Re: [PATCH v2 3/3] pwm: imx27: workaround of the pwm output bug when decrease the duty cycle

[Frank Li <Frank.li@nxp.com>]

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@nxp.com> wrote:
> > >
> > > From: Clark Wang <xiaoning.wang@nxp.com>
> > >
> > > 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@pengutronix.de/
> > >    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@gmx.net/
> > >    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.

Frank

>
> > >          writel(period_cycles, imx->mmio_base + MX3_PWMPR);