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 <gre...@linuxfoundation.org> 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 > > <gre...@linuxfoundation.org> 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