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Subject: Re: Policy to keep USB ports powered in low-power states
To:     Duncan Laurie <dlaurie@google.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc:     Nick Crews <ncrews@chromium.org>, linux-usb@vger.kernel.org,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Daniel Kurtz <djkurtz@google.com>
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From:   Enric Balletbo i Serra <enric.balletbo@collabora.com>
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Date:   Tue, 27 Aug 2019 17:29:56 +0200
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Hi,

On 16/8/19 19:02, Duncan Laurie wrote:
> On Fri, Aug 16, 2019 at 2:12 AM Greg Kroah-Hartman
> <gregkh@linuxfoundation.org> wrote:
>>
>> On Thu, Aug 15, 2019 at 05:42:05PM -0600, Duncan Laurie wrote:
>>> On Wed, Aug 14, 2019 at 6:08 PM Nick Crews <ncrews@chromium.org> wrote:
>>>>
>>>> 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@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
>>>>>> <gregkh@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?
>>>>
>>>
>>>
>>> I would say it is somewhat similar to the USB port power control which
>>> eventually calls into usb_acpi_set_power_state() but in this case it only
>>> affects the behavior when the system is NOT running.
>>
>> Ok, if this is when the system is not running, why does Linux need to be
>> involved at all?
>>
>> And if Linux is running, why not just follow the USB spec and not create
>> your own craziness?
>>
>>> This design has a standalone USB charge power controller on the board
>>> that passes through the USB2 D+/D- pins from one port and is able to do
>>> BC1.2 negotiation when the host controller is not powered, assuming
>>> the chip has been enabled by the Embedded Controller.
>>
>> So it does follow the spec?  Or does not?  I can't determine here.
>>
> 
> 
> I didn't realize the part had a public datasheet:
> https://www.dialog-semiconductor.com/sites/default/files/xslgc55544cr105_09292017.pdf
> 
> It is really only concerned with following the BC1.2 spec and not
> interfering with the USB protocol part.
> 

Without knowing the internal design, but having more infor now, looks to me that
should be modelled more as a kind of power supply? Maybe something similar to
UCS1002-2 device (drivers/power/supply/ucs1002_power.c) but behind the EC?

Cheers,
 Enric

> 
>> If the EC is in charge of all of this, why does Linux need to get
>> involved?
>>
> 
> Only because we are looking to expose a policy to control the behavior
> of this chip at the OS level.
> 
> Most systems would put this in as an option in the BIOS but we do
> not have setup menus on Chrome OS and we want to have the policy
> controlled directly, preferably without resorting to an opaque interface
> to a userspace utility.
> 
> To that end we have added a number of different EC controls and are
> looking to fit them into the appropriate subsystems wherever possible.
> As you can see they don't always fit naturally..
> 
> 
>>>>> 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?
>>>
>>> We could add a Device Property to the affected USB port in ACPI and
>>> describe it that way, similar to other properties like 'vcc-supply', 'clocks',
>>> 'vbus-detect', etc and hook it into the phy-generic driver.
>>>
>>> However I'm not clear on whether the phy driver binding works with XHCI
>>> when using ACPI, so this may not be an appropriate place either.
>>
>> Why would you have DT involved if you are using acpi?  :)
>>
> 
> This would come in via the _DSD method of passing parameters to
> specific ACPI devices.
> 
> 
> -duncan
>