linux-kernel - [PATCH] workaround regression in ina2xx introduced by cb47755725da("time: Prevent undefined behaviour in timespec64_to

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Date:   Sat, 11 Sep 2021 12:36:23 +0100
From:   Iain Hunter <drhunter95@...il.com>
To:     unlisted-recipients:; (no To-header on input)
Cc:     lothar.felten@...il.com, iain@...terembedded.co.uk,
        Jonathan Cameron <jic23@...nel.org>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Alexandru Ardelean <alexandru.ardelean@...log.com>,
        Matt Ranostay <matt.ranostay@...sulko.com>,
        Gwendal Grignou <gwendal@...omium.org>,
        linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH] workaround regression in ina2xx introduced by cb47755725da("time: Prevent undefined behaviour in timespec64_to_ns()")

From: Iain Hunter <iain@...terembedded.co.uk>

That change adds an error check to avoid saturation during multiplication
to calculate nano seconds in timespec64_to_ns().
In ina2xx_capture_thread() a timespec64 structure is used to calculate
the delta time until the next sample time. This delta can be negative if
the next sample time was in the past. In the -1 case timespec64_to_ns()
now clamps the -1 second value to KTIME_MAX. This essentially puts ina2xx
thread to sleep forever.
Proposed patch is to replace the call to timespec64_to_ns() with the
contents of that function without the overflow test introduced by the
commit (ie revert to pre kernel 5.4 behaviour)

Signed-off-by: Iain Hunter <iain@...terembedded.co.uk>
---
 drivers/iio/adc/ina2xx-adc.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/iio/adc/ina2xx-adc.c b/drivers/iio/adc/ina2xx-adc.c
index a4b2ff9e0..ba3e98fde 100644
--- a/drivers/iio/adc/ina2xx-adc.c
+++ b/drivers/iio/adc/ina2xx-adc.c
@@ -777,6 +777,7 @@ static int ina2xx_capture_thread(void *data)
 	int ret;
 	struct timespec64 next, now, delta;
 	s64 delay_us;
+	s64 delta_ns;
 
 	/*
 	 * Poll a bit faster than the chip internal Fs, in case
@@ -818,7 +819,8 @@ static int ina2xx_capture_thread(void *data)
 		do {
 			timespec64_add_ns(&next, 1000 * sampling_us);
 			delta = timespec64_sub(next, now);
-			delay_us = div_s64(timespec64_to_ns(&delta), 1000);
+			delta_ns = (((s64)delta.tv_sec) * NSEC_PER_SEC)+delta.tv_nsec;
+			delay_us = div_s64(delta_ns, 1000);
 		} while (delay_us <= 0);
 
 		usleep_range(delay_us, (delay_us * 3) >> 1);
-- 
2.17.1