linux-kernel - Re: [PATCH] usb-musb: keep VBUS on when device is disconnected

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Date:   Mon, 27 Mar 2017 12:15:34 -0500
From:   Bin Liu <b-liu@...com>
To:     Tony Lindgren <tony@...mide.com>
CC:     Moreno Bartalucci <moreno.bartalucci@...norama.it>,
        Lars Melin <larsm17@...il.com>,
        "linux-omap@...r.kernel.org" <linux-omap@...r.kernel.org>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Alessio Igor Bogani <abogani@...nel.org>
Subject: Re: [PATCH] usb-musb: keep VBUS on when device is disconnected

On Mon, Mar 27, 2017 at 09:59:47AM -0700, Tony Lindgren wrote:
> * Moreno Bartalucci <moreno.bartalucci@...norama.it> [170327 09:23]:
> > If I understood your patch, however, if the device (anyone, not just my one) takes longer to switch, VBUS is deasserted anyway.
> 
> Yeah some of them can take at least 10 seconds even to enumerate.
> So probably we need to have to have some longer timeout set for
> OTG_STATE_A_WAIT_BCON, like 20 or 30 seconds.
> 
> > Although this patch is working for me, personally I would prefer a solution which would not deassert VBUS. At least on a host only port. Honestly I don’t know how a dual role port should work.
> 
> It's been really long time since I read the OTG spec. There
> may be some diagram showing the required timeouts in the spec
> if there is one for VBUS.
> 
> Maybe we need some property to specify vbus-always-on-in-host-mode?

The MUSB otg state machine has been changed in many place since the last
time I looked at it, and I am not sure how exactly it works now.

If the $subject patch can correctly keep the VBUS on for host-only mode,
we can somehow use dr_modei value to distinguish the mode. We don't have
to create a new vbus-always-on-in-host-mode flag. VBUS has to be always
on in host-only mode anyway, until some error condition happens.

Regards,
-Bin.