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

[Paul Kocialkowski <contact@paulk.fr>]

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Hi,

Le vendredi 20 avril 2018 à 09:25 -0500, Bin Liu a écrit :
> 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?

> > 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?

Correct.

> > 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.

Cheers,

-- 
Paul Kocialkowski,

developer of free digital technology and hardware support.

Website: https://www.paulk.fr/
Coding blog: https://code.paulk.fr/
Git repositories: https://git.paulk.fr/ https://git.code.paulk.fr/

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