[PATCH v3 2/7] iopoll: Do not use timekeeping in read_poll_timeout_atomic()

[Geert Uytterhoeven]

read_poll_timeout_atomic() uses ktime_get() to implement the timeout
feature, just like its non-atomic counterpart.  However, there are
several issues with this, due to its use in atomic contexts:

  1. When called in the s2ram path (as typically done by clock or PM
     domain drivers), timekeeping may be suspended, triggering the
     WARN_ON(timekeeping_suspended) in ktime_get():

	WARNING: CPU: 0 PID: 654 at kernel/time/timekeeping.c:843 ktime_get+0x28/0x78

     Calling ktime_get_mono_fast_ns() instead of ktime_get() would get
     rid of that warning.  However, that would break timeout handling,
     as (at least on systems with an ARM architectured timer), the time
     returned by ktime_get_mono_fast_ns() does not advance while
     timekeeping is suspended.
     Interestingly, (on the same ARM systems) the time returned by
     ktime_get() does advance while timekeeping is suspended, despite
     the warning.

  2. Depending on the actual clock source, and especially before a
     high-resolution clocksource (e.g. the ARM architectured timer)
     becomes available, time may not advance in atomic contexts, thus
     breaking timeout handling.

Fix this by abandoning the idea that one can rely on timekeeping to
implement timeout handling in all atomic contexts, and switch from a
global time-based to a locally-estimated timeout handling.  In most
(all?) cases the timeout condition is exceptional and an error
condition, hence any additional delays due to underestimating wall clock
time are irrelevant.

Signed-off-by: Geert Uytterhoeven <geert+renesas at glider.be>
Acked-by: Arnd Bergmann <arnd at arndb.de>
Reviewed-by: Tony Lindgren <tony at atomide.com>
Reviewed-by: Ulf Hansson <ulf.hansson at linaro.org>
---
The first issue was seen with the rcar-sysc driver in the BSP, as the
BSP contains modifications to the resume sequence of various PM Domains.

v3:
  - Add Acked-by, Reviewed-by,
  - Add comment about not using timekeeping, and its impact,

v2:
  - New.
---
 include/linux/iopoll.h | 22 +++++++++++++++++-----
 1 file changed, 17 insertions(+), 5 deletions(-)

diff --git a/include/linux/iopoll.h b/include/linux/iopoll.h
index 0417360a6db9b0d6..19a7b00baff43595 100644
--- a/include/linux/iopoll.h
+++ b/include/linux/iopoll.h
@@ -74,6 +74,10 @@
  * Returns 0 on success and -ETIMEDOUT upon a timeout. In either
  * case, the last read value at @args is stored in @val.
  *
+ * This macro does not rely on timekeeping.  Hence it is safe to call even when
+ * timekeeping is suspended, at the expense of an underestimation of wall clock
+ * time, which is rather minimal with a non-zero delay_us.
+ *
  * When available, you'll probably want to use one of the specialized
  * macros defined below rather than this macro directly.
  */
@@ -81,22 +85,30 @@
 					delay_before_read, args...) \
 ({ \
 	u64 __timeout_us = (timeout_us); \
+	s64 __left_ns = __timeout_us * NSEC_PER_USEC; \
 	unsigned long __delay_us = (delay_us); \
-	ktime_t __timeout = ktime_add_us(ktime_get(), __timeout_us); \
-	if (delay_before_read && __delay_us) \
+	u64 __delay_ns = __delay_us * NSEC_PER_USEC; \
+	if (delay_before_read && __delay_us) { \
 		udelay(__delay_us); \
+		if (__timeout_us) \
+			__left_ns -= __delay_ns; \
+	} \
 	for (;;) { \
 		(val) = op(args); \
 		if (cond) \
 			break; \
-		if (__timeout_us && \
-		    ktime_compare(ktime_get(), __timeout) > 0) { \
+		if (__timeout_us && __left_ns < 0) { \
 			(val) = op(args); \
 			break; \
 		} \
-		if (__delay_us) \
+		if (__delay_us) { \
 			udelay(__delay_us); \
+			if (__timeout_us) \
+				__left_ns -= __delay_ns; \
+		} \
 		cpu_relax(); \
+		if (__timeout_us) \
+			__left_ns--; \
 	} \
 	(cond) ? 0 : -ETIMEDOUT; \
 })
-- 
2.34.1