Re: [PATCH v2 3/3] pwm: imx27: workaround of the pwm output bug when decrease the duty cycle
From: Marek Vasut
Date: Thu Sep 05 2024 - 15:10:05 EST
On 9/5/24 8:54 PM, Frank Li wrote:
On Thu, Sep 05, 2024 at 08:26:34PM +0200, Marek Vasut wrote:
On 9/5/24 7:12 PM, Fabio Estevam wrote:
Adding Marek.
On Mon, Jul 15, 2024 at 5:30 PM Frank Li <Frank.Li@xxxxxxx> wrote:
From: Clark Wang <xiaoning.wang@xxxxxxx>
Implement workaround for ERR051198
(https://www.nxp.com/docs/en/errata/IMX8MN_0N14Y.pdf)
PWM output may not function correctly if the FIFO is empty when a new SAR
value is programmed
Description:
When the PWM FIFO is empty, a new value programmed to the PWM Sample
register (PWM_PWMSAR) will be directly applied even if the current timer
period has not expired. If the new SAMPLE value programmed in the
PWM_PWMSAR register is less than the previous value, and the PWM counter
register (PWM_PWMCNR) that contains the current COUNT value is greater
than the new programmed SAMPLE value, the current period will not flip
the level. This may result in an output pulse with a duty cycle of 100%.
Workaround:
Program the current SAMPLE value in the PWM_PWMSAR register before
updating the new duty cycle to the SAMPLE value in the PWM_PWMSAR
register. This will ensure that the new SAMPLE value is modified during
a non-empty FIFO, and can be successfully updated after the period
expires.
Frank, could you submit this patch separately ? The 1/3 and 2/3 are
unrelated as far as I can tell ?
---
Change from v1 to v2
- address comments in https://lore.kernel.org/linux-pwm/20211221095053.uz4qbnhdqziftymw@xxxxxxxxxxxxxx/
About disable/enable pwm instead of disable/enable irq:
Some pmw periphal may sensitive to period. Disable/enable pwm will
increase period, althouhg it is okay for most case, such as LED backlight
or FAN speed. But some device such servo may require strict period.
- address comments in https://lore.kernel.org/linux-pwm/d72d1ae5-0378-4bac-8b77-0bb69f55accd@xxxxxxx/
Using official errata number
fix typo 'filp'
add {} for else
I supposed fixed all previous issues, let me know if I missed one.
---
drivers/pwm/pwm-imx27.c | 52 ++++++++++++++++++++++++++++++++++++++++++++++++-
1 file changed, 51 insertions(+), 1 deletion(-)
diff --git a/drivers/pwm/pwm-imx27.c b/drivers/pwm/pwm-imx27.c
index 253afe94c4776..e12eaaebe3499 100644
--- a/drivers/pwm/pwm-imx27.c
+++ b/drivers/pwm/pwm-imx27.c
@@ -27,6 +27,7 @@
#define MX3_PWMSR 0x04 /* PWM Status Register */
#define MX3_PWMSAR 0x0C /* PWM Sample Register */
#define MX3_PWMPR 0x10 /* PWM Period Register */
+#define MX3_PWMCNR 0x14 /* PWM Counter Register */
#define MX3_PWMCR_FWM GENMASK(27, 26)
#define MX3_PWMCR_STOPEN BIT(25)
@@ -232,8 +233,11 @@ static int pwm_imx27_apply(struct pwm_chip *chip, struct pwm_device *pwm,
{
unsigned long period_cycles, duty_cycles, prescale;
struct pwm_imx27_chip *imx = to_pwm_imx27_chip(chip);
+ void __iomem *reg_sar = imx->mmio_base + MX3_PWMSAR;
unsigned long long c;
unsigned long long clkrate;
+ unsigned long flags;
+ int val;
int ret;
u32 cr;
@@ -274,7 +278,53 @@ static int pwm_imx27_apply(struct pwm_chip *chip, struct pwm_device *pwm,
pwm_imx27_sw_reset(chip);
}
- writel(duty_cycles, imx->mmio_base + MX3_PWMSAR);
+ /*
+ * This is a limited workaround. When the SAR FIFO is empty, the new
+ * write value will be directly applied to SAR even the current period
+ * is not over.
+ *
+ * If the new SAR value is less than the old one, and the counter is
+ * greater than the new SAR value, the current period will not filp
+ * the level. This will result in a pulse with a duty cycle of 100%.
+ * So, writing the current value of the SAR to SAR here before updating
+ * the new SAR value can avoid this issue.
+ *
+ * Add a spin lock and turn off the interrupt to ensure that the
+ * real-time performance can be guaranteed as much as possible when
+ * operating the following operations.
+ *
+ * 1. Add a threshold of 1.5us. If the time T between the read current
+ * count value CNR and the end of the cycle is less than 1.5us, wait
+ * for T to be longer than 1.5us before updating the SAR register.
+ * This is to avoid the situation that when the first SAR is written,
+ * the current cycle just ends and the SAR FIFO that just be written
+ * is emptied again.
+ *
+ * 2. Use __raw_writel() to minimize the interval between two writes to
+ * the SAR register to increase the fastest pwm frequency supported.
+ *
+ * When the PWM period is longer than 2us(or <500KHz), this workaround
+ * can solve this problem.
+ */
+ val = FIELD_GET(MX3_PWMSR_FIFOAV, readl_relaxed(imx->mmio_base + MX3_PWMSR));
+ if (duty_cycles < imx->duty_cycle && val < MX3_PWMSR_FIFOAV_2WORDS) {
+ c = clkrate * 1500;
+ do_div(c, NSEC_PER_SEC);
+
+ local_irq_save(flags);
+ if (state->period >= 2000)
+ readl_poll_timeout_atomic(imx->mmio_base + MX3_PWMCNR, val,
+ period_cycles - val >= c, 0, 10);
+
+ val = FIELD_GET(MX3_PWMSR_FIFOAV, readl_relaxed(imx->mmio_base + MX3_PWMSR));
+ if (!val)
+ writel_relaxed(imx->duty_cycle, reg_sar);
+ writel_relaxed(duty_cycles, reg_sar);
+ local_irq_restore(flags);
+ } else {
+ writel_relaxed(duty_cycles, reg_sar);
+ }
Why so complicated ? Can't this be simplified to this ?
const u32 sar[3] = { old_sar, old_sar, new_sar };
val = FIELD_GET(MX3_PWMSR_FIFOAV, readl_relaxed(imx->mmio_base +
MX3_PWMSR));
switch (val) {
case MX3_PWMSR_FIFOAV_EMPTY:
case MX3_PWMSR_FIFOAV_1WORD:
writesl(duty_cycles, sar, 3);
break;
case MX3_PWMSR_FIFOAV_2WORDS:
writesl(duty_cycles, sar + 1, 2);
break;
default: // 3 words in FIFO
writel(new_sar, duty_cycles);
}
The MX3_PWMSR_FIFOAV_EMPTY/MX3_PWMSR_FIFOAV_1WORD case will effectively
trigger three consecutive 'str' instructions:
1: str PWMSAR, old_sar
2: str PWMSAR, old_sar
3: str PWMSAR, new_sar
If the PWM cycle ends before 1:, then PWM will reload old value, then pick
old value from 1:, 2: and then new value from 3: -- the FIFO will never be
empty.
If the PWM cycle ends after 1:, then PWM will pick up old value from 1:
which is now in FIFO, 2: and then new value from 3: -- the FIFO will never
be empty.
The MX3_PWMSR_FIFOAV_2WORDS and default: case is there to prevent FIFO
overflow which may lock up the PWM. In case of MX3_PWMSR_FIFOAV_2WORDS there
are two words in the FIFO, write two more, old SAR value and new SAR value.
In case of default, there must be at least one free slot in the PWM FIFO
because pwm_imx27_wait_fifo_slot() which polled the FIFO for free slot
above, so there is no danger of FIFO being empty or FIFO overflow.
Maybe this can still be isolated to "if (duty_cycles < imx->duty_cycle)" .
What do you think ?
Reasonable. Let me test it.
Thank you.
I have MX8MN locally, so if you need RB/TB for V3, let me know.
Will I be able to reproduce it on another iMX too? Like MX8MP or MX8MM
(they are a bit easier to work with for me) ?
Could you include "how to reproduce" in the commit message ? Something
easy like:
- Write this and that sysfs attribute file with old value
- Write this and that sysfs attribute file with new value
- Observe this on a scope