Re: [PATCH/RFT] arm64: dts: renesas: r8a77990: Add I2C-DVFS device node

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

Hi Simon,

On Thu, Nov 8, 2018 at 12:52 PM Simon Horman <horms@verge.net.au> wrote:
> On Thu, Nov 08, 2018 at 11:27:31AM +0100, Geert Uytterhoeven wrote:
> > On Thu, Nov 8, 2018 at 11:22 AM Simon Horman <horms@verge.net.au> wrote:
> > > On Wed, Nov 07, 2018 at 01:30:10PM +0100, Simon Horman wrote:
> > > > On Mon, Nov 05, 2018 at 09:52:48AM +0100, Geert Uytterhoeven wrote:
> > > > > On Sat, Oct 20, 2018 at 11:35 PM Yoshihiro Kaneko <ykaneko0929@gmail.com> wrote:
> > > > > > From: Takeshi Kihara <takeshi.kihara.df@renesas.com>
> > > > > >
> > > > > > This patch adds I2C-DVFS device node for the R8A77990 SoC.
> > > > > >
> > > > > > Signed-off-by: Takeshi Kihara <takeshi.kihara.df@renesas.com>
> > > > > > Signed-off-by: Yoshihiro Kaneko <ykaneko0929@gmail.com>
> > > > >
> > > > > Thanks for your patch!
> > > > >
> > > > > > --- a/arch/arm64/boot/dts/renesas/r8a77990.dtsi
> > > > > > +++ b/arch/arm64/boot/dts/renesas/r8a77990.dtsi
> >
> > > > > >         };
> > > > > >
> > > > > >         cpus {
> > > > > > @@ -337,6 +338,22 @@
> > > > > >                         reg = <0 0xe6060000 0 0x508>;
> > > > > >                 };
> > > > > >
> > > > > > +               i2c_dvfs: i2c@e60b0000 {
> > > > > > +                       #address-cells = <1>;
> > > > > > +                       #size-cells = <0>;
> > > > > > +                       compatible = "renesas,iic-r8a77990",
> > > > >
> > > > > "renesas,iic-r8a77990" is not yet documented.
> > > >
> > > > Thanks, as per my comment below I wonder if as well as documenting
> > > > "renesas,iic-r8a77990" we also to teach the driver about it.
> > > >
> > > > >
> > > > > > +                                    "renesas,rcar-gen3-iic",
> > > > > > +                                    "renesas,rmobile-iic";
> > > > >
> > > > > Also, IIC on R-Car E3 does not have the automatic transmission registers.
> > > > > Does this affect claiming compatibility with the family-specific or generic
> > > > > compatible values?
> > > >
> > > > My cursory reading of the driver indicates that only register that is
> > > > used by it but documented as not existing on E3 is ICSTART (offset 0x70).
> > > >
> > > > It seems to me that we should confirm with Renesas that the register does
> > > > indeed not exist - as this patch comes from the BSP which implies it is
> > > > being used there. And if it does not exist we should try teaching the
> > > > driver not to use ICSTART via the "renesas,iic-r8a77990" compat string.
> > > > What do you think?
> > >
> > > Further examination by Wolfram Sang has shown that the code in question is
> > > only used on the r8a7740. I don't think there is any compatibility issue
> > > for r8a77990 when using the current driver.
> >
> > "When using the current driver".
> > Do we want to claim compatibility with "renesas,rcar-gen3-iic" and/or
> > "renesas,rmobile-iic"?
>
> Thinking a bit more since I wrote my previous email.
>
> It seems that the r8a77990 has a different feature set to other R-Car Gen3
> SoCs but that the current driver implementation "renesas,rcar-gen3-iic"
> and "renesas,rmobile-iic" do not exceed that feature set.
>
> In future we could expand the features supported by the driver for other
> Gen3 SoCs beyond what is supported for r8a77990. If we chose to describe
> r8a77990 as compatible with renesas,rcar-gen3-iic then that implies those
> new features would not be activated by matching on "renesas,rcar-gen3-iic",
> though we could choose to control that using soc_match. Conversely if we
> decide that r8a77990 is not compatible with renesas,rcar-gen3-iic then we
> have more freedom to move with regards to adding features not supported by
> r8a77990 to renesas,rcar-gen3-iic in future.
>
> So perhaps it would be wise to be conservative and, for now,
> not document r8a77990 as being compatible with renesas,iic-r8a77990.
>
> I'm tempted to say that renesas,rmobile-iic is a lowest common denominator
> and r8a77990 can be documented as being compatible with it. But we could
> say the same of renesas,rcar-gen3-iic.
>
> What do you think?

That matches my thoughts.

Given R-Mobile A1 does have the register, I think r8a77990 should not claim
compatibility with it.

Of course all this assumes there's no bug in the datasheet w.r.t. r8a77990...

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