[PATCH v3] pwm: imx-tpm: reset counter if CMOD is 0

[Laurentiu Mihalcea <laurentiumihalcea111@gmail.com>]

From: Laurentiu Mihalcea <laurentiu.mihalcea@nxp.com>

As per the i.MX93 TRM, section 67.3.2.1 "MOD register update", the value
of the TPM counter does NOT get updated when writing MOD.MOD unless
SC.CMOD != 0. Therefore, with the current code, assuming the following
sequence:

	1) pwm_disable()
	2) pwm_apply_might_sleep() /* period is changed here */
	3) pwm_enable()

and assuming only one channel is active, if CNT.COUNT is higher than the
MOD.MOD value written during the pwm_apply_might_sleep() call then, when
re-enabling the PWM during pwm_enable(), the counter will end up resetting
after UINT32_MAX - CNT.COUNT + MOD.MOD cycles instead of MOD.MOD cycles as
normally expected.

Fix this problem by forcing a reset of the TPM counter before MOD.MOD is
written.

Fixes: 738a1cfec2ed ("pwm: Add i.MX TPM PWM driver support")
Cc: stable@vger.kernel.org
Signed-off-by: Laurentiu Mihalcea <laurentiu.mihalcea@nxp.com>
---
Changes in v3:
  - added Fixes and Cc tags
  - rephrased the newly introduced comment such that the bit is understandable
  w/o having to check the TRM.
  - Link to v2: https://lore.kernel.org/lkml/20250714123634.6442-1-laurentiumihalcea111@gmail.com/

Changes in v2:
  - dropped the "VERY IMPORTANT" bit as per Uwe's suggestion.
  - Link to v1: https://lore.kernel.org/lkml/20250701220147.1007786-1-laurentiumihalcea111@gmail.com/

 drivers/pwm/pwm-imx-tpm.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/drivers/pwm/pwm-imx-tpm.c b/drivers/pwm/pwm-imx-tpm.c
index 7ee7b65b9b90..5b399de16d60 100644
--- a/drivers/pwm/pwm-imx-tpm.c
+++ b/drivers/pwm/pwm-imx-tpm.c
@@ -204,6 +204,15 @@ static int pwm_imx_tpm_apply_hw(struct pwm_chip *chip,
 		val |= FIELD_PREP(PWM_IMX_TPM_SC_PS, p->prescale);
 		writel(val, tpm->base + PWM_IMX_TPM_SC);
 
+		/*
+		 * if the counter is disabled (CMOD == 0), programming the new
+		 * period length (MOD) will not reset the counter (CNT). If
+		 * CNT.COUNT happens to be bigger than the new MOD value then
+		 * the counter will end up being reset way too late. Therefore,
+		 * manually reset it to 0.
+		 */
+		if (!cmod)
+			writel(0x0, tpm->base + PWM_IMX_TPM_CNT);
 		/*
 		 * set period count:
 		 * if the PWM is disabled (CMOD[1:0] = 2b00), then MOD register
-- 
2.34.1