Re: [PATCH 1/2] pwm: mediatek: Add support for MT7986

[Sam Shih]

On Tue, 2022-10-25 at 12:57 +0100, Daniel Golle wrote:
> On Tue, Oct 25, 2022 at 02:35:43PM +0800, Sam Shih wrote:
> > Hi Daniel:
> > 
> > On Fri, 2022-10-21 at 16:24 +0100, Daniel Golle wrote:
> > > Add support for PWM on MT7986 which has 2 PWM channels, one of
> > > them
> > > is
> > > typically used for a temperature controlled fan.
> > > 
> > > Signed-off-by: Daniel Golle <daniel@xxxxxxxxxxxxxx>
> > > ---
> > >  drivers/pwm/pwm-mediatek.c | 7 +++++++
> > >  1 file changed, 7 insertions(+)
> > > 
> > > diff --git a/drivers/pwm/pwm-mediatek.c b/drivers/pwm/pwm-
> > > mediatek.c
> > > index 6901a44dc428de..2219cba033e348 100644
> > > --- a/drivers/pwm/pwm-mediatek.c
> > > +++ b/drivers/pwm/pwm-mediatek.c
> > > @@ -329,6 +329,12 @@ static const struct pwm_mediatek_of_data
> > > mt8365_pwm_data = {
> > >  	.has_ck_26m_sel = true,
> > >  };
> > >  
> > > +static const struct pwm_mediatek_of_data mt7986_pwm_data = {
> > > +	.num_pwms = 2,
> > > +	.pwm45_fixup = false,
> > > +	.has_ck_26m_sel = true,
> > 
> > For MT7986 SoC, I think the value of "has_ck_26m_sel" should be
> > 'false'
> 
> That's a bit surprising, please explain why.
> 

The clock tree of MT7981/MT7986 PWM BCLK is as bellow:
PLL --> topckgen fix-factors --> TOP_PWM_SEL (topckgen clock mux) -->
--
> CLK_INFRA_PWM_BSEL (infra clock mux) --> PWM BCLK (gate)

We do have the clock multiplexer to select the source clock for PWM_BCLK
https://github.com/torvalds/linux/blob/master/drivers/clk/mediatek/clk-mt7986-infracfg.c#L63

In my knowledge, the pwm hardware of MT7981/MT7986 SoC should ignore
this register directtly, but we still keep the register for backword
compatibility.

In fact, the MT7986 SoC is also working whether 'has_ck_26m_sel' is
'true' or 'false'.

Going back to the definition of 'has_ck_26m_sel', if it means
"PWM_CK_26M_SEL" register exists or not, we should use 'true', but if
it means clock from 26M clock or BUS clock, we should use 'false'

> Reading the commit adding .has_ck_26m_sel field:
> > commit 0c0ead76235db0bcfaab83f04db546995449d002
> > Author: Fabien Parent <fparent@xxxxxxxxxxxx>
> > Date:   Mon Oct 19 16:07:02 2020 +0200
> > 
> > pwm: mediatek: Always use bus clock
> > 
> > The MediaTek PWM IP can sometimes use the 26 MHz source clock to
> > generate the PWM signal, but the driver currently assumes that we
> > always
> > use the PWM bus clock to generate the PWM signal.
> > 
> > This commit modifies the PWM driver in order to force the PWM IP to
> > always use the bus clock as source clock.
> > 
> > I do not have the datasheet of all the MediaTek SoC, so I don't
> > know if
> > the register to choose the source clock is present in all the SoCs
> > or
> > only in subset. As a consequence I made this change optional by
> > using a
> > platform data paremeter to says whether this register is supported
> > or
> > not. On all the SoCs I don't have the datasheet (MT2712, MT7622,
> > MT7623,
> > MT7628, MT7629) I kept the behavior to be the same as before this
> > change.
> > 
> > Signed-off-by: Fabien Parent <fparent@xxxxxxxxxxxx>
> > Reviewed-by: Matthias Brugger <matthias.bgg@xxxxxxxxx>
> > Signed-off-by: Thierry Reding <thierry.reding@xxxxxxxxx>
> 
> From MT7986 datasheet:
> > 0x10048210 PWM_CK_26M_SEL PWM BCLK Selection
> > Reset value 0x00000001
> > Description
> > 0: Select bus CLK as BCLK
> > 1: Select 26M fix CLK as BCLK
> So after reset, the 26M clock is selected by default.
> 
> In pwm-mediatek.c I read:
> > #define PWM_CK_26M_SEL          0x210
> > ...
> >         /* Make sure we use the bus clock and not the 26MHz clock
> > */
> >         if (pc->soc->has_ck_26m_sel)
> >                 writel(0, pc->regs + PWM_CK_26M_SEL);
> 
> So this PWM_CK_26M_SEL register does exist on MT7986 and has the
> same address as expected by the driver ($PWM_BASE + 0x210).
> The default value selected after reset (0x00000001) matches the
> problem and solution described in the commit description
> "pwm: mediatek: Always use bus clock".
> 
> Sidenode: I've tried with both, .has_ck_26m_sel = true as well as
> .has_ck_26m_sel = false. Both do work, but the behavior is slightly
> different, again matching the commit description above.

What is the difference between the two?

I tried to config the pwm ch0 period=1000000 and duty=500000,
Modify PWM_CK_26M_SEL, and measure the output waveform, the waveform
keep the same.


Regards,
Sam