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

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Message-ID: <20170327175536.GP10760@atomide.com>
Date:   Mon, 27 Mar 2017 10:55:37 -0700
From:   Tony Lindgren <tony@...mide.com>
To:     Bin Liu <b-liu@...com>,
        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

* Bin Liu <b-liu@...com> [170327 10:17]:
> 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.

Yup.. I looked up the timers in the OTG spec and they are described
in chapter "8.5.5.2" as a_wait_bcon_tmo or a_wait_bcon_tmr. But
I could not find any values for them.

Anyways, clearly we want things working with real devices :)

> 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.

Yeh and it seems PM still works with the $subject patch also for
host mode. So maybe that's enough to fix the issue.

Also I don't have any idea why for ages we have been writing
0 to devctl there.. Maybe we've had a bug there that only now
shows up when we idle things.

Anyways, for the $subject patch related to MUSB runtime PM:

Tested-by: Tony Lindgren <tony@...mide.com>