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

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Date:   Tue, 28 Mar 2017 08:10:00 +0200
From:   Moreno Bartalucci <moreno.bartalucci@...norama.it>
To:     Bin Liu <b-liu@...com>
Cc:     Tony Lindgren <tony@...mide.com>, 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

> Il giorno 27 mar 2017, alle ore 19:15, Bin Liu <b-liu@...com> ha scritto:
> 
> […]
> 
> 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.
> 

During my research, I used this patch to try to print the status of the usb port:

--- a/drivers/usb/musb/musb_dsps.c	2017-03-13 09:34:31.000000000 +0100
+++ b/drivers/usb/musb/musb_dsps.c	2017-03-13 09:36:02.000000000 +0100
@@ -245,6 +245,8 @@ static void otg_timer(unsigned long _mus
 	dev_dbg(musb->controller, "Poll devctl %02x (%s)\n", devctl,
 				usb_otg_state_string(musb->xceiv->otg->state));

+	dev_emerg(musb->controller, "musb->xceiv->otg->state=%s, musb->port_mode=%d\n", usb_otg_state_string(musb->xceiv->otg->state),(int)musb->port_mode);
+
 	spin_lock_irqsave(&musb->lock, flags);
 	switch (musb->xceiv->otg->state) {
 	case OTG_STATE_A_WAIT_BCON:

Unless I did something wrong, maybe it’s worth to notice that before the patch that apparently introduced this bug (2f3fd2c5bde1f94513c3dc311ae64494085ec371) I got nothing printed anywhere.
With that patch applied, instead, I got the line printed in dmesg.
I might be wrong but my assumption is that without that patch otg_timer was never called.
If this is true, it would explain why writing 0 on DEVCTL didn’t bother anything: it never happened.

Regards,

Moreno