linux-kernel - [PATCH v2] iio: gts-helper: Fix division loop

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Message-ID: <ZbJvoM3NJd_Uh_s6@drtxq0yyyyyyyyyyyyybt-3.rev.dnainternet.fi>
Date: Thu, 25 Jan 2024 16:26:40 +0200
From: Matti Vaittinen <mazziesaccount@...il.com>
To: Matti Vaittinen <matti.vaittinen@...rohmeurope.com>,
	Matti Vaittinen <mazziesaccount@...il.com>
Cc: Jonathan Cameron <jic23@...nel.org>,
	Lars-Peter Clausen <lars@...afoo.de>,
	Matti Vaittinen <mazziesaccount@...il.com>,
	linux-kernel@...r.kernel.org,
	David Laight <David.Laight@...lab.com>,
	Subhajit Ghosh <subhajit.ghosh@...aklogic.com>,
	linux-iio@...r.kernel.org
Subject: [PATCH v2] iio: gts-helper: Fix division loop

The loop based 64bit division may run for a long time when dividend is a
lot bigger than the divider. Replace the division loop by the
div64_u64() which implementation may be significantly faster.

Signed-off-by: Matti Vaittinen <mazziesaccount@...il.com>
Fixes: 38416c28e168 ("iio: light: Add gain-time-scale helpers")

---
This change was earlier applied and reverted as it confusingly lacked of
the removal of the overflow check (which is only needed when we do
looping "while (full > scale * (u64)tmp)". As this loop got removed, the
check got also obsolete and leaving it to the code caused some
confusion.

So, I marked this as a v2, where v1 is the reverted change discussed
here:
https://lore.kernel.org/linux-iio/ZZZ7pJBGkTdFFqiY@dc78bmyyyyyyyyyyyyydt-3.rev.dnainternet.fi/

Revision history:
v1 => v2:
 - Drop the obsolete overflow check
 - Rebased on top of the v6.8-rc1

iio: gts: loop fix fix
---
 drivers/iio/industrialio-gts-helper.c | 15 +--------------
 1 file changed, 1 insertion(+), 14 deletions(-)

diff --git a/drivers/iio/industrialio-gts-helper.c b/drivers/iio/industrialio-gts-helper.c
index 7653261d2dc2..b51eb6cb766f 100644
--- a/drivers/iio/industrialio-gts-helper.c
+++ b/drivers/iio/industrialio-gts-helper.c
@@ -34,24 +34,11 @@
 static int iio_gts_get_gain(const u64 max, const u64 scale)
 {
 	u64 full = max;
-	int tmp = 1;
 
 	if (scale > full || !scale)
 		return -EINVAL;
 
-	if (U64_MAX - full < scale) {
-		/* Risk of overflow */
-		if (full - scale < scale)
-			return 1;
-
-		full -= scale;
-		tmp++;
-	}
-
-	while (full > scale * (u64)tmp)
-		tmp++;
-
-	return tmp;
+	return div64_u64(full, scale);
 }
 
 /**

base-commit: 6613476e225e090cc9aad49be7fa504e290dd33d
-- 
2.43.0


-- 
Matti Vaittinen, Linux device drivers
ROHM Semiconductors, Finland SWDC
Kiviharjunlenkki 1E
90220 OULU
FINLAND

~~~ "I don't think so," said Rene Descartes. Just then he vanished ~~~
Simon says - in Latin please.
~~~ "non cogito me" dixit Rene Descarte, deinde evanescavit ~~~
Thanks to Simon Glass for the translation =] 

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