00/13] Runtime PM for Thunderbolt on Macs

[Andreas Noever]

On Tue, Jun 14, 2016 at 10:22 PM, Bjorn Helgaas <helgaas@xxxxxxxxxx> 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@xxxxxxxxxx> wrote:
>> > [+cc linux-kernel]
>> >
>> > On Sat, May 21, 2016 at 11:48:42AM +0200, Andreas Noever wrote:
>> >>
>> >> Signed-off-by: Andreas Noever <andreas.noever@xxxxxxxxx>
>> >>
>> >> Tested on MacBookPro10,1
>> >>
>> >> On Fri, May 13, 2016 at 1:15 PM, Lukas Wunner <lukas@xxxxxxxxx> 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?

Regards,
Andreas


> Bjorn