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

[Roger Quadros <rogerq@ti.com>]

Chanwoo,

On 29/05/15 15:15, Chanwoo Choi wrote:
> 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.

I don't see any issues with extcon_get_cable_line_state(). I would prefer that
than changing existing APIS.

User space users will still fail in the dynamic role swap case [3]. But probably nobody
cares about it as nobody seems to be complaining or using role-swap :)
So we can choose to ignore that and solve it when someone complains.

cheers,
-roger