Re: [PATCH v2 3/4] drivers: clk: renesas: rzg2l-cpg: Add support to handle coupled clocks

[Geert Uytterhoeven <geert@linux-m68k.org>]

Hi Biju,

On Thu, Aug 12, 2021 at 9:00 AM Biju Das <biju.das.jz@bp.renesas.com> wrote:
> > Subject: Re: [PATCH v2 3/4] drivers: clk: renesas: rzg2l-cpg: Add support
> > to handle coupled clocks
> > On Tue, Jul 27, 2021 at 4:18 PM Biju Das <biju.das.jz@bp.renesas.com>
> > wrote:
> > > The AXI and CHI clocks use the same register bit for controlling clock
> > > output. Add a new clock type for coupled clocks, which sets the
> > > CPG_CLKON_ETH.CLK[01]_ON bit when at least one clock is enabled, and
> > > clears the bit only when both clocks are disabled.
> > >
> > > Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
> > > Reviewed-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
> >
> > Thanks for your patch!
> >
> > > --- a/drivers/clk/renesas/rzg2l-cpg.c
> > > +++ b/drivers/clk/renesas/rzg2l-cpg.c
> > > @@ -333,12 +333,16 @@ rzg2l_cpg_register_core_clk(const struct
> > cpg_core_clk *core,
> > >   * @hw: handle between common and hardware-specific interfaces
> > >   * @off: register offset
> > >   * @bit: ON/MON bit
> > > + * @is_coupled: flag to indicate coupled clock
> > > + * @on_cnt: ON count for coupled clocks
> > >   * @priv: CPG/MSTP private data
> > >   */
> > >  struct mstp_clock {
> > >         struct clk_hw hw;
> > >         u16 off;
> > >         u8 bit;
> > > +       bool is_coupled;
> > > +       u8 on_cnt;
> >
> > While u8 is probably sufficient, you may want to use unsigned int, as
> > there will be a gap anyway due to alignment rules.
> >
> > >         struct rzg2l_cpg_priv *priv;
> > >  };
> > >
> > > @@ -392,11 +396,37 @@ static int rzg2l_mod_clock_endisable(struct
> > > clk_hw *hw, bool enable)
> > >
> > >  static int rzg2l_mod_clock_enable(struct clk_hw *hw)  {
> > > +       struct mstp_clock *clock = to_mod_clock(hw);
> > > +       struct rzg2l_cpg_priv *priv = clock->priv;
> > > +       unsigned long flags;
> > > +
> > > +       spin_lock_irqsave(&priv->rmw_lock, flags);
> > > +       clock->on_cnt++;
> > > +       if (clock->is_coupled && clock->on_cnt > 1) {
> > > +               spin_unlock_irqrestore(&priv->rmw_lock, flags);
> > > +               return 1;
> > > +       }
> > > +
> > > +       spin_unlock_irqrestore(&priv->rmw_lock, flags);
> >
> > I think you can avoid taking the spinlock and touching the counter if the
> > is_coupled flag is not set.
>
> OK.
>
> >
> > > +
> > >         return rzg2l_mod_clock_endisable(hw, true);  }
> >
> > However, I'm wondering how this can work?
> >
> >       DEF_COUPLED("eth0_axi", R9A07G044_ETH0_CLK_AXI, R9A07G044_CLK_M0,
> >                               0x57c, 0),
> >       DEF_COUPLED("eth0_chi", R9A07G044_ETH0_CLK_CHI, R9A07G044_CLK_ZT,
> >                               0x57c, 0),
> >
> > This will create 2 independent clocks, each with their own mstp_clock
> > structure that has the is_coupled flag set.  Hence each clock has its own
> > counter. Shouldn't the counter be shared?
> > Am I missing something?
>
> Oops. You are correct. I need to add this counter to priv instead of mstp_clocks.

On second thought, a counter is overkill. A simple flag should be
sufficient, as the clk core only calls .{en,dis}able() when the clock is
{dis,en}enabled.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-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