Re: [PATCH v5 2/3] pwm: add MediaTek display PWM driver support

[YH Huang <yh.huang@mediatek.com>]

On Fri, 2015-07-17 at 01:18 +0800, Daniel Kurtz wrote:
> On Fri, Jul 17, 2015 at 12:44 AM, YH Huang <yh.huang@mediatek.com> wrote:
> > On Thu, 2015-07-16 at 23:21 +0800, Daniel Kurtz wrote:
> >> On Thu, Jul 16, 2015 at 3:17 PM, YH Huang <yh.huang@mediatek.com> wrote:
> >> > On Thu, 2015-07-16 at 14:54 +0800, Daniel Kurtz wrote:
> >> >> On Thu, Jul 16, 2015 at 1:38 PM, YH Huang <yh.huang@mediatek.com> wrote:
> >> >> > On Wed, 2015-07-15 at 23:59 +0800, YH Huang wrote:
> >> >> >> On Mon, 2015-07-13 at 18:19 +0800, Daniel Kurtz wrote:
> >> >> >> > On Mon, Jul 13, 2015 at 5:04 PM, YH Huang <yh.huang@mediatek.com> wrote:
> >> >> >> > > +#ifdef CONFIG_PM_SLEEP
> >> >> >> > > +static int mtk_disp_pwm_suspend(struct device *dev)
> >> >> >> > > +{
> >> >> >> > > +       struct mtk_disp_pwm *mdp = dev_get_drvdata(dev);
> >> >> >> > > +
> >> >> >> > > +       clk_disable_unprepare(mdp->clk_main);
> >> >> >> > > +       clk_disable_unprepare(mdp->clk_mm);
> >> >> >> > > +
> >> >> >> > > +       return 0;
> >> >> >> > > +}
> >> >> >> > > +
> >> >> >> > > +static int mtk_disp_pwm_resume(struct device *dev)
> >> >> >> > > +{
> >> >> >> > > +       struct mtk_disp_pwm *mdp = dev_get_drvdata(dev);
> >> >> >> > > +       int ret;
> >> >> >> > > +
> >> >> >> > > +       ret = clk_prepare_enable(mdp->clk_main);
> >> >> >> > > +       if (ret < 0)
> >> >> >> > > +               return ret;
> >> >> >> > > +
> >> >> >> > > +       ret = clk_prepare_enable(mdp->clk_mm);
> >> >> >> > > +       if (ret < 0) {
> >> >> >> > > +               clk_disable_unprepare(mdp->clk_main);
> >> >> >> > > +               return ret;
> >> >> >> > > +       }
> >> >> >> > > +
> >> >> >> >
> >> >> >> > Don't you also have to restore the PWM rate and frequency?
> >> >> >> >
> >> >> >> > Is it possible to save power at runtime by leaving mdp->clk_mm enabled
> >> >> >> > (to generate the PWM signal), but disable mdp->clk_main (clock
> >> >> >> > required to access PWM registers)?
> >> >> >>
> >> >> >> The pwm-backlight driver will restore the data.
> >> >> >>
> >> >> >> After I try to disable anyone of the two clocks at runtime, the
> >> >> >> backlight doesn't work well(no immediate update or losing backlight).
> >> >> >> So we need to keep both clock enabled.
> >>
> >> Do you mean you see backlight glitch because the clocks / backlight
> >> were *already on* during the first config (Perhaps left on by the
> >> bootloader)?
> >> I don't know how to solve that problem.
> >> Maybe Thierry does.
> >>
> >> In any case, this is a minor issue; we really shouldn't hold up
> >> landing the driver to optimize when the clocks are enabled/disabled
> >> :-). I'm happy enough with what you have in this patch.
> >
> > Sorry for my terrible expression. Let me try again.
> > 1. We want to disable unnecessary clock at runtime.
> > But, I get backlight glitch when I disable clk_main or clk_mm in
> > mtk_disp_pwm_config(). So both clocks are necessary and we don't disable
> > them at runtime.
> >
> > 2. Because pwm-backlight driver calls mtk_disp_pwm_config() before
> > mtk_disp_pwm_enable(), we will lose the first config if clocks are
> > enabled in mtk_disp_pwm_enable(). I prefer to enable clocks in probe
> > function. Samsung did the same way in their pwm driver.
> 
> I don't understand why you will "lose the first config if clocks are
> enabled in mtk_disp_pwm_enable().  I don't believe registers will lose
> their values just because you turn enable/disable clocks.
> 
> Perhaps I wasn't clear with what I was proposing which is something like this:
> 
> mtk_disp_pwm_config()
> {
>   clk_enable();
>   /* write registers */
>   clk_disable();
> }
> 
> mtk_disp_enable()
> {
>   clk_enable();
>   /* write enable bit */
> }
> 
> mtk_disp_disable()
> {
>   /* clear enable bit */
>   clk_disable();
> }
> 
> 
> In this way, if mtk_disp_pwm_config() is called when the pwm is
> disabled, we will temporarily enable the clocks long enough to update
> the register values.  These values should take effect the next time
> the PWM is enabled.  We then disable the clocks and wait for the PWM
> to be enabled.
> 
> If mtk_disp_pwm_config() is called when the pwm is already enabled, we
> will increment the enable count on the clocks, but then we decrement
> it again immediately.

I think there is something wrong about mdp->clk_mm(generate the PWM
signal) and mdp->clk_main(access PWM registers).
If we want to write the register, we should enable both clocks before.
I try it in this way.

mtk_disp_pwm_config()
{
   clk_enable(mdp->clk_main);
   clk_enable(mdp->clk_mm);
   /* write registers */
   clk_disable(mdp->clk_mm);
   clk_disable(mdp->clk_main);
}
with
A:
mtk_disp_enable()
{
   clk_enable(mdp->clk_main);
   clk_enable(mdp->clk_mm);
   /* write enable bit */
   clk_disable(mdp->clk_mm);
}
or
B:
mtk_disp_enable()
{
   clk_enable(mdp->clk_main);
   clk_enable(mdp->clk_mm);
   /* write enable bit */
   clk_disable(mdp->clk_main);
}

I both get backlight glitch with "A" or "B".
So I think we should keep clocks enabled.

Regards,
YH Huang