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Date:   Thu, 21 Mar 2019 17:41:38 +0100
From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     Maxime Ripard <maxime.ripard@bootlin.com>
Cc:     Bin Liu <b-liu@ti.com>, Paul Kocialkowski <contact@paulk.fr>,
        Paul Kocialkowski <paul.kocialkowski@bootlin.com>,
        linux-kernel@vger.kernel.org, linux-usb@vger.kernel.org,
        Chen-Yu Tsai <wens@csie.org>
Subject: Re: [PATCH] usb: musb: Support gadget mode when the port is set to
 dual role
Message-ID: <20190321164138.GB11121@kroah.com>
References: <20180328215213.29538-1-contact@paulk.fr>
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On Thu, Mar 21, 2019 at 02:01:33PM +0100, Maxime Ripard wrote:
> Hi,
> 
> I'm reviving this thread a bit, because I encountered this bug today.
> 
> On Thu, Mar 21, 2019 at 11:02:10AM +0100, Bin Liu wrote:
> > On Sat, Apr 21, 2018 at 12:59:23PM +0200, Paul Kocialkowski wrote:
> > > Hi,
> > >
> > > Le vendredi 20 avril 2018 ? 09:25 -0500, Bin Liu a ?crit :
> > > > 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@paulk.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.
> > >
> > > Yes you're right, I didn't make myself very clear here. I didn't
> > > investigate the udc_start call path much since it was apparently not the
> > > culprit.
> > >
> > > > 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().
> > >
> > > It does work under that scenario, although my used case here is using
> > > musb with DUAL_ROLE but no gadget driver loaded. That it, I want the
> > > musb_start call to originate from the virtual hub, not from the gadget
> > > side.
> > >
> > > > > 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.
> > >
> > > It makes sense that the port should be in b_idle state by default, but
> > > here it fails to switch to host mode when the ID pin detects that it
> > > should. Or does b_idle state entail that a gadget must be loaded (per
> > > the USB spec), and thus nothing should (ever) happen until that happens?
> > >
> > > I find it really odd to need a gadget device to trigger host mode.
> > > This patch does fix the issue, but I am puzzled as to why it is needed
> > > in the first place. The comment above it mentions that "In OTG mode we
> > > have to wait until we loaded a gadget. We don't really need a gadget if
> > > we operate as a host but we should not start a session as a device
> > > without a gadget or else we explode.", which is apparently compatible
> > > with my use case: a gadget is not really needed and I'm not trying to
> > > start a session as a device without a gadget loaded.
> > >
> > > What do you think?
> >
> > Okay, this came down to an argument that whether we should require
> > loading a gadget driver on a dual-role port to work in host mode,
> > which is currently required on musb since a long long time ago.
> >
> > I understand the requirement is kinda unnecessary, but since it already
> > exists on musb stack for a long time, I don't plan to change it. Because I
> > cannot think of a use case in real products that doesn't automatically
> > load a gadget function on the dual-role port.
> >
> > If you can explain a use case in real world (not a engineering lab) that
> > the gadget driver will not be loaded at linux booting up, but later
> > based on user's input, I will reconsider my decision. To remove this
> > requirement from musb stack, the work is more than this patch.
> 
> I have one for you: we're working on a device that boots pretty fast,
> and therefore are pushing as much things as we can to modules. It
> includes gadgets, the musb driver and glue, etc. That doesn't sound
> way very different from what a generic distro would do as well.
> 
> At boot, the various modules for the hardware are loaded
> automatically: the musb glue, the musb core, our USB PHY, etc. We end
> up in a situation where the musb driver is loaded and reported to work
> properly. The USB cable to the OTG port (in peripheral) might or might
> not be connected, it's kind of irrelevant.
> 
> The gadgets, however, are not loaded automatically.
> 
> Now comes a user that wants to use musb as a host, and connect a
> proper USB adapter, that wires the ID pin properly. In our case, the
> phy detects it, reports the mode change, and .... nothing.
> 
> That doesn't really look like an engineering lab setup to me.

I agree, that sounds like a valid setup.

Also realize that Android is pushing to have all drivers as modules, so
you will start to see a whole lot more devices out there be modular
instead of statically built kernels.  So issues like this are good to
resolve :)

thanks,

greg k-h