From: Arnd Bergmann <arnd@arndb.de>
To: Alan Stern <stern@rowland.harvard.edu>
Cc: Tina Ruchandani <ruchandani.tina@gmail.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        linux-usb@vger.kernel.org, linux-kernel@vger.kernel.org,
        Pete Zaitcev <zaitcev@redhat.com>
Subject: Re: [PATCH] USB: usbmon: Remove timeval usage for timestamp
Date: Tue, 05 May 2015 17:33:03 +0200
Message-ID: <4657648.5HP9obPUcU@wuerfel>
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On Tuesday 05 May 2015 10:59:32 Alan Stern wrote:
> On Tue, 5 May 2015, Arnd Bergmann wrote:
> 
> > Your conversion looks entirely correct, but the original code is a bit
> > odd here as it does not use the entire range of the 32-bit microsecond
> > value, and counts from 0 to 4096000000us instead of the more intuitive
> > 0 to 4294967296 us range before wrapping around.
> > 
> > If we change the code to 
> > 
> > static inline unsigned int mon_get_timestamp(void)
> > {
> > 	return ktime_to_us(ktime_get_real());
> > }
> > 
> > it might be more obvious what is going on, but it would slightly change
> > the output in the debugfs file to use the full range. Do we know what
> > behavior is expected by normal user space here? Pete Zaitcev submitted
> > a patch for this behavior in 2010, he might remember something about it.
> 
> I don't know of any programs that use the timestamp value, but if some 
> do exist then the way overflow works should not be changed.
> 
> In my experience, the timestamps are used by humans reading the usbmon
> output.  Overflow is rare, but when it does occur, a human finds it
> much easier to wrap from 4095.999999 seconds to 0.000000 than to wrap
> from 4294.967295 to 0.000000.
> 
> (Also, in the rare cases where usbmon timestamps have to be matched up
> with printk timestamps, it's easier to figure the relative offset when
> overflow affects only the seconds, not the fractions of a second.)

Ok, got it.

> > I also wonder if we should make the output use monotonic time instead
> > of real time (so change it to ktime_get_ts64() or ktime_get()). The effect
> > of that would be to keep the time ticking monotonically across a concurrent
> > settimeofday() call.
> 
> That seems reasonable to me.  The absolute values of the timestamps are 
> practically meaningless; only the differences are important.

Right, although it would also mean you could no longer match up the
microseconds with the printk timestamps.

	Arnd
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