path: root/drivers/pwm
diff options
authorThomas Hebb <tommyhebb@gmail.com>2018-06-06 17:42:10 (GMT)
committerThierry Reding <thierry.reding@gmail.com>2018-07-09 16:57:03 (GMT)
commit4de445cb431eb4b473936396436d819a0724735c (patch)
tree9a3838763cdb9ba785a1b1e0af8da57d47be73fc /drivers/pwm
parentce397d215ccd07b8ae3f71db689aedb85d56ab40 (diff)
pwm: berlin: Don't use broken prescaler values
The Berlin PWM driver is currently broken on at least BG2CD. The symptoms manifest as a very non-linear and erratic mapping from the duty cycle configured in software to the duty cycle produced by hardware. The cause of the bug is software's configuration of the prescaler, and in particular its usage of the six prescaler values between the minimum value of 1 and the maximum value of 4096. As it turns out, these six values do not actually slow down the PWM clock; rather, they emulate slowing down the clock by internally multiplying the value of TCNT. This would be a fine trick, if not for the fact that the internal, scaled TCNT value has no extra bits beyond the 16 already exposed to software in the register. What this means is that, for a prescaler of 4, the software must ensure that the top two bits of TCNT are not set, because hardware will chop them off; for a prescaler of 8, the top three bits must not be set, and so forth. Software does not currently ensure this, resulting in a TCNT several orders of magnitude lower than intended any time one of those six prescalers are selected. Because hardware chops off the high bits in its internal shift, the middle six prescalers don't actually allow *anything* that the first doesn't. In fact, they are strictly worse than the first, since the internal shift of TCNT prevents software from setting the low bits, decreasing the resolution, without providing any extra high bits. By skipping the useless prescalers entirely, this patch both fixes the driver's behavior and increases its performance (since, when the 4096 prescaler is selected, it now does only a single shift rather than the seven successive divisions it did before). Tested on BG2CD. Signed-off-by: Thomas Hebb <tommyhebb@gmail.com> Signed-off-by: Thierry Reding <thierry.reding@gmail.com>
Diffstat (limited to 'drivers/pwm')
1 files changed, 25 insertions, 20 deletions
diff --git a/drivers/pwm/pwm-berlin.c b/drivers/pwm/pwm-berlin.c
index 771859a..7c8d6a1 100644
--- a/drivers/pwm/pwm-berlin.c
+++ b/drivers/pwm/pwm-berlin.c
@@ -21,8 +21,18 @@
#define BERLIN_PWM_EN 0x0
+ * The prescaler claims to support 8 different moduli, configured using the
+ * low three bits of PWM_CONTROL. (Sequentially, they are 1, 4, 8, 16, 64,
+ * 256, 1024, and 4096.) However, the moduli from 4 to 1024 appear to be
+ * implemented by internally shifting TCNT left without adding additional
+ * bits. So, the max TCNT that actually works for a modulus of 4 is 0x3fff;
+ * for 8, 0x1fff; and so on. This means that those moduli are entirely
+ * useless, as we could just do the shift ourselves. The 4096 modulus is
+ * implemented with a real prescaler, so we do use that, but we treat it
+ * as a flag instead of pretending the modulus is actually configurable.
+ */
+#define BERLIN_PWM_PRESCALE_4096 0x7
#define BERLIN_PWM_DUTY 0x8
#define BERLIN_PWM_TCNT 0xc
@@ -46,10 +56,6 @@ static inline struct berlin_pwm_chip *to_berlin_pwm_chip(struct pwm_chip *chip)
return container_of(chip, struct berlin_pwm_chip, chip);
-static const u32 prescaler_table[] = {
- 1, 4, 8, 16, 64, 256, 1024, 4096
static inline u32 berlin_pwm_readl(struct berlin_pwm_chip *chip,
unsigned int channel, unsigned long offset)
@@ -86,33 +92,32 @@ static int berlin_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm_dev,
int duty_ns, int period_ns)
struct berlin_pwm_chip *pwm = to_berlin_pwm_chip(chip);
- unsigned int prescale;
+ bool prescale_4096 = false;
u32 value, duty, period;
- u64 cycles, tmp;
+ u64 cycles;
cycles = clk_get_rate(pwm->clk);
cycles *= period_ns;
do_div(cycles, NSEC_PER_SEC);
- for (prescale = 0; prescale < ARRAY_SIZE(prescaler_table); prescale++) {
- tmp = cycles;
- do_div(tmp, prescaler_table[prescale]);
+ if (cycles > BERLIN_PWM_MAX_TCNT) {
+ prescale_4096 = true;
+ cycles >>= 12; // Prescaled by 4096
- if (tmp <= BERLIN_PWM_MAX_TCNT)
- break;
+ if (cycles > BERLIN_PWM_MAX_TCNT)
+ return -ERANGE;
- if (tmp > BERLIN_PWM_MAX_TCNT)
- return -ERANGE;
- period = tmp;
- cycles = tmp * duty_ns;
+ period = cycles;
+ cycles *= duty_ns;
do_div(cycles, period_ns);
duty = cycles;
value = berlin_pwm_readl(pwm, pwm_dev->hwpwm, BERLIN_PWM_CONTROL);
- value |= prescale;
+ if (prescale_4096)
+ value |= BERLIN_PWM_PRESCALE_4096;
+ else
+ value &= ~BERLIN_PWM_PRESCALE_4096;
berlin_pwm_writel(pwm, pwm_dev->hwpwm, value, BERLIN_PWM_CONTROL);
berlin_pwm_writel(pwm, pwm_dev->hwpwm, duty, BERLIN_PWM_DUTY);

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