linux-kernel - Re: [PATCH v2 1/2] usb: dwc2: host: Fix missing device insertions

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Date:	Mon, 16 Nov 2015 15:31:55 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Doug Anderson <dianders@...omium.org>
cc:	Felipe Balbi <balbi@...com>, John Youn <John.Youn@...opsys.com>,
	Yunzhi Li <lyz@...k-chips.com>,
	Heiko Stübner <heiko@...ech.de>,
	"open list:ARM/Rockchip SoC..." <linux-rockchip@...ts.infradead.org>,
	Julius Werner <jwerner@...omium.org>,
	"Herrero, Gregory" <gregory.herrero@...el.com>,
	"Kaukab, Yousaf" <yousaf.kaukab@...el.com>,
	Dinh Nguyen <dinguyen@...nsource.altera.com>,
	John Youn <johnyoun@...opsys.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/2] usb: dwc2: host: Fix missing device insertions

On Mon, 16 Nov 2015, Doug Anderson wrote:

> Alan,
> 
> On Mon, Nov 16, 2015 at 11:31 AM, Alan Stern <stern@...land.harvard.edu> wrote:
> > On Mon, 16 Nov 2015, Doug Anderson wrote:
> >
> >> ---
> >>
> >> usb: dwc2: host: Fix missing device insertions
> >>
> >> If you've got your interrupt signals bouncing a bit as you insert your
> >> USB device, you might end up in a state when the device is connected but
> >> the driver doesn't know it.
> >>
> >> Specifically, the observed order is:
> >>  1. hardware sees connect
> >>  2. hardware sees disconnect
> >>  3. hardware sees connect
> >>  4. dwc2_port_intr() - clears connect interrupt
> >>  5. dwc2_handle_common_intr() - calls dwc2_hcd_disconnect()
> >>
> >> Now you'll be stuck with the cable plugged in and no further interrupts
> >> coming in but the driver will think we're disconnected.
> >>
> >> We'll fix this by checking for the missing connect interrupt and
> >> re-connecting after the disconnect is posted.  We don't skip the
> >> disconnect because if there is a transitory disconnect we really want to
> >> de-enumerate and re-enumerate.
> >
> > Why do you need to do anything special here?  Normally a driver's
> > interrupt handler should query the hardware status after clearing the
> > interrupt source.  That way no transitions ever get missed.
> >
> > In your example, at step 5 the dwc2 driver would check the port status
> > and see that it currently is connected.  Therefore the driver would
> > pass a "connect status changed" event to the USB core and set the port
> > status to "connected".  No extra checking is needed, and transitory
> > connects or disconnects get handled correctly.
> 
> Things are pretty ugly at the moment because the dwc2 interrupt
> handler is split in two.  There's dwc2_handle_hcd_intr() and
> dwc2_handle_common_intr().  Both are registered for the same "shared"
> IRQ.  If I had to guess, I'd say that this is probably because someone
> wanted to assign the ".irq" field in the "struct hc_driver" for the
> host controller but that they also needed the other interrupt handler
> to handle things shared between host and gadget mode.
> 
> In any case, one of these two interrupt routines handles connect and
> the other disconnect.  That's pretty ugly but means that you can't
> just rely on standard techniques for keeping things in sync.

Okay, that is rather weird.  Still, I don't see why it should matter.  

Fundamentally there's no difference between a "connect" interrupt and a
"disconnect" interrupt.  They should both do exactly the same things:
clear the interrupt source, post a "connection change" event, and set
the driver's connect status based on the hardware's current state.  
The second and third parts can be handled by a shared subroutine.

If you think of these things as "connect changed" interrupts rather 
than as "connect" and "disconnect" interrupts, it makes a lot of sense.

> It would probably be a pretty reasonable idea to try to clean that up
> more, but that would be a very major and intrusive change.

Maybe so -- I'll take your word for it since I'm not at all familiar 
with the dwc2 code.

Alan Stern

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