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From: Felipe Balbi <>
To: ext David Brownell <>
Subject: Re: [PATCH] USB: OTG: Fix weirdnesses on enumerating partial otg devices
Date: Wed, 13 Feb 2008 11:29:05 -0500	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On Tue, Feb 12, 2008 at 12:32:34PM -0800, David Brownell wrote:
> > > > > > Some devices claim
> > > > > > to be b_host even though they have an a_connector attached to it.
> > > > > 
> > > > > Why not just fix that bug?  Remember that's Linux code.
> > > > 
> > > > The device claiming to be b_host is not linux based.
> > > 
> > > Wrong ... the meaning of that flag is:  *THIS* system is a
> > > Linux-USB OTG device which came up in B-peripheral role, and
> > > then through the magic of HNP morphed into the B-host role.
> > 
> > Don't be sarcastic I won't even fall into what Jean Delvare told you.
> I see no sarcasm there.  People seem to want different degrees

Ok, sorry here. Misunderstood the point of "magic"

> of handholding when being told they're wrong.  First time gently,
> second time less so ...
> I think there are limits to how long you can realistically expect
> someone (in this case, me) to let that drag on.
> (You object to "magic"?  Most LKML readers know squat about OTG,
> and that seemed like a succinct introduction to what it does.)
> > > Linux is acting as a host at that point.  So either it's
> > > being the A-host, or the B-host.  That flag says which.  If
> > > the other device has the A-connector, yet we're the B-host,
> > > then right now it is acting as an A-peripheral.  That's
> > > exactly what HNP is designed to do.
> > 
> > True, but allowing hnp nowing we're b_host doesn't sound correct.
> Why not?  The B-host can't proceed in that case.  The mechanism
> for not-proceeding is inside this:
>         if (!is_targeted(udev)) {
>                 if (udev->bus->b_hnp_enable || udev->bus->is_b_host) {
>                         err = usb_port_suspend(udev);
>                         if (err < 0)
>                                 dev_dbg(&udev->dev, "HNP fail, %d\n", err);
>                 }
>                 err = -ENOTSUPP;
>                 goto fail;
>         }
> Your proposal is to strike the "is_b_host" check.  In  terms of the
> OTG (1.3) state machine, that removes a b_host --> b_peripheral
> state transition.

Not at all, we're just not doing transition right now.

> BUT:  notice it doesn't replace it with the ONLY other valid state
> transition, b_host --> b_idle.   In fact, that can not be initiated
> by the B-device...
> So the current code *is* correct.
> > If 
> > we're b_host it can get two scenarios we would get to fall into this
> > state:
> > 
> > 	1. We asked to become host; or
> This is always the case... the other side can enable HNP all
> it wants, but it won't proceed unless the b-peripheral does
> its side of the dance.  (In terms of spec:  b_bus_req must be
> true before HNP can proceed.)
> > 	2. A_device allowed us become host because it can't handle us.
> We actually can't know why it started HNP; it may well have
> had other reasons.  All we know is that the roles switched.

Got your point.

> Maybe it finished its initial task, and decided to give our
> side the opportunity to accomplish some tasks it couldn't
> know about in advance.
> > In both cases, HNP is quite useless happening again, don't you think?
> No.  You have no way of knowing what the A-device may or may not
> do when it becomes host again.  It's getting a chance to do
> more work, in conformance with what OTG is supposed to do.
> > We should at least try to enumerate a_peripheral, if it can't due to
> > our power capabilities i.e. 100mA, we'll fall into overcurrent
> > protection and we'll suspend the bus and disconnect, which would give
> > a_device another change to enumerate us.
> > 
> > This sound much better to me.
> You're overlooking something:  this is the "!is_targeted(udev)"
> case, so we have already enumerated the a_peripheral as much as
> we can.  There is *nothing* more we can do with it.  Sitting
> around in the b_host state would be utterly pointless.

And you're overlooking something: tpl is not an enumeration blocker at
all. The only blockers are drivers and power limits.
Imagine what happens on n810, we have 3 mass storage devices listed on
its tpl: 770, n800 and itself. Besides that, we allow enumeration of any
mass storage device as long as it draws less than 100mA. These tips I
got from otg chairman himself and from OTG Clarification document, you
can find it in So, what I'm trying to say is if
a_device let us become host, we shold at least try to do, let's call it,
full enumeration. Which means attaching any unlisted mass storage device
should work.

> > > So it can happen again, to go back into the b_peripheral state.
> > > (And of course, we can't set the b_hnp_enable flag in a device
> > > which is in the a_peripheral state...)
> > 
> > it shouldn't happen again at this point. If we're b_host, we should try to
> > enumerate a_peripheral. If it draws more than 100mA the session should
> > end, otherwise we'd fall into hnp loops until we start getting hub port
> > errors and linux decide to suspend roothub and end the session.
> Notice that because we're b_host we are not supplying power.  :)

True, sorry. :-p

> Re HNP loops, go look again at the B-device state diagram.  Now
> tell me how the B-device could terminate such a loop ... it can't.
> But look at the A-device diagram ... it obviously *CAN* terminate
> such a loop (by turning off VBUS power).

aka ending the session, yeah. But once again I'm still thinking we
should try to enumerate the device and, in mass storage case, start scsi
emulation, mount fs and so on.

> Though I don't think Linux currently *will* terminate that loop.
> It needs to disable port power, which is still problematic.  And
> also it would need to *decide* to disable port power.

But then a-device should end the session, not b. If we really can't use
that a_peripheral, we suspend and roles swtich again. In this case
a_host should know we're still that non-working b_peripheral and end the

> > > Seems like the root cause of the problem here is that you
> > > have some correctly functioning code, but for some reason
> > > you're surprised by that correct functioning.  (Maybe this
> > > is a case where neither device is on the other's whitelist?)
> > 
> > True, neither is in other's whitelist, but the point is:
> > 
> >  1. Currently standard-a device supports hnp only on A states
> That would be a bug; the a_peripheral --> a_wait_bcon transition
> has always been part of OTG.

OTG 1.3 page 25:

OTG products are required to support HNP as an A-device. OTG products
must support HNP as a B-device if they can support any OTG product as
a peripheral. OTG products that cannot support any OTG product
as a peripheral are not required to support HNP as a B-device.

> >  2. Currently standard-b device supports hnp on both roles
> Correct; that conforms with OTG requirements.
> >  3. A-device enumerates b-device, checks it's not tpl'ed and allow hnp
> OK ...
> >  4. B-device become b_host, checks a_peripheral is not on its tpl, even
> >     though it has support for that device class
> If it supports a device class I'd say that should be included
> in the TPL ("whitelist").  Though if it tries to do that, it's
> an explicit violation of the OTG spec (sect 3.3):

We can't support classes. What I'm trying to say is we have usb mass
storage support enabled in kernel and we can handle _any_ mass storage
devices as long as they meet otg requirements.

> 	The Targeted Peripheral List shall not list supported
> 	USB Classes or ???similar??? devices.
> >     and it draws less than  100mA.
> Irrelevent, though possibly the Linux code there needs to be
> taught to ignore the power constraints when it is_b_host ...
> because the B-device is never supplying power to the A-device,
> even when it's in the b_host state.
> > According to otg tpl clarification, they should work in this scenario,
> > shouldn't they ? Or should they stay in an hnp loop until someone
> > decides to end the session ?
> Shouldn't work, because your points 1 and 4 violate specs.

Not true, check again Chapter 6 Host Negotiation Protocol and OTG
Clarification regarding TPL.

> Though it would make sense to me to have the host record
> whether it already gave up on the peripheral once ... and
> if it did, then power it off.

Good catch. That'd be nice.

	- Balbi

  reply	other threads:[~2008-02-13  9:29 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2008-02-12 17:00 Felipe Balbi
2008-02-12 10:28 ` David Brownell
2008-02-12 17:45   ` Felipe Balbi
2008-02-12 13:02     ` David Brownell
2008-02-12 20:22       ` Felipe Balbi
2008-02-12 20:32         ` David Brownell
2008-02-13 16:29           ` Felipe Balbi [this message]
2008-02-13 21:36             ` David Brownell
2008-02-14  5:03               ` Felipe Balbi

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