linux-kernel - Re: [PATCH V3] leds: trigger: Introduce an USB port trigger

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Date:   Thu, 25 Aug 2016 20:39:24 +0200
From:   Bjørn Mork <bjorn@...k.no>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     Rafał Miłecki <zajec5@...il.com>,
        Richard Purdie <rpurdie@...ys.net>,
        Jacek Anaszewski <j.anaszewski@...sung.com>,
        Felipe Balbi <balbi@...nel.org>,
        Peter Chen <hzpeterchen@...il.com>,
        "linux-usb\@vger.kernel.org" <linux-usb@...r.kernel.org>,
        Rafał Miłecki 
        <rafal@...ecki.pl>, Jonathan Corbet <corbet@....net>,
        Ezequiel Garcia <ezequiel@...guardiasur.com.ar>,
        Matthias Brugger <mbrugger@...e.com>,
        Boris Brezillon <boris.brezillon@...e-electrons.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Stephan Linz <linz@...pro.net>,
        "open list\:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        "open list\:LED SUBSYSTEM" <linux-leds@...r.kernel.org>
Subject: Re: [PATCH V3] leds: trigger: Introduce an USB port trigger

Greg KH <gregkh@...uxfoundation.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