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Subject: Re: [PATCH V3] leds: trigger: Introduce an USB port trigger
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Date: Thu, 25 Aug 2016 20:39:24 +0200
In-Reply-To: <20160825125305.GB15567@kroah.com> (Greg KH's message of "Thu, 25
        Aug 2016 08:53:05 -0400")
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Greg KH <gregkh@linuxfoundation.org> writes:
> On Thu, Aug 25, 2016 at 07:14:52AM +0200, Rafał Miłecki wrote:
>>
>> Good question. I would like to extend this USB port trigger in the
>> future by reacting to USB activity. This involves playing with URBs
>> and I believe that at that point it'd be getting too much USB specific
>> to /rule them all/.
>
> Oh that's never going to work, sorry.  USB "activity" happens deep in
> USB host controller hardware, not up at the kernel level at all.  It's
> just too fast, and would take up too much CPU to do it otherwise.  Heck,
> even old UHCI 1.1 USB controllers did this.
>
> USB "activity" happens all the time, look at a USB bus analyzer if you
> want to see what goes on.  Teasing out what is "real data" and what is
> just "normal bus activity" is impossible at the kernel level,

OK, I am going to play stupid again: Does this mean that usbmon is
impossible?

> and really
> makes no sense at all (your led would just be "on" all the time.)

I don't see how this is any different from e.g. the network activity
triggers.

FWIW, on my system "cat /sys/kernel/debug/usb/usbmon/0u" can be silent
for long periods, and show a rush of lines whenever there is USB
activity.  I imagine that a LED triggered by a hook in a similar place
would be useful on systems with LEDs for that sort of stuff.


Bjørn