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

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Date:   Thu, 11 May 2017 15:44:06 -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 Thu, May 11, 2017 at 01:23:06PM -0700, Tony Lindgren wrote:
> * Bin Liu <b-liu@...com> [170511 13:05]:
> > On Thu, May 11, 2017 at 12:38:11PM -0700, Tony Lindgren wrote:
> > > 
> > > I wonder if just keeping VBUS on longer in OTG_STATE_A_WAIT_BCON
> > > solves this issue?
> > 
> > We don't cut VBUS intentionally for host mode (when devctl=0x19). The
> > VBUS got cut in this case only because when a device is detached, the
> > otg state changes from A_HOST -> A_WAIT_BCON, then otg_timer() cuts VBUS
> > and sets the state to A_IDLE, then next otg_timer() turns on VBUS (I
> > haven't check how the otg state becomes A_WAIT_VRISE from here).
> > 
> > Not sure how to *easily* keep VBUS here, without adding condition check
> > in otg_timer() for TG_STATE_A_WAIT_BCON.
> > 
> > > solves this issue? It seems the issue is with modems that get
> > > reconfigured after the initial enumeration?
> > 
> > Idealy we shouldn't cut VBUS at all in this case for host-only. I am
> > looking for a small patch to solve this, if possilbe ;)
> 
> Maybe try something like below, compile tested only. I don't
> think I have any USB modem here to test with.

The patch below doesn't help. In device detach by the time
dsps_check_status() is called, the otg state is already A_WAIT_BCON, set
by musb_root_disconnect() in musb_stage0_irq() when handling DISCONNECT
interrupt.

> 
> > > We could poll for new devices every 2 seconds, if anything is seen
> > > on the bus, keep VBUS on at least 20 seconds, then if nothing is
> > > found, poll every 2 seconds again.
> > 
> > I am not sure this is relevant, VBUS is constantly on once the otg stage
> > becomes A_HOST -> A_WAIT_BCON -> A_IDLE -> A_WAIT_VRISE, within a
> > second.
> 
> Yup, but it sounds like once the modem changes mode, it disappears
> from the USB bus for a long enough time where we go to A_WAIT_BCON
> again. Or else I'm misunderstanding what's going on.

I don't think it is about how long the modem disappears. When detach happens,
DISCONNECT interrupt happens, then dsps_check_status() is scheduled with
state A_WAIT_BCON, then VBUS got cut.

BTY, I didn't debug with a modem, just with a device detach. Tring to
see how to not cut vbus at all in dsps_check_status().

> 
> It's also possible that we have dsps_check_status() getting called
> again on disconnect before the new 20 second period is over, I did
> not check for that yet.

After a few trials, I start to think about a little cleanup in the otg
state machine in the musb drivers, I think we need to somehow
distinguish between normal and error conditions for A_WAIT_BCON state.

Regards,
-Bin.

> 
> Regards,
> 
> Tony
> 
> 8< -----------------------
> diff --git a/drivers/usb/musb/musb_dsps.c b/drivers/usb/musb/musb_dsps.c
> --- a/drivers/usb/musb/musb_dsps.c
> +++ b/drivers/usb/musb/musb_dsps.c
> @@ -270,6 +270,10 @@ static int dsps_check_status(struct musb *musb, void *unused)
>  		musb_writel(musb->ctrl_base, wrp->coreintr_set,
>  			    MUSB_INTR_VBUSERROR << wrp->usb_shift);
>  		break;
> +	case OTG_STATE_A_HOST:
> +		if (glue->vbus_irq)
> +			dsps_mod_timer(glue, 20000);	/* 20s */
> +		break;
>  	default:
>  		break;
>  	}
> -- 
> 2.13.0