linux-kernel - Re: [PATCH v10 5/6] usb:cdns3 Add Cadence USB3 DRD Driver

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Date:   Tue, 13 Aug 2019 10:48:03 +0300
From:   Roger Quadros <rogerq@...com>
To:     Chunfeng Yun <chunfeng.yun@...iatek.com>
CC:     Felipe Balbi <felipe.balbi@...ux.intel.com>,
        Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
        Pawel Laszczak <pawell@...ence.com>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "jbergsagel@...com" <jbergsagel@...com>,
        "nsekhar@...com" <nsekhar@...com>, "nm@...com" <nm@...com>,
        Suresh Punnoose <sureshp@...ence.com>,
        Jayshri Dajiram Pawar <jpawar@...ence.com>,
        Rahul Kumar <kurahul@...ence.com>,
        Anil Joy Varughese <aniljoy@...ence.com>
Subject: Re: [PATCH v10 5/6] usb:cdns3 Add Cadence USB3 DRD Driver



On 13/08/2019 10:30, Chunfeng Yun wrote:
> On Mon, 2019-08-12 at 16:04 +0300, Roger Quadros wrote:
>>
>> On 12/08/2019 15:46, Felipe Balbi wrote:
>>>
>>> Hi,
>>>
>>> Roger Quadros <rogerq@...com> writes:
>>>>> The sysfs file we expose from the class for the role switches is
>>>>> primarily meant for supporting proprietary protocols that require us
>>>>> to basically override the connector USB data role. The default role
>>>>> should always be selected in the drivers.
>>>>
>>>> OK. Let's take this example
>>>> - Port is dual-role port micro AB.
>>>> - microAB to type-A adapter is connected which pulls ID low. port transitions
>>>> to "host" role by the controller driver.
>>>> - proprietary protocol want to switch role to device role so writes "device" to
>>>> mode switch sysfs. port transitions to "device" role.
>>>>
>>>> Now, how does controller driver know to fall back to HW based role switching?
>>>
>>> Use a 'disconnect' or 'suspend' event to go reset it? But that should,
>>> probably, be done at kernel space, no?
>>>
>>
>> Yes that could be one option.
>> So after a disconnect, sysfs role should reflect actual hardware role. correct?
> 
> Maybe it's difficult to support both HW based role switch and SW based
> role switch by sysfs at the same if the HW's FSM rely on, such as, the
> state of Vbus pin or ID pin. Likes the upper example, when user writes
> "device" to mode switch sysfs, the driver should skip the HW state of ID
> pin, due to it's state is Low, or force it as High.
> 

We do need a clear way of indicating that SW wants to override so HW
state is ignored.

> Another option way is that introduces a property in DTS to indicate the
> way the driver want to use (HW based or SW based, usb_role_switch
> doesn't provide this information for the controller driver), but is not
> flexible enough.

That is not good enough for us. We need both HW and SW based role switching.

Can we introduce a new state (e.g. "auto") in usb_role_switch. This would
explicitly indicate the driver to do HW based switching.

This way we don't need to depend on connect/disconnect events and can
do role switch tests even without cable/device connected.

-- 
cheers,
-roger
Texas Instruments Finland Oy, Porkkalankatu 22, 00180 Helsinki.
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