linux-kernel - [PATCH] pwm: samsung: fix to use lowest div for large enough modulation bits

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Date:	Tue, 02 Aug 2016 19:16:46 +0900
From:	Seung-Woo Kim <sw0312.kim@...sung.com>
To:	linux-samsung-soc@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	k.kozlowski@...sung.com, thierry.reding@...il.com,
	linux-pwm@...r.kernel.org
Cc:	jy0922.shim@...sung.com, sw0312.kim@...sung.com
Subject: [PATCH] pwm: samsung: fix to use lowest div for large enough
 modulation bits

>From pwm_samsung_calc_tin(), there is routine to find the lowest
divider possible to generate lower frequency than requested one.
But it is always possible to generate requested frequency with
large enough modulation bits, so this patch fixes to use lowest
div for the case. This patch removes following UBSAN warning:

   UBSAN: Undefined behaviour in drivers/pwm/pwm-samsung.c:197:13
   shift exponent 32 is too large for 32-bit type 'long unsigned int'
   [...]
   [<c0670248>] (ubsan_epilogue) from [<c06707b4>] (__ubsan_handle_shift_out_of_bounds+0xd8/0x120)
   [<c06707b4>] (__ubsan_handle_shift_out_of_bounds) from [<c0694b28>] (pwm_samsung_config+0x508/0x6a4)
   [<c0694b28>] (pwm_samsung_config) from [<c069286c>] (pwm_apply_state+0x174/0x40c)
   [<c069286c>] (pwm_apply_state) from [<c0b2e070>] (pwm_fan_probe+0xc8/0x488)
   [<c0b2e070>] (pwm_fan_probe) from [<c07ba8b0>] (platform_drv_probe+0x70/0x150)
   [...]

Signed-off-by: Seung-Woo Kim <sw0312.kim@...sung.com>
---
The UBSAN warning from ARM is reported with the patch in following link:
https://patchwork.kernel.org/patch/9189575/
---
 drivers/pwm/pwm-samsung.c |   10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/drivers/pwm/pwm-samsung.c b/drivers/pwm/pwm-samsung.c
index ada2d32..ff0def6 100644
--- a/drivers/pwm/pwm-samsung.c
+++ b/drivers/pwm/pwm-samsung.c
@@ -193,9 +193,13 @@ static unsigned long pwm_samsung_calc_tin(struct samsung_pwm_chip *chip,
 	 * divider settings and choose the lowest divisor that can generate
 	 * frequencies lower than requested.
 	 */
-	for (div = variant->div_base; div < 4; ++div)
-		if ((rate >> (variant->bits + div)) < freq)
-			break;
+	if (fls(rate) <= variant->bits) {
+		div = variant->div_base;
+	} else {
+		for (div = variant->div_base; div < 4; ++div)
+			if ((rate >> (variant->bits + div)) < freq)
+				break;
+	}
 
 	pwm_samsung_set_divisor(chip, chan, BIT(div));
 
-- 
1.7.9.5