[08/16] pwm: crc: Fix off-by-one error in the clock-divider calculations

Message ID 20200606202601.48410-9-hdegoede@redhat.com
State New
Headers show
Series
  • [01/16] ACPI / LPSS: Resume Cherry Trail PWM controller in no-irq phase
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Commit Message

Hans de Goede June 6, 2020, 8:25 p.m.
The CRC PWM controller has a clock-divider which divides the clock with
a value between 1-128. But as can seen from the PWM_DIV_CLK_xxx
defines, this range maps to a register value of 0-127.

So after calculating the clock-divider we must subtract 1 to get the
register value, unless the requested frequency was so high that the
calculation has already resulted in a (rounded) divider value of 0.

Note that before this fix, setting a period of PWM_MAX_PERIOD_NS which
corresponds to the max. divider value of 128 could have resulted in a
bug where the code would use 128 as divider-register value which would
have resulted in an actual divider value of 0 (and the enable bit being
set). A rounding error stopped this bug from actually happen. This
same rounding error means that after the subtraction of 1 it is impossible
to set the divider to 128. Also bump PWM_MAX_PERIOD_NS by 1 ns to allow
setting a divider of 128 (register-value 127).

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
---
 drivers/pwm/pwm-crc.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

Patch

diff --git a/drivers/pwm/pwm-crc.c b/drivers/pwm/pwm-crc.c
index 43fc912c1fe9..5ba2a65c524c 100644
--- a/drivers/pwm/pwm-crc.c
+++ b/drivers/pwm/pwm-crc.c
@@ -22,7 +22,7 @@ 
 #define PWM_MAX_LEVEL		0xFF
 
 #define PWM_BASE_CLK_MHZ	6	/* 6 MHz */
-#define PWM_MAX_PERIOD_NS	5461333	/* 183 Hz */
+#define PWM_MAX_PERIOD_NS	5461334	/* 183 Hz */
 
 #define NSEC_PER_MHZ		1000
 
@@ -75,6 +75,9 @@  static int crc_pwm_config(struct pwm_chip *c, struct pwm_device *pwm,
 		/* changing the clk divisor, need to disable fisrt */
 		crc_pwm_disable(c, pwm);
 		clk_div = PWM_BASE_CLK_MHZ * period_ns / (256 * NSEC_PER_MHZ);
+		/* clk_div 1 - 128, maps to register values 0-127 */
+		if (clk_div > 0)
+			clk_div--;
 
 		regmap_write(crc_pwm->regmap, PWM0_CLK_DIV,
 					clk_div | PWM_OUTPUT_ENABLE);