netdev - Re: [PATCH net v3] rswitch: Fix PHY station management clock setting

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Date: Thu, 28 Sep 2023 09:32:58 +0200
From: Geert Uytterhoeven <geert@...ux-m68k.org>
To: Yoshihiro Shimoda <yoshihiro.shimoda.uh@...esas.com>
Cc: Andrew Lunn <andrew@...n.ch>, "s.shtylyov@....ru" <s.shtylyov@....ru>, 
	"davem@...emloft.net" <davem@...emloft.net>, "edumazet@...gle.com" <edumazet@...gle.com>, 
	"kuba@...nel.org" <kuba@...nel.org>, "pabeni@...hat.com" <pabeni@...hat.com>, 
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>, 
	"linux-renesas-soc@...r.kernel.org" <linux-renesas-soc@...r.kernel.org>, Tam Nguyen <tam.nguyen.xa@...esas.com>, 
	Kuninori Morimoto <kuninori.morimoto.gx@...esas.com>
Subject: Re: [PATCH net v3] rswitch: Fix PHY station management clock setting

Hi Shimoda-san,

On Thu, Sep 28, 2023 at 4:13 AM Yoshihiro Shimoda
<yoshihiro.shimoda.uh@...esas.com> wrote:
> > From: Andrew Lunn, Sent: Wednesday, September 27, 2023 9:44 PM
> > > +   /* MPIC.PSMCS = (clk [MHz] / (MDC frequency [MHz] * 2) - 1.
> > > +    * Calculating PSMCS value as MDC frequency = 2.5MHz. So, multiply
> > > +    * both the numerator and the denominator by 10.
> > > +    */
> > > +   etha->psmcs = clk_get_rate(priv->clk) / 100000 / (25 * 2) - 1;
> > >  }
> > >
> > >  static int rswitch_device_alloc(struct rswitch_private *priv, int index)
> > > @@ -1900,6 +1907,10 @@ static int renesas_eth_sw_probe(struct platform_device *pdev)
> > >             return -ENOMEM;
> > >     spin_lock_init(&priv->lock);
> > >
> > > +   priv->clk = devm_clk_get(&pdev->dev, NULL);
> > > +   if (IS_ERR(priv->clk))
> > > +           return PTR_ERR(priv->clk);
> > > +
> >
> > /**
> >  * clk_get_rate - obtain the current clock rate (in Hz) for a clock source.
> >  *              This is only valid once the clock source has been enabled.

Whether clk_get_rate() works when the clock is still disabled actually
depends on the clock driver implementation/hardware.  It's not
guaranteed, so generic code cannot make that assumption.
It should work fine on all Renesas on-SoC clock generators.

> >  * @clk: clock source
> >  */
> > unsigned long clk_get_rate(struct clk *clk);
> >
> > I don't see the clock being enabled anywhere.
>
> Since GENPD_FLAG_PM_CLK is set in the drivers/pmdomain/renesas/rcar-gen4-sysc.c,
> pm_runtime_get_sync() will enable the clock. That's why this code works correctly
> without clk_enable() calling.
>
> > Also, is the clock documented in the device tree binding?
>
> Yes, but this is a "clocks" property only though. In the dt-bindings doc:
> ---
> examples:
> ...
>         clocks = <&cpg CPG_MOD 1505>;
> ---
>
> And, in the drivers/clk/renesas/r8a779f0-cpg-mssr.c:
> ---
>         DEF_FIXED("rsw2",       R8A779F0_CLK_RSW2,      CLK_PLL5_DIV2,  5, 1),
> ...
>         DEF_MOD("rswitch2",     1505,   R8A779F0_CLK_RSW2),
> ---
> So, the device will get the paranet clock " R8A779F0_CLK_RSW2".
> And according to the clk_summary in the debugfs:
> ---
> # grep rsw /sys/kernel/debug/clk/clk_summary
>              rsw2                     1        1        0   320000000          0     0  50000         Y
>                 rswitch2              1        1        0   320000000          0     0  50000         Y
> ---
>
> I found that i2c-rcar.c and pwm-rcar.c are also the same implementation
> which call clk_get_rate() without clk_enable(). But, perhaps, we should enable
> the clock by clk API?
>
> To Geert-san, do you have any opinion?

As the device is part of a clock domain, the clock is managed through
Runtime PM, and there is no need to enable or disable manually.
Just make sure to call pm_runtime_resume_and_get() before accessing
any register of the device.

Calling clk_get_rate() at any time is fine.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds