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

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

Hi Geert,

Thanks for the feedback.


> kernel@vger.kernel.org>; Prabhakar <prabhakar.csengg@gmail.com>
> Subject: Re: [PATCH v3] clk: renesas: r8a774c0: Add RPC clocks
> 
> Hi Biju,
> 
> On Mon, Nov 16, 2020 at 10:03 AM Biju Das <biju.das.jz@bp.renesas.com>
> wrote:
> > > Subject: Re: [PATCH v3] clk: renesas: r8a774c0: Add RPC clocks 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
> 
> > > > @@ -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.
> 
> What is a wrong configuration? According to the RPCSRC docs, DIV[4:3]
> select both the parent clock and the divider.
> All four possible values are valid.

OK. It is clear now.

Regards,
Biju

> MD[4:1] select the boot device, and determine the _initial values_ of the
> DIV[4:0] bits.  Nothing in the documentation says they cannot be changed
> later, after which the DIV[4:3] bits no longer match MD[4:1].
> 
> If you want to be really sure, you can change the bits, and measure the
> impact on the RPC clock signal.
> 
> A long time ago, I did a similar thing for PLL0CR.CKSEL on R-Car D3, and
> looked at the impact on the serial console (albeit with remote access,
> i.e. no console output if kernel and hardware didn't agree ;-)
> 
> 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