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

From: Alan Stern
Date: Mon Nov 16 2015 - 14:31:52 EST

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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