Date:   Wed, 16 Nov 2022 18:46:10 +0100
From:   Emil Renner Berthing <emil.renner.berthing@...onical.com>
To:     Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
Cc:     Thierry Reding <thierry.reding@...il.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Atish Patra <atishp@...shpatra.org>,
        "Wesley W. Terpstra" <wesley@...ive.com>,
        linux-pwm@...r.kernel.org, linux-riscv@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] pwm: sifive: Always let the first pwm_apply_state succeed

On Wed, 16 Nov 2022 at 18:41, Emil Renner Berthing
<emil.renner.berthing@...onical.com> wrote:
>
> On Wed, 9 Nov 2022 at 16:33, Uwe Kleine-König
> <u.kleine-koenig@...gutronix.de> wrote:
> >
> > On Wed, Nov 09, 2022 at 01:45:43PM +0100, Emil Renner Berthing wrote:
> > > On Wed, 9 Nov 2022 at 13:01, Uwe Kleine-König
> > > <u.kleine-koenig@...gutronix.de> wrote:
> > > >
> > > > Hello Emil,
> > > >
> > > > On Wed, Nov 09, 2022 at 12:37:24PM +0100, Emil Renner Berthing wrote:
> > > > > Commit 2cfe9bbec56ea579135cdd92409fff371841904f added support for the
> > > > > RGB and green PWM controlled LEDs on the HiFive Unmatched board
> > > > > managed by the leds-pwm-multicolor and leds-pwm drivers respectively.
> > > > > All three colours of the RGB LED and the green LED run from different
> > > > > lines of the same PWM, but with the same period so this works fine when
> > > > > the LED drivers are loaded one after the other.
> > > > >
> > > > > Unfortunately it does expose a race in the PWM driver when both LED
> > > > > drivers are loaded at roughly the same time. Here is an example:
> > > > >
> > > > >   |          Thread A           |          Thread B           |
> > > > >   |  led_pwm_mc_probe           |  led_pwm_probe              |
> > > > >   |    devm_fwnode_pwm_get      |                             |
> > > > >   |      pwm_sifive_request     |                             |
> > > > >   |        ddata->user_count++  |                             |
> > > > >   |                             |    devm_fwnode_pwm_get      |
> > > > >   |                             |      pwm_sifive_request     |
> > > > >   |                             |        ddata->user_count++  |
> > > > >   |         ...                 |          ...                |
> > > > >   |    pwm_state_apply          |    pwm_state_apply          |
> > > > >   |      pwm_sifive_apply       |      pwm_sifive_apply       |
> > > > >
> > > > > Now both calls to pwm_sifive_apply will see that ddata->approx_period,
> > > > > initially 0, is different from the requested period and the clock needs
> > > > > to be updated. But since ddata->user_count >= 2 both calls will fail
> > > > > with -EBUSY, which will then cause both LED drivers to fail to probe.
> > > > >
> > > > > Fix it by letting the first call to pwm_sifive_apply update the clock
> > > > > even when ddata->user_count != 1.
> > > > >
> > > > > Fixes: 9e37a53eb051 ("pwm: sifive: Add a driver for SiFive SoC PWM")
> > > > > Signed-off-by: Emil Renner Berthing <emil.renner.berthing@...onical.com>
> > > > > ---
> > > > >  drivers/pwm/pwm-sifive.c | 8 +++++++-
> > > > >  1 file changed, 7 insertions(+), 1 deletion(-)
> > > > >
> > > > > diff --git a/drivers/pwm/pwm-sifive.c b/drivers/pwm/pwm-sifive.c
> > > > > index 2d4fa5e5fdd4..b3c60ec72a6e 100644
> > > > > --- a/drivers/pwm/pwm-sifive.c
> > > > > +++ b/drivers/pwm/pwm-sifive.c
> > > > > @@ -159,7 +159,13 @@ static int pwm_sifive_apply(struct pwm_chip *chip, struct pwm_device *pwm,
> > > > >
> > > > >       mutex_lock(&ddata->lock);
> > > > >       if (state->period != ddata->approx_period) {
> > > > > -             if (ddata->user_count != 1) {
> > > > > +             /*
> > > > > +              * Don't let a 2nd user change the period underneath the 1st user.
> > > > > +              * However if ddate->approx_period == 0 this is the first time we set
> > > > > +              * any period, so let whoever gets here first set the period so other
> > > > > +              * users who agree on the period won't fail.
> > > > > +              */
> > > > > +             if (ddata->user_count != 1 && ddata->approx_period) {
> > > >
> > > > While I'm convinced this works, we'd get some more uniform behaviour
> > > > compared to other hardwares with similar restrictions if you lock the
> > > > period on enabling the PWM instead of at request time. See for example
> > > > drivers/pwm/pwm-pca9685.c.
> > >
> > > Hmm.. that driver uses a pwms_enabled bitmap rather than a user count,
> > > but it still sets the bit in the request method and refuses to change
> > > period in the apply method if more than 1 bit is set.
> >
> > Note there are two different bitmaps. The one modified in .request is
> > for gpio stuff and the other in .apply() for locking the common period
> > length.
>
> Yeah, there is the pwms_enabled and pwms_inuse bitmaps, but
> pwms_enabled is used both in .request and .apply.

Oh, I think you might have looked at the pca9685_pwm_gpio_request
function and not pca9685_pwm_request.

> > > So as far as I
> > > can tell it still suffers from the same race. However using a bitmap
> > > instead of a user count would let us handle everything in the apply
> > > method if we don't set the bit in the request method, but then the
> > > behaviour would still be different. In any case it would still be a
> > > large change to this driver.
> > >
> > > How about we merge this bug fix that can easily be backported first
> > > and then look at how it should be handled properly?
> >
> > I thought it wouldn't be that hard to do it right from the start,
> > but I admit it's harder than I expected to get right. My prototype looks
> > as follows:
>
> This works for me (modulo the two extra {'s). I'd still prefer merging
> the simpler version and then this on top for ease of backporting, but
> as long as the race is fixed I'm fine. Will you send a cleaned up
> version of this?
>
> /Emil
>
> > diff --git a/drivers/pwm/pwm-sifive.c b/drivers/pwm/pwm-sifive.c
> > index 2d4fa5e5fdd4..89846d95bfc0 100644
> > --- a/drivers/pwm/pwm-sifive.c
> > +++ b/drivers/pwm/pwm-sifive.c
> > @@ -41,13 +41,13 @@
> >
> >  struct pwm_sifive_ddata {
> >         struct pwm_chip chip;
> > -       struct mutex lock; /* lock to protect user_count and approx_period */
> > +       struct mutex lock; /* lock to protect approx_period */
> >         struct notifier_block notifier;
> >         struct clk *clk;
> >         void __iomem *regs;
> >         unsigned int real_period;
> >         unsigned int approx_period;
> > -       int user_count;
> > +       DECLARE_BITMAP(pwms_enabled, 4);
> >  };
> >
> >  static inline
> > @@ -59,10 +59,16 @@ struct pwm_sifive_ddata *pwm_sifive_chip_to_ddata(struct pwm_chip *c)
> >  static int pwm_sifive_request(struct pwm_chip *chip, struct pwm_device *pwm)
> >  {
> >         struct pwm_sifive_ddata *ddata = pwm_sifive_chip_to_ddata(chip);
> > +       u32 val = readl(ddata->regs + PWM_SIFIVE_PWMCFG);
> >
> > -       mutex_lock(&ddata->lock);
> > -       ddata->user_count++;
> > -       mutex_unlock(&ddata->lock);
> > +       if (val & PWM_SIFIVE_PWMCFG_EN_ALWAYS) {
> > +               val = readl(ddata->regs + PWM_SIFIVE_PWMCMP(pwm->hwpwm));
> > +               if (val > 0) {
> > +                       mutex_lock(&ddata->lock);
> > +                       __set_bit(pwm->hwpwm, ddata->pwms_enabled);
> > +                       mutex_unlock(&ddata->lock);
> > +               }
> > +       }
> >
> >         return 0;
> >  }
> > @@ -72,7 +78,7 @@ static void pwm_sifive_free(struct pwm_chip *chip, struct pwm_device *pwm)
> >         struct pwm_sifive_ddata *ddata = pwm_sifive_chip_to_ddata(chip);
> >
> >         mutex_lock(&ddata->lock);
> > -       ddata->user_count--;
> > +       __clear_bit(pwm->hwpwm, ddata->pwms_enabled);
> >         mutex_unlock(&ddata->lock);
> >  }
> >
> > @@ -158,11 +164,18 @@ static int pwm_sifive_apply(struct pwm_chip *chip, struct pwm_device *pwm,
> >         frac = min(frac, (1U << PWM_SIFIVE_CMPWIDTH) - 1);
> >
> >         mutex_lock(&ddata->lock);
> > +
> > +       if (state->enabled) {
> > +               __set_bit(pwm->hwpwm, ddata->pwms_enabled);
> > +
> >         if (state->period != ddata->approx_period) {
> > -               if (ddata->user_count != 1) {
> > +               if (bitmap_weight(ddata->pwms_enabled, 4) > 1) {
> > +                       if (!enabled) {
> > +                               __clear_bit(pwm->hwpwm, ddata->pwms_enabled);
> >                         mutex_unlock(&ddata->lock);
> >                         return -EBUSY;
> >                 }
> > +
> >                 ddata->approx_period = state->period;
> >                 pwm_sifive_update_clock(ddata, clk_get_rate(ddata->clk));
> >         }
> > @@ -177,14 +190,23 @@ static int pwm_sifive_apply(struct pwm_chip *chip, struct pwm_device *pwm,
> >                 ret = clk_enable(ddata->clk);
> >                 if (ret) {
> >                         dev_err(ddata->chip.dev, "Enable clk failed\n");
> > +                       if (state->enabled) {
> > +                               mutex_lock(&ddata->lock);
> > +                               __clear_bit(pwm->hwpwm, ddata->pwms_enabled);
> > +                               mutex_unlock(&ddata->lock);
> > +                       }
> >                         return ret;
> >                 }
> >         }
> >
> >         writel(frac, ddata->regs + PWM_SIFIVE_PWMCMP(pwm->hwpwm));
> >
> > -       if (!state->enabled)
> > +       if (!state->enabled) {
> > +               mutex_lock(&ddata->lock);
> > +               __clear_bit(pwm->hwpwm, ddata->pwms_enabled);
> > +               mutex_unlock(&ddata->lock);
> >                 clk_disable(ddata->clk);
> > +       }
> >
> >         return 0;
> >  }
> >
> > Best regards
> > Uwe
> >
> > --
> > Pengutronix e.K.                           | Uwe Kleine-König            |
> > Industrial Linux Solutions                 | https://www.pengutronix.de/ |

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