linux-kernel - Re: [PATCH] usb: musb: Support gadget mode when the port is set to dual role

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Date:   Fri, 22 Mar 2019 09:04:04 -0500
From:   Bin Liu <b-liu@...com>
To:     Maxime Ripard <maxime.ripard@...tlin.com>
CC:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Paul Kocialkowski <contact@...lk.fr>,
        Paul Kocialkowski <paul.kocialkowski@...tlin.com>,
        <linux-kernel@...r.kernel.org>, <linux-usb@...r.kernel.org>,
        Chen-Yu Tsai <wens@...e.org>
Subject: Re: [PATCH] usb: musb: Support gadget mode when the port is set to
 dual role

Hi all,

On Fri, Mar 22, 2019 at 02:46:30PM +0100, Maxime Ripard wrote:
> >
> > Again, think about an embedded product,
> 
> That's all I'm thinking about.
> 
> > if dr_mode is 'otg' which indicates the peripheral mode will be used
> > at some point
> 
> No, it indicates that it *might* be used at some point, based on a
> number of external factors, including:
>   - Whether or not the user has plugged something in the connected USB
>     connector
>   - If they did so, how the ID pin has been wired (and therefore, is
>     a device or a host on the other end)
>   - And how the system designer decided to configure their kernel and
>     userspace.
> 
> > when and how to load the gadget driver if it is not loaded
> > automatically when Linux boots up? the end user doesn't have access
> > to the console.
> 
> An application could load it. And really, we start seeing SoCs in more
> and more pc-like devices, including with mUSB, so I don't think we
> should be making assumptions here.
> 
> How do you think Fedora, Ubuntu or Debian would behave here?
> 
> > > Because no other controller requires it and therefore it's not
> > > standard and violates the principle of least surprise?
> >
> > I know no other controller does this, but this doesn't mean it is not
> > standard.
> 
> I'm pretty sure that would be the definition of "standard", or of a
> norm at least.
> 
> > > And even without taking this into account, there's also the fact that
> > > while the *hardware* can do dual role, the software might decide
> > > otherwise. If I don't want to have support for any gadget (at all) in
> > > the end system, then why should I be forced to compile and load
> > > something I don't even want to use in the first place?
> >
> > then dr_mode should be set to 'host' instead, you don't have to load a
> > gadget if peripheral mode will never be used.
> 
> No. The hardware is perfectly capable of using OTG. The software has
> been configured not to.

I don't think the argument will lead to anywhere. Let's stop arguing
here, so that you can spend time to fix the driver if you want to.

Regards,
-Bin.