[PATCH v2] drm/etnaviv: correct timeout calculation

[Lucas Stach]

From: Russell King <rmk+kernel at armlinux.org.uk>

The old way did clamp the jiffy conversion and thus caused the timeouts
to become negative after some time. Also it didn't work with userspace
which actually fills the upper 32bits of the 64bit timestamp value.

clock_gettime() is 32-bit on 32-bit architectures. Using 64-bit timespec
math, like we do in this commit, means that when a wrap occurs, the
specified timeout goes into the past and we can't request a timeout in
the future. As the Linux implementation of CLOCK_MONOTONIC is reasonable
and starts at 0, the first such timer wrap will occur after approx. 68
years of system uptime.

Signed-off-by: Russell King <rmk+kernel at armlinux.org.uk>
Signed-off-by: Lucas Stach <l.stach at pengutronix.de>
---
v2:
- add Russell as author and add Sign-off
- amend commit message to carry the overflow on 32bit issue
---
 drivers/gpu/drm/etnaviv/etnaviv_drv.h | 25 +++++++++++++++++--------
 1 file changed, 17 insertions(+), 8 deletions(-)

diff --git a/drivers/gpu/drm/etnaviv/etnaviv_drv.h b/drivers/gpu/drm/etnaviv/etnaviv_drv.h
index ddb17ee565e9..17a43da98fb9 100644
--- a/drivers/gpu/drm/etnaviv/etnaviv_drv.h
+++ b/drivers/gpu/drm/etnaviv/etnaviv_drv.h
@@ -26,6 +26,7 @@
 #include <linux/pm_runtime.h>
 #include <linux/slab.h>
 #include <linux/list.h>
+#include <linux/time64.h>
 #include <linux/types.h>
 #include <linux/sizes.h>
 
@@ -132,19 +133,27 @@ static inline bool fence_after_eq(u32 a, u32 b)
 	return (s32)(a - b) >= 0;
 }
 
+/*
+ * Etnaviv timeouts are specified wrt CLOCK_MONOTONIC, not jiffies.
+ * We need to calculate the timeout in terms of number of jiffies
+ * between the specified timeout and the current CLOCK_MONOTONIC time.
+ */
 static inline unsigned long etnaviv_timeout_to_jiffies(
 	const struct timespec *timeout)
 {
-	unsigned long timeout_jiffies = timespec_to_jiffies(timeout);
-	unsigned long start_jiffies = jiffies;
-	unsigned long remaining_jiffies;
+	struct timespec64 ts, to;
+
+	to = timespec_to_timespec64(*timeout);
+
+	ktime_get_ts64(&ts);
+
+	/* timeouts before "now" have already expired */
+	if (timespec64_compare(&to, &ts) <= 0)
+		return 0;
 
-	if (time_after(start_jiffies, timeout_jiffies))
-		remaining_jiffies = 0;
-	else
-		remaining_jiffies = timeout_jiffies - start_jiffies;
+	ts = timespec64_sub(to, ts);
 
-	return remaining_jiffies;
+	return timespec64_to_jiffies(&ts);
 }
 
 #endif /* __ETNAVIV_DRV_H__ */
-- 
2.16.1