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

[Andreas Noever]

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.


> I know I don't understand the whole situation, so somebody please tell
> me why I'm being unreasonable here.
>
>> > A Thunderbolt controller appears to the OS as a set of PCI devices:  One
>> > NHI (Native Host Interface) and multiple bridges.  Power is cut to the
>> > entire set of devices:
>> >
>> >   (Root Port) ---- Upstream Bridge --+-- Downstream Bridge 0 ---- NHI
>> >                                      +-- Downstream Bridge 1 --
>> >                                      +-- Downstream Bridge 2 --
>> >                                      ...
>> >
>> > v1 of this series shoehorned power control into the NHI driver.  This
>> > violated the Linux pm model which assumes that a child cannot resume
>> > before its parent. As seen above, the NHI is a child, so the child cut
>> > power to the bridges above it.
>> >
>> > v2 resolves this by positioning power control on the controller's
>> > topmost device, which is the upstream bridge.  That is achieved by
>> > binding to it as a Thunderbolt port service driver.  portdrv already
>> > calls down to each service driver on ->suspend and ->resume and I
>> > extended that scheme to further PM callbacks.  E.g. when the upstream
>> > bridge is runtime suspended, portdrv invokes the ->runtime_suspend
>> > callback of each attached service driver, and the Thunderbolt service
>> > driver's callback in turn invokes the ACPI method to cut power to the
>> > controller.
>> >
>> >
>> > For such a nonstandard ACPI-based PM mechanism one would normally assign
>> > a dev_pm_domain to the upstream bridge which overrides the PCI subsystem
>> > PM callbacks.  But that's not an option here because dev_pm_domain_set()
>> > can only be called during driver probe.  The driver is portdrv and
>> > obviously loads earlier than the thunderbolt port service driver.
>> > So one has to make do with the PCI subsystem PM callbacks.
>> >
>> > The PCI core currently assumes that devices can only be put into D3cold
>> > by the platform, i.e. using the standard ACPI _PSx methods.  I extended
>> > the PCI core so that it can deal with devices which are put into D3cold
>> > by the driver callbacks.  It turns out only two changes are needed to
>> > make this work, and they are in patches [09/13] and [10/13].  Runtime
>> > suspend works out of the box, but runtime resume tries to set the device
>> > power state *before* invoking the driver callback, and this goes awry
>> > since the device is still in D3cold.  I solved this by returning an error
>> > in pci_raw_set_power_state() if the device's current_state is D3cold
>> > (patch [09/13]).
>> >
>> > Theoretically it would also be possible to patch the missing _PSx methods
>> > into the ACPI namespace at runtime but I suspect it wouldn't be pretty:
>> > I think I'd have to include pre-compiled AML methods in the kernel and
>> > modify those blobs at runtime (adjust GPE number etc) before patching
>> > them into the namespace.
>> >
>> >
>> > To make direct-complete work for such non-platform-power-managed devices,
>> > I also had to modify pci_target_state() (patch [10/13]).
>> >
>> > Finally, I wanted to avoid the mandatory runtime resume after direct-
>> > complete which was introduced by Rafael with 58a1fbbb2ee8 ("PM / PCI /
>> > ACPI: Kick devices that might have been reset by firmware"), so I added
>> > the possibility to opt out of it (patch [11/13]).
>> >
>> >
>> > I've pushed these patches to GitHub where they can be reviewed more
>> > comfortably with green/red highlighting:
>> > https://github.com/l1k/linux/commits/thunderbolt_runpm_v2
>> >
>> > For reference, here's a link to v1:
>> > http://thread.gmane.org/gmane.linux.power-management.general/73197
>> >
>> > Thanks in advance for your comments.
>> >
>> > Lukas
>> >
>> >
>> > Lukas Wunner (13):
>> >   PCI: Recognize Thunderbolt devices
>> >   PCI: Allow D3 for Thunderbolt ports
>> >   PCI: Add Thunderbolt portdrv service type
>> >   PCI: Generalize portdrv pm iterator
>> >   PCI: Use portdrv pm iterator on further callbacks
>> >   PCI: pciehp: Support runtime pm
>> >   PCI: pciehp: Ignore interrupts during D3cold
>> >   PCI: Allow runtime PM for Thunderbolt hotplug ports on Macs
>> >   PCI: Do not write to PM control register while in D3cold
>> >   PCI: Avoid going from D3cold to D3hot for system sleep
>> >   PM / sleep: Allow opt-out from runtime resume after direct-complete
>> >   thunderbolt: Support runtime pm on upstream bridge
>> >   thunderbolt: Support runtime pm on NHI
>> >
>> >  drivers/base/power/generic_ops.c  |   3 +-
>> >  drivers/pci/hotplug/pciehp_ctrl.c |   9 +-
>> >  drivers/pci/hotplug/pciehp_hpc.c  |   4 +
>> >  drivers/pci/pci.c                 |  50 ++----
>> >  drivers/pci/pci.h                 |   2 +
>> >  drivers/pci/pcie/portdrv.h        |   6 +-
>> >  drivers/pci/pcie/portdrv_core.c   |  47 +-----
>> >  drivers/pci/pcie/portdrv_pci.c    |  88 ++++++++--
>> >  drivers/pci/probe.c               |  17 ++
>> >  drivers/thunderbolt/Kconfig       |   4 +-
>> >  drivers/thunderbolt/Makefile      |   4 +-
>> >  drivers/thunderbolt/nhi.c         |  32 +++-
>> >  drivers/thunderbolt/switch.c      |   9 +
>> >  drivers/thunderbolt/tb.c          |  13 ++
>> >  drivers/thunderbolt/upstream.c    | 345 ++++++++++++++++++++++++++++++++++++++
>> >  include/linux/pci.h               |   1 +
>> >  include/linux/pcieport_if.h       |   7 +
>> >  include/linux/pm.h                |   1 +
>> >  18 files changed, 539 insertions(+), 103 deletions(-)
>> >  create mode 100644 drivers/thunderbolt/upstream.c
>> >
>> > --
>> > 2.8.1
>> >