Re: [PATCH v2 0/2] extcon: Inform the state of both ID and VBUS pin for USB

[Chanwoo Choi <cw00.choi@samsung.com>]

Dear all,

On 05/29/2015 07:53 PM, Chanwoo Choi wrote:
> Hi Peter,
> 
> On 05/29/2015 10:22 AM, Peter Chen wrote:
>> On Thu, May 28, 2015 at 05:23:57PM +0300, Roger Quadros wrote:
>>> +Peter & Li,
>>>
>>> Ivan,
>>>
>>> On 28/05/15 11:45, Ivan T. Ivanov wrote:
>>>>
>>>> Hi Chanwoo,
>>>>
>>>> On Wed, 2015-05-27 at 21:15 +0900, Chanwoo Choi wrote:
>>>>> Previously, I discussed how to inform the changed state of both ID
>>>>> and VBUS pin for USB connector on patch-set[1].
>>>>> [1] https://lkml.org/lkml/2015/4/2/310
>>>>>
>>>>> So, this patch adds the extcon_set_cable_line_state() function to inform
>>>>> the additional state of external connectors without additional register/
>>>>> unregister functions. This function uses the existing notifier chain
>>>>> which is registered by extcon_register_notifier() / extcon_register_interest().
>>>>>
>>>>> The extcon_set_cable_line_state() can inform the new state of both
>>>>> ID and VBUS pin state through extcon_set_cable_line_state().
>>>>>
>>>>> For exmaple:
>>>>> - On extcon-usb-gpio.c as extcon provider driver as following:
>>>>>         static void usb_extcon_detect_cable(struct work_struct *work)
>>>>>         {
>>>>>                 ...
>>>>>                 /* check ID and update cable state */
>>>>>                 id = gpiod_get_value_cansleep(info->id_gpiod);
>>>>>                 if (id) {
>>>>>                         extcon_set_cable_state_(info->edev, EXTCON_USB_HOST, false);
>>>>>                         extcon_set_cable_state_(info->edev, EXTCON_USB, true);
>>>>>
>>>>>                         extcon_set_cable_line_state(info->edev, EXTCON_USB,
>>>>>                                                         EXTCON_USB_ID_HIGH);
>>>>
>>>> I am getting more and more confused :-). Why EXTCON_USB is now used for ID notifications?
>>>> It should be EXTCON_USB_HOST, no? Why we need another function, framework already have
>>>> required information from the function one line above, do I miss something?
>>>
>>> This is because the existing EXTCON_USB_HOST and EXTCON_USB do not capture all
>>> the 4 states of ID and VBUS pins that we need for a real USB driver to work.
>>>
>>> It looks like it was designed from user space users perspective where they are
>>> only interested in USB role. i.e. host or peripheral.
>>>
>>> Right now we are mixing both ID/VBUS and HOST/Peripheral states.
>>> This will break when we consider OTG role switching.
>>> With role switching, the USB device might start as a peripheral but switch role to host
>>> on the fly and the existing setup (including these patches) can't cater to that
>>> if user space is relying on EXTCON_USB_HOST/EXTCON_USB events.
>>> Because they are hard-wired to the ID pin state which doesn't change during
>>> role switch without cable switch.
>>>
>>> The USB driver doesn't care about EXTCON_USB_HOST/peripheral states.
>>> It just needs ID/VBUS states and should decide the Host/Peripheral state from
>>> that and other inputs (like HNP/user request/etc).
>>>
>>> The flow could be like this
>>>
>>> (extcon-usb-driver) -> [ID/VBUS states] -> (USB driver) -> [HOST/Peripheral states]
>>
>> Agree. Chanwoo, USB driver knows better than extcon driver about USB
>> role (host/peripheral), so the app should use USB interface to know it,
>> in fact, I don't be aware any use case needs to know USB role?
>> Are there any users for EXTCON_USB and EXTCON_USB_HOST currently?
> 
> You're right. But, extcon can just distinguish the type of external connectors
> and inform the type to the user-space and extcon consumer driver in kernel-space.
> 
> When USB mouse or keyboard is attached, user-space can check the state of externel connector which is attaced to the H/W target as following:
> - /sys/class/extcon/extconX/cable.0/name -> USB
> - /sys/class/extcon/extconX/cable.0/state -> 0
> - /sys/class/extcon/extconX/cable.1/name -> USB-HOST
> - /sys/class/extcon/extconX/cable.1/state -> 1
> 

On last month, Robert Baldyga sent the patch[1] for extcon-usb-gpio driver.
[1] https://lkml.org/lkml/2015/4/2/311
- [PATCH v3 2/4] extcon: usb-gpio: add support for VBUS detection

This patch[1] included a following table which show the type of external connectors
according to both ID and VBUS pin state.

 State              |    ID   |   VBUS
----------------------------------------
 [1] USB            |    H    |    H
 [2] none           |    H    |    L
 [3] USB & USB-HOST |    L    |    H
 [4] USB-HOST       |    L    |    L

So, I think that extcon-usb-gpio.c could set below two status with set:
- the state of whether USB/USB-HOST are attached or detached -> send the state to both kernel-space and user-space.
: extcon_set_cable_state_()
- the h/w line state of both ID and VBUS pin state -> send the state to only kernel space.
: extcon_set_cable_line_state()

[1]
 State              |    ID   |   VBUS
----------------------------------------
 [1] USB            |    H    |    H

extcon_set_cable_state_(edev, EXTCON_USB, true);
extcon_set_cable_state_(edev, EXTCON_USB_HOST, false);

extcon_set_cable_line_state(edev, EXTCON_USB, EXTCON_USB_ID_HIGH);
extcon_set_cable_line_state(edev, EXTCON_USB, EXTCON_USB_VBUS_HIGH);


[2]
 State              |    ID   |   VBUS
----------------------------------------
 [2] none           |    H    |    L

extcon_set_cable_state_(edev, EXTCON_USB, false);
extcon_set_cable_state_(edev, EXTCON_USB_HOST, false);

extcon_set_cable_line_state(edev, EXTCON_USB, EXTCON_USB_ID_HIGH);
extcon_set_cable_line_state(edev, EXTCON_USB, EXTCON_USB_VBUS_LOW);


[3]
 State              |    ID   |   VBUS
----------------------------------------
 [3] USB & USB-HOST |    L    |    H

extcon_set_cable_state_(edev, EXTCON_USB, false);
extcon_set_cable_state_(edev, EXTCON_USB_HOST, true);

extcon_set_cable_line_state(edev, EXTCON_USB, EXTCON_USB_ID_LOW);
extcon_set_cable_line_state(edev, EXTCON_USB, EXTCON_USB_VBUS_HIGH);


[4]
 State              |    ID   |   VBUS
----------------------------------------
 [4] USB-HOST       |    L    |    L

extcon_set_cable_state_(edev, EXTCON_USB, false);
extcon_set_cable_state_(edev, EXTCON_USB_HOST, true);

extcon_set_cable_line_state(edev, EXTCON_USB, EXTCON_USB_ID_LOW);
extcon_set_cable_line_state(edev, EXTCON_USB, EXTCON_USB_VBUS_LOW);

If I know the wrong knowledge, please let me know your comment and advice.

Thanks,
Chanwoo Choi