Merge branch 'timecounter'
Richard Cochran says:
====================
Time Counter fixes and improvements
Several PTP Hardware Clock (PHC) drivers implement the clock in
software using the timecounter/cyclecounter code. This series adds one
simple improvement and one more subtle fix to the shared timecounter
facility. Credit for this series goes to Janusz Użycki, who pointed
the issues out to me off list.
Patch #1 simply move the timecounter code into its own file. When
working on this series, it was really annoying to see half the kernel
recompile after every tweak to the timecounter stuff. There is no
reason to keep this together with the clocksource code.
Patch #2 implements an improved adjtime() method, and patches 3-10
convert all of the drivers over to the new method.
Patch #11 fixes a subtle but important issue with the timecounter WRT
frequency adjustment. As it stands now, a timecounter based PHC will
exhibit a variable frequency resolution (and variable time error)
depending on how often the clock is read.
In timecounter_read_delta(), the expression
(delta * cc->mult) >> cc->shift;
can lose resolution from the adjusted value of 'mult'. If the value
of 'delta' is too small, then small changes in 'mult' have no effect.
However, if the delta value is large enough, then small changes in
'mult' will have an effect.
Reading the clock too often means smaller 'delta' values which in turn
will spoil the fine adjustments made to 'mult'. Up until now, this
effect did not show up in my testing. The following example explains
why.
The CPTS has an input clock of 250 MHz, and the clock source uses
mult=0x80000000 and shift=29, making the ticks to nanoseconds
conversion like this:
ticks * 2^31
------------
2^29
Imagine what happens if the clock is read every 10 milliseconds. Ten
milliseconds are about 2500000 ticks, which corresponds to about 21
bits. The product in the numerator has then 52 bits. After the shift
operation, 23 bits are preserved. This results in a frequency
adjustment resolution of about 0.1 ppm (not _too_ bad.)
A frequency resolution of 1 ppm requires 20 bits.
A frequency resolution of 1 ppb requires 30 bits.
For the 250 MHz CPTS clock, reading every 4 seconds yields a 1 ppb
resolution (which is the finest that our API allows).
However, the error can be much higher if the clock is read too often
or if time stamps occur close in time to read operations. In general
it is really not acceptable to allow the rate of clock readings to
influence the clock accuracy.
====================
Signed-off-by: David S. Miller <davem@davemloft.net>
