Re: [PATCH] usb: musb: return -ESHUTDOWN in urb when three-strikes error happened

[Bin Liu <b-liu@ti.com>]

On Thu, May 14, 2020 at 10:02:59AM -0500, Bin Liu wrote:
> On Thu, May 14, 2020 at 10:40:53AM -0400, Alan Stern wrote:
> > On Thu, May 14, 2020 at 09:28:03AM -0500, Bin Liu wrote:
> > > On Wed, May 13, 2020 at 09:32:05PM -0400, Alan Stern wrote:
> > > > On Wed, May 13, 2020 at 04:36:20PM -0500, Bin Liu wrote:
> > > > > When a USB device attached to a hub got disconnected, MUSB controller
> > > > > generates RXCSR_RX_ERROR interrupt for the 3-strikes-out error.
> > > > > 
> > > > > Currently the MUSB host driver returns -EPROTO in current URB, then the
> > > > > USB device driver could immediately resubmit the URB which causes MUSB
> > > > > generate RXCSR_RX_ERROR interrupt again. This circle causes interrupt
> > > > > storm then the hub never got a chance to report the USB device detach.
> > > > > 
> > > > > To fix the interrupt storm, change the URB return code to -ESHUTDOWN for
> > > > > MUSB_RXCSR_H_ERROR interrupt, so that the USB device driver will not
> > > > > immediately resubmit the URB.
> > > > > 
> > > > > Signed-off-by: Bin Liu <b-liu@ti.com>
> > > > 
> > > > Strictly speaking, this is not the right thing to do.  It goes against 
> > > > the API described in error-codes.rst.  A better approach would be to fix 
> > > 
> > > error-codes.rst says:
> > > 
> > > -ESHUTDOWN              The device or host controller has been
> > > 			disabled due to some problem that could not
> > > 			be worked around, such as a physical
> > > 			disconnect.
> > > 
> > > So -ESHUTDOWN is applicable in this case - the device is disconnected
> > > behind a hub.
> > 
> > Yes, but you don't _know_ that the device was disconnected.  All you 
> > know is that there was a 3-strikes error.  Other problems can cause such 
> > errors (noise, for example).
> 
> Yes, I know this. But we don't have a solution then. I cannot add
> resubmit delay in those ~500 device drivers.

By the way I don't think noise could last long enough to cause 3-strikes
error. A shortest USB packet is about 3-bytes long, a noise should be
just a glitch, it won't last at least 3-bytes long to supress the bus
and 3 times on the exact timing when the host expecting a response
packet. I cannot think of any other reason which can cause the 3-strikes
error other than the device is off the bus.

-Bin.