by YH Huang

[permalink] [raw]

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

On Fri, 2015-07-17 at 14:59 +0800, Daniel Kurtz wrote:
> On Fri, Jul 17, 2015 at 2:35 PM, YH Huang <[email protected]> wrote:
> >
> > On Fri, 2015-07-17 at 01:18 +0800, Daniel Kurtz wrote:
> > > On Fri, Jul 17, 2015 at 12:44 AM, YH Huang <[email protected]> wrote:
> > > > On Thu, 2015-07-16 at 23:21 +0800, Daniel Kurtz wrote:
> > > >> On Thu, Jul 16, 2015 at 3:17 PM, YH Huang <[email protected]> wrote:
> > > >> > On Thu, 2015-07-16 at 14:54 +0800, Daniel Kurtz wrote:
> > > >> >> On Thu, Jul 16, 2015 at 1:38 PM, YH Huang <[email protected]> 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 <[email protected]> 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.
>
>
> Ok, but what about this:
>
> mtk_disp_pwm_config()
> {
> clk_enable(mdp->clk_main);
> /* write registers */
> clk_disable(mdp->clk_main);
> }
>
> mtk_disp_enable()
> {
> clk_enable(mdp->clk_main);
> clk_enable(mdp->clk_mm);
> /* write enable bit */
> }

The pwm-backlight driver update status with this order:
pwm_config(pb->pwm, duty_cycle, pb->period);
pwm_backlight_power_on(pb, brightness);

So we call mtk_disp_pwm_config() first and then mtk_disp_enable().

At the first time we call mtk_disp_pwm_config() from the situation that
PWM is disable, we can't write registers because mdp->clk_mm isn't
enabled yet.

Revising code like this can get the right behavior.
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);
}

Regards,
YH Huang