linux-kernel - Re: [PATCH v10 2/7] usb: mux: add generic code for dual role port mux

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Date:	Tue, 7 Jun 2016 17:53:31 +0800
From:	Lu Baolu <baolu.lu@...ux.intel.com>
To:	Jun Li <jun.li@....com>, Roger Quadros <rogerq@...com>,
	Peter Chen <hzpeterchen@...il.com>
Cc:	"felipe.balbi@...ux.intel.com" <felipe.balbi@...ux.intel.com>,
	Mathias Nyman <mathias.nyman@...el.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Lee Jones <lee.jones@...aro.org>,
	Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
	Liam Girdwood <lgirdwood@...il.com>,
	Mark Brown <broonie@...nel.org>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v10 2/7] usb: mux: add generic code for dual role port mux

Hi,

On 06/07/2016 11:03 AM, Jun Li wrote:
> Hi Roger
>
>>  
>> For Mux devices implementing dual-role, the mux device driver _must_ use
>> OTG/dual-role core API so that a common ABI is presented to user space for
>> OTG/dual-role.
> That's the only point we have concern, do dual role switch through
> OTG/dual-role core, not do it by itself.
>
>> I haven't yet looked at the mux framework but if we take care of the above
>> point then we are not introducing any redundancy.
>>
> Roger, actually this is my worry on OTG core: those dual role switch
> users just tends to do it simply by itself(straightforward and easy),
> not through the OTG core(some complicated in first look),

I'm sorry, but I'm really confused.

Why do we need to drop "straightforward and easy", but have to run
an *unnecessary* OTG state machine? Don't you think that will (1) add
*unnecessary* software complexity; (2) increase *unnecessary* memory
footprint; and (3) increase the debugging efforts?

> this is just an example for us to convince people to select a better
> way:)

Sure. Let's take my case for an example.

My system has a third-party port mux, which is not part any USB controllers.
Also, my system doesn't have any DRD capable devices. I need a
"straightforward and easy" driver for it. Otherwise, the system could not be
waken up from system suspend.

But you said I must run an unnecessary OTG state machine, even thought it
has nothing to do with my system, only because the two sides of my port
mux device is a host and peripheral controller.

Why?

Best regards,
Lu Baolu