RE: [PATCH v3] clk: renesas: r8a774c0: Add RPC clocks

[Biju Das <biju.das.jz@bp.renesas.com>]

Hi Geert,

> Subject: Re: [PATCH v3] clk: renesas: r8a774c0: Add RPC clocks
> 
> Hi Prabhakar,
> 
> On Tue, Nov 10, 2020 at 1:56 PM Lad Prabhakar <prabhakar.mahadev-
> lad.rj@bp.renesas.com> wrote:
> > Describe the RPCSRC internal clock and the RPC[D2] clocks derived from
> > it, as well as the RPC-IF module clock, in the RZ/G2E (R8A774C0)
> > CPG/MSSR driver.
> >
> > Add new clk type CLK_TYPE_GEN3E3_RPCSRC to register rpcsrc as a fixed
> > clock on R-Car Gen3 E3 (and also RZ/G2E which is identical to E3 SoC),
> > parent and the divider is set based on the register value
> > CPG_RPCCKCR[4:3] (parent is cross verified against MD[4:1] pins) which
> > has been set prior to booting the kernel.
> >
> > MD[4] MD[3] MD[2] MD[1]
> >   0     0     0    1     -> RPCSRC CLK source is PLL1
> >   0     0     1    1     -> RPCSRC CLK source is PLL1
> >   0     1     0    0     -> RPCSRC CLK source is PLL1
> >   1     0     1    1     -> RPCSRC CLK source is PLL1
> >   x     x     x    x     -> For any other values RPCSRC CLK source is
> PLL0
> >
> > Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
> > Reviewed-by: Biju Das <biju.das.jz@bp.renesas.com>
> 
> Thanks for your patch!
> 
> > ---
> > v2->v3
> > * Implemented as a fixed clock
> 
> Sounds fine to me.  If we ever need to configure this clock from Linux,
> the driver can be changed.
> 
> > --- a/drivers/clk/renesas/rcar-gen3-cpg.c
> > +++ b/drivers/clk/renesas/rcar-gen3-cpg.c
> > @@ -427,6 +427,19 @@ static struct clk * __init
> cpg_sd_clk_register(const char *name,
> >         return clk;
> >  }
> >
> > +static bool __init cpg_rpcsrc_e3_parent_is_pll0(u32 mode) {
> > +       unsigned int e3_rpcsrc = (mode & GENMASK(4, 1)) >> 1;
> > +       unsigned int pll1[] = { 0x1, 0x3, 0x4, 0xb, };
> > +       int i;
> > +
> > +       for (i = 0; i < ARRAY_SIZE(pll1); i++)
> > +               if (e3_rpcsrc == pll1[i])
> > +                       return false;
> 
> Did you know gcc (version 9.3.0) generates smaller code for:
> 
>         switch (e3_rpcsrc) {
>         case 0x1:
>         case 0x3:
>         case 0x4:
>         case 0xb:
>                 return false;
> 
>         default:
>                 return true;
>         }
> 
> ?
> 
> > @@ -696,6 +709,42 @@ struct clk * __init
> rcar_gen3_cpg_clk_register(struct device *dev,
> >                                                   cpg_rpcsrc_div_table,
> >                                                   &cpg_lock);
> >
> > +       case CLK_TYPE_GEN3E3_RPCSRC:
> > +               /*
> > +                * Register RPCSRC as fixed factor clock based on the
> > +                * MD[4:1] pins and CPG_RPCCKCR[4:3] register value for
> > +                * which has been set prior to booting the kernel.
> > +                */
> > +
> > +               value = (readl(base + CPG_RPCCKCR) & GENMASK(4, 3)) >>
> 3;
> > +               if (cpg_rpcsrc_e3_parent_is_pll0(cpg_mode)) {
> > +                       if (value != 2)
> > +                               return ERR_PTR(-EINVAL);
> > +               } else {
> > +                       if (value == 2)
> > +                               return ERR_PTR(-EINVAL);
> > +               }
> 
> IMHO this cross-verification is not needed, and harmful: it prevents the
> boot loader from changing the configuration, which I think is a valid use
> case.

But this check validates, whether bootloader done wrong configuration or not?
For eg:- PLL1 and setting wrong divider value in RPCCKCR.

It allows bootloader for changing right configurations. I may be wrong. Please correct me if I am wrong.

Regards,
Biju


> > +
> > +               switch (value) {
> > +               case 0:
> > +                       div = 5;
> > +                       break;
> > +               case 1:
> > +                       div = 3;
> > +                       break;
> > +               case 2:
> > +                       parent = clks[core->parent >> 16];
> > +                       if (IS_ERR(parent))
> > +                               return ERR_CAST(parent);
> > +                       div = 8;
> 
> R-Car D3 is very similar, but uses div = 5 instead of 8.
> Perhaps this value can be retrieved from cpg_core_clk.div?
> Of course, we can do that later, when D3 support is added.
> 
> > +                       break;
> > +               case 3:
> > +               default:
> > +                       div = 2;
> > +                       break;
> > +               }
> > +               break;
> > +
> >         case CLK_TYPE_GEN3_RPC:
> >                 return cpg_rpc_clk_register(core->name, base,
> >                                             __clk_get_name(parent),
> > notifiers);
> 
> The rest looks good to me.
> 
> 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