Re: Policy to keep USB ports powered in low-power states

[Nick Crews]

Adding Duncan Laurie who I think has some more intimate knowledge
of how this is implemented in HW. Duncan, could you correct or elaborate
on my answers below as you see fit? Also, sorry if I make some beginner
mistakes here, I'm just getting familiar with the USB subsystem, and thanks for
your patience.

On Wed, Aug 14, 2019 at 3:20 PM Greg Kroah-Hartman
<gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
>
> On Wed, Aug 14, 2019 at 02:12:07PM -0600, Nick Crews wrote:
> > Thanks for the fast response!
> >
> > On Tue, Aug 13, 2019 at 12:02 AM Greg Kroah-Hartman
> > <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
> > >
> > > On Mon, Aug 12, 2019 at 06:08:43PM -0600, Nick Crews wrote:
> > > > Hi Greg!
> > >
> > > Hi!
> > >
> > > First off, please fix your email client to not send html so that vger
> > > does not reject your messages :)
> >
> > Thanks, should be good now.
> >
> > >
> > > > I am working on a Chrome OS device that supports a policy called "USB Power
> > > > Share," which allows users to turn the laptop into a charge pack for their
> > > > phone. When the policy is enabled, power will be supplied to the USB ports
> > > > even when the system is in low power states such as S3 and S5. When
> > > > disabled, then no power will be supplied in S3 and S5. I wrote a driver
> > > > <https://lore.kernel.org/patchwork/patch/1062995/> for this already as part
> > > > of drivers/platform/chrome/, but Enric Balletbo i Serra, the maintainer,
> > > > had the reasonable suggestion of trying to move this into the USB subsystem.
> > >
> > > Correct suggestion.
> > >
> > > > Has anything like this been done before? Do you have any preliminary
> > > > thoughts on this before I start writing code? A few things that I haven't
> > > > figured out yet:
> > > > - How to make this feature only available on certain devices. Using device
> > > > tree? Kconfig? Making a separate driver just for this device that plugs
> > > > into the USB core?
> > > > - The feature is only supported on some USB ports, so we need a way of
> > > > filtering on a per-port basis.
> > >
> > > Look at the drivers/usb/typec/ code, I think that should do everything
> > > you need here as this is a typec standard functionality, right?
> >
> > Unfortunately this is for USB 2.0 ports, so it's not type-C.
> > Is the type-C code still worth looking at?
>
> If this is for USB 2, does it use the "non-standard" hub commands to
> turn on and off power?  If so, why not just use the usbreset userspace
> program for that?

It does not use the standard hub commands. The USB ports are controlled
by an Embedded Controller (EC), so to control this policy we send a command
to the EC. Since the command to send to the EC is very specific, this would need
to go into a "hub driver" unique for these Wilco devices. We would make it so
that the normal hub registration is intercepted by something that sees this is a
Wilco device, and instead register the hub as a "wilco-hub", which has its own
special "power_share" sysfs attribute, but still is treated as a normal USB hub
otherwise?

>
> And how are you turning a USB 2 port into a power source?  That feels
> really odd given the spec.  Is this part of the standard somewhere or
> just a firmware/hardware hack that you are adding to a device?

The EC twiddles something in the port' HW so that the port turns into a
DCP (Dedicated Charging Port) and only supplies power, not data. So I
think yes, this is a bit of a hack that does not conform to the spec.

>
> Is there some port information in the firmware that describes this
> functionality?  If so, can you expose it through sysfs to the port that
> way?

[I'm not sure I'm answering your question, but] I believe that we could
make the BIOS firmware describe the USB ports' capabilities, and the
kernel's behavior would be gated upon what the firmware reports. I see
that struct usb_port already contains a "quirks" field, should we add a
POWER_SHARE quirk to include/linux/usb/quirks.h? I would guess that
should that should be reserved for quirks shared between many USB
devices/hubs?

Thanks,
Nick

>
> thanks,
>
> greg k-h