Message-ID: <dc8894f5-8960-4e0f-93ff-47f305cd902e@gmail.com>
Date: Sun, 9 Mar 2025 21:57:21 +0000
From: Colin Evans <colin.evans.parkstone@...il.com>
To: Alan Stern <stern@...land.harvard.edu>
Cc: eichest@...il.com, francesco.dolcini@...adex.com,
 gregkh@...uxfoundation.org, linux-kernel@...r.kernel.org,
 linux-usb@...r.kernel.org, stable@...r.kernel.org,
 stefan.eichenberger@...adex.com
Subject: Re: [PATCH v1] usb: core: fix pipe creation for get_bMaxPacketSize0

On 09/03/2025 21:01, Alan Stern wrote:
> On Sat, Mar 08, 2025 at 11:19:22PM +0000, Colin Evans wrote:
>> I believe I have the information requested. The output of usbmon for the "problem" scenario is
>> large, I hope it doesn't exceed any email attachment limits, but if it does I will have to work
>> out another way to share it.
>>
>> It may be that 30s of data is more than is needed. If that's the case I can easily run a shorter
>> usbmon cycle.
> It is a lot more than needed, but that's okay.
>
>> Additional Observations
>> -----------------------
>> It appears that having pretty much any external device plugged into a motherboard port connected
>> to the _problem_ controller is enough to suppress the stream of "usb usb2-port4: Cannot enable.
>> Maybe the USB cable is bad?" dmesg errors.
>>
>> For these tests the results named "working" had a USB2.0 memory stick plugged
>   into one
>> of the top 4 USB ports on the motherboard, while the "problem" results didn't.
>>
>> For info- the older machine that exhibits this problem ("machine 1") also shows device manager
>> errors if booted into Windows 10, suggesting that machine may in fact have a motherboard
>> hardware fault.
>>
>> However "machine 2" (which is less than 2 weeks old), shows no errors when booted into Windows.
> Well, I have no idea what Windows is doing on that machine.
>
> The usbmon trace shows that port 4 on bus 2 generates a continual
> stream of link-state-change events, constantly interrupting the system
> and consuming computational resources.  That's why the performance
> goes way down.
>
> I can't tell what's causing those link-state changes.  It _looks_ like
> what you would get if there was an intermittent electrical connection
> causing random voltage fluctuations.  Whatever the cause is, plugging
> in the memory stick does seem to suppress those changes; they don't
> show up at all in the "working" trace.
>
> In theory, turning off power to port 4 might stop all the events from
> being reported.  You can try this to see if it works:
>
> 	echo 1 >/sys/bus/usb/devices/2-0:1.0/usb2-port4/disable
>
> Alan Stern

Thank you, that is very helpful, for a couple of reasons.

"Machine 2" is a new build, so if (as it sounds) the motherboard has a 
hardware problem, then I need to
look into returning it.

BTW- it seems I spoke too soon about the USB stick suppressing the 
error. After a couple of reboots with
it in place the problem re-occurred. It does seem that connecting a hub 
(switch) is the  only way
to reliably stop the error. The switch has a bunch of wiring connected 
to USB peripherals and other
machines. I would have guessed that might make the likelihood of picking 
up electrical noise
actually worse, but that seems not to be the case here.

"Machine 1" is several years old, it's actually the guts of the same PC 
that was upgraded to make M/c 2.
It's not usable, or sellable, with this performance hit happening. I 
have tried all the external USB ports
on this machine and not found the failing controller, my guess is it's 
going to be one that supports
some of the on-board USB headers.

I had been looking on the web for a way to shut down the problem port, 
or worst case the whole hub,
however all the Linux examples I found worked by either-

a) Preventing the loading of the driver for the chipset, by type. 
However that would kill all ports supported by
     the same type of controller, and this motherboard has multiple 
controllers of the same type onboard.

b) Shutting down a port by searching for the connected device 
identifier. However in these cases there
     _are_ no connected devlces, the fault happens when the controller 
is not connected to anything.

Hopefully the command you recommended will do the trick, I will let you 
know.

Would I be correct in thinking this would need to be run at every boot, 
some time after device enumeration,
or would it need to be run after every re-enumeration of devices after a 
USB device is connected /
disconnected? Not sure how to achieve that.

I very much appreciate your help in identifying the fault. Thank you.

Regards: C Evans

-- 

*Mr. C. J. Evans
Parkstone
Davis Street
Hurst
RG10 0TH

Tel: 0118 9340297
Mob: 07434 491984
*

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