iopoll: Use ktime_get() instead of jiffies for timeout calculations

Presently, small timeout values will round up to 1 jiffy which,
on CONFIG_HZ=100 systems, corresponds to a full 10 milliseconds.
This is undesirable for drivers specifying timeouts of only a
few 10's of microseconds (which is common), since they may end
up tight-loop spinning for hundreds or thousands of times longer
than expected before reporting a timeout.

Additionally, jiffies cannot be reliably used with time_after()
when the value of jiffies is small (like 1). In rare but real
scenarios, jiffies may actually increment by more than 1 at a
time. Specifically, this will occur if interrupts are disabled
on for more than 1 jiffy (10 milliseconds for CONFIG_HZ=100) on
the CPU responsible for incrementing jiffies.

If interrupts are re-enabled on that CPU between the time the
iopoll code (on another CPU) calculates the timeout value of
jiffies and when the time_after() comparison is made, then the
iopoll APIs may return -ETIMEDOUT prematurely, even though the
specified timeout has not actually expired.

Using ktime_get() avoid this problem (which arguably also needs
a generic fix to avoid similar problems for other code which use
jiffies to calculate timeouts).

Signed-off-by: Matt Wagantall <mattw@codeaurora.org>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>
Signed-off-by: engstk <eng.stk@sapo.pt>
iopoll: change uS to us in comments

S is the symbol for Siemens but we're referring to seconds in this
file. Change uS to us to avoid confusion.

This is based on feedback from upstream:
http://lists.infradead.org/pipermail/linux-arm-kernel/2014-June/268025.html

Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>
Signed-off-by: engstk <eng.stk@sapo.pt>
iopoll: Use ktime_get() instead of jiffies for timeout calculations - fix

credits to @dorimanx

Signed-off-by: engstk <eng.stk@sapo.pt>
pktgen: clean up ktime_t helpers

Some years ago, the ktime_t helper functions ktime_now() and ktime_lt()
have been introduced. Instead of defining them inside pktgen.c, they
should either use ktime_t library functions or, if not available, they
should be defined in ktime.h, so that also others can benefit from them.
ktime_compare() is introduced with a similar notion as in timespec_compare().

Signed-off-by: Daniel Borkmann <daniel.borkmann@tik.ee.ethz.ch>
Cc: Cong Wang <xiyou.wangcong@gmail.com>
Cc: Stephen Hemminger <shemminger@vyatta.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Matt Wagantall <mattw@codeaurora.org>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>
Signed-off-by: engstk <eng.stk@sapo.pt>