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

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Message-ID: <20180420142524.GB29011@uda0271908>
Date:   Fri, 20 Apr 2018 09:25:24 -0500
From:   Bin Liu <b-liu@...com>
To:     Paul Kocialkowski <paul.kocialkowski@...tlin.com>
CC:     Maxime Ripard <maxime.ripard@...tlin.com>,
        <linux-kernel@...r.kernel.org>, <linux-usb@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Chen-Yu Tsai <wens@...e.org>,
        Paul Kocialkowski <contact@...lk.fr>
Subject: Re: [PATCH] usb: musb: Support gadget mode when the port is set to
 dual role

On Thu, Mar 29, 2018 at 01:57:24PM +0200, Paul Kocialkowski wrote:
> Hi,
> 
> On Thu, 2018-03-29 at 11:23 +0200, Maxime Ripard wrote:
> > On Wed, Mar 28, 2018 at 11:52:13PM +0200, Paul Kocialkowski wrote:
> > > This allows dual-role ports to be reported as having gadget mode by
> > > the
> > > musb_has_gadget helper. This is required to enable MUSB at all with
> > > MUSB
> > > glue layers that set the port mode to MUSB_PORT_MODE_DUAL_ROLE at
> > > init.
> > > 
> > > Most notably, this allows calling musb_start when needed in the
> > > virtual
> > > MUSB root HUB, regardless of whether the current mode should be
> > > gadget
> > > or host.
> > > 
> > > This fixes USB OTG on Allwinner devices that I could test it with,
> > > mainly A20 devices.
> > > 
> > > Signed-off-by: Paul Kocialkowski <contact@...lk.fr>
> > 
> > Surely there's more to it than that. The gadget mode of A20 boards
> > have been working in the past, including when compiling with mUSB
> > setup as dual role.
> >
> > Is this a regression since a particular commit? Or is there another,
> > deeper issue overlooked in the commit log?
> 
> The root of the issue here is that musb_start is not called at any point
> without this patch. My understanding of the flow is the following: when
> the PHY detects that there was a VBUS/ID change, it will notify its
> listeners (mainly the musb sunxi glue layer). This will then schedule
> the driver's work (sunxi_musb_work), which does nothing since the
> SUNXI_MUSB_FL_ENABLED bit was never set. This bit is only set after
> calling sunxi_musb_enable, which is called from musb_platform_enable,
> that originates from musb_start.
> 
> Currently I see two places where musb_start is called:
> * musb_virthub
> * musb_gadget
> 
> In the latter case, it is in turn called from udc_start, which should
> probably (correct me if I'm wrong) happen later in the call chain than
> ID/VBUS change notification time.

I don't think it is correct that udc_start() is triggered by ID/VBUS
events, but I don't have an Allwinner platform to verify the callflow.

Have you tried to load with a gadget driver? When a gadget function is
bound to UDC, udc_start() is triggered, which in turn calls
musb_start().

> 
> In the former case, musb_start is called in the root controller hub
> control, when setting the USB_PORT_FEAT_POWER feature. This looks
> perfectly legit and IMO this is where it should be initially calling
> musb_start in the dual role case. The kernel is indeed setting the

No actually. A dual-role port should be in b_idle state by default, so
logically all actions should go to the gadget path until the port
switches to host mode.

> feature, only that it fails to enable musb without this patch.
> 
> First, I'd like to make sure that this understanding of the flow is
> correct as I may have missed something here. Does it make sense?

I am guessing you didn't load a gadget driver when testing?

> Then, it seems that the offending commit is: be9d39881fc4f
> ("usb: musb: host: rely on port_mode to call musb_start()")
> 
> That itself fixed: ae44df2e21b5
> ("usb: musb: call musb_start() only once in OTG mode")
> 
> Still, this commit was authored in June 2015, so almost 3 years ago.
> In the meantime, the sunxi driver has received feature improvements, so
> it seems hard to believe that it was broken all this time...
> 
> Cheers,

Regards,
-Bin.