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

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Date:	Mon, 16 Nov 2015 14:31:13 -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:

> ---
> 
> 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.

Alan Stern

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