pwm: imx-tpm: Reset counter if CMOD is 0

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>
Link: https://lore.kernel.org/r/20250728194144.22884-1-laurentiumihalcea111@gmail.com
Signed-off-by: Uwe Kleine-König <ukleinek@kernel.org>

diff --git a/drivers/pwm/pwm-imx-tpm.c b/drivers/pwm/pwm-imx-tpm.c
index 7ee7b65b9b90c576d3dca57eb69b981c51981da1..5b399de16d6040a416b7babfaed59f197110f5ce 100644 (file)
--- 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