Re: [PATCH v4 07/11] pwm: imx: Provide atomic PWM support for i.MX PWMv2

Boris Brezillon
2017-01-10T07:59:13Z

On Mon, 09 Jan 2017 19:14:43 -0800
Stefan Agner <stefan@agner.ch> wrote:

> >>
> >> >
> >> > But, while reviewing your patch I realized this was actually unneeded
> >> > (see the explanation in my previous review).
> >> >
> >> > >
> >> > > Now it depends on cstate.enabled flag.
> >> > >
> >> > > So we end up with
> >> > >
> >> > > if (state.enabled && !cstate.enabled)
> >> > > clk_preapre_enable();
> >> >
> >> > Yep, and that's correct.
> >>
> >> And following patch:
> >> http://patchwork.ozlabs.org/patch/709510/
> >>
> >> address this issue.
> >
> > Yes, that was needed because the enable/disable path were not
> > separated, and we were unconditionally writing to the IP registers
> > even when the PWM was already disabled (which is probably the case
> > generating the fault reported by Stefan). This is not the case anymore,
> > but let's wait for Stefan to confirm this.
>
> With v4 as is, the kernel crashes/hangs on i.MX 7.
>
> The function imx_pwm_apply_v2 gets first called with state->enabled 0,
> cstate->enabled 0. This branches to else and leads to a register access
> with clocks disabled (and if that would succeed, also an unbalanced
> disable?...)
>
> With the proposed change plus the additional change in the else branch
> it works for me:
>
> @@ -192,19 +193,20 @@ static int imx_pwm_apply_v2(struct pwm_chip *chip,
> struct pwm_device *pwm,
> else
> period_cycles = 0;
>
> - ret = clk_prepare_enable(imx->clk_per);
> - if (ret)
> - return ret;
> -
> /*
> * Wait for a free FIFO slot if the PWM is already
> enabled, and
> * flush the FIFO if the PWM was disabled and is about
> to be
> * enabled.
> */
> - if (cstate.enabled)
> + if (cstate.enabled) {
> imx_pwm_wait_fifo_slot(chip, pwm);
> - else if (state->enabled)
> + } else if (state->enabled) {
> + ret = clk_prepare_enable(imx->clk_per);
> + if (ret)
> + return ret;
> +
> imx_pwm_sw_reset(chip);
> + }
>
> writel(duty_cycles, imx->mmio_base + MX3_PWMSAR);
> writel(period_cycles, imx->mmio_base + MX3_PWMPR);
> @@ -218,7 +220,7 @@ static int imx_pwm_apply_v2(struct pwm_chip *chip,
> struct pwm_device *pwm,
> cr |= MX3_PWMCR_POUTC;
>
> writel(cr, imx->mmio_base + MX3_PWMCR);
> - } else {
> + } else if (cstate.enabled) {
> writel(0, imx->mmio_base + MX3_PWMCR);
>
> clk_disable_unprepare(imx->clk_per);
>
>
> This would not disable a disabled PWM anymore, I guess at normal use not
> a problem. Only at bootup it could end up left on, but I guess if we
> care about boot time transition we should implement get_state, but
> something which we can do in a follow up patch.

Yep, that's a different problem which could be addressed by
implementing ->get_state(). Note that you don't necessary want to
disable the PWM at boot time, in some situation, when the PWM is
driving a critical device (like the VDDIODDR regulator), you want the
transition between the bootloader/firmware and Linux to be as smooth as
possible. Actually, 'initial state retrieval' and 'atomic changes'
were added to handle this case.

Stefan, one last thing, can you apply patch 2 alone and check if it
doesn't introduce a regression?

Thanks,

Boris