linux-kernel - Re: [PATCH 03/17] ARM: dts: r8a7742: Add I2C and IIC support

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Date:   Fri, 22 May 2020 20:14:45 +0100
From:   "Lad, Prabhakar" <prabhakar.csengg@...il.com>
To:     Wolfram Sang <wsa+renesas@...g-engineering.com>
Cc:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        Lad Prabhakar <prabhakar.mahadev-lad.rj@...renesas.com>,
        Jens Axboe <axboe@...nel.dk>, Rob Herring <robh+dt@...nel.org>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Sergei Shtylyov <sergei.shtylyov@...entembedded.com>,
        "David S. Miller" <davem@...emloft.net>,
        Wim Van Sebroeck <wim@...ux-watchdog.org>,
        Guenter Roeck <linux@...ck-us.net>, linux-ide@...r.kernel.org,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
        Linux I2C <linux-i2c@...r.kernel.org>,
        Linux MMC List <linux-mmc@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>,
        Linux Watchdog Mailing List <linux-watchdog@...r.kernel.org>
Subject: Re: [PATCH 03/17] ARM: dts: r8a7742: Add I2C and IIC support

Hi Wolfram,

On Mon, May 18, 2020 at 11:10 AM Geert Uytterhoeven
<geert@...ux-m68k.org> wrote:
>
> Hi Wolfram,
>
> On Mon, May 18, 2020 at 11:26 AM Wolfram Sang
> <wsa+renesas@...g-engineering.com> wrote:
> > > > However, both versions (with and without automatic transmission) are
> > > > described with the same "renesas,iic-r8a7742" compatible. Is it possible
> > > > to detect the reduced variant at runtime somehow?
> > > >
> > > I couldn't find anything the manual that would be useful to detect at runtime.
>
> Hence if we really need that (see below), we need a quirk based on compatible
> value + base address.
>
> > > > My concern is that the peculiarity of this SoC might be forgotten if we
> > > > describe it like this and ever add "automatic transmissions" somewhen.
> > > >
> > > Agreed.
> >
> > Well, I guess reading from a register which is supposed to not be there
> > on the modified IP core is too hackish.
>
> According to the Hardware User's Manual Rev. 1.00, the registers do exist
> on all RZ/G1, except for RZ/G1E (see below).
>
>    "(automatic transmission can be used as a hardware function, but this is
>     not meaningful for actual use cases)."
>
> (whatever that comment may mean?)
>
> > Leaves us with a seperate compatible entry for it?
>
> On R-Car E3 and RZ/G2E, which have a single IIC instance, we
> handled that by:
>
>         The r8a77990 (R-Car E3) and r8a774c0 (RZ/G2E)
>         controllers are not considered compatible with
>         "renesas,rcar-gen3-iic" or "renesas,rmobile-iic"
>         due to the absence of automatic transmission registers.
>
> On R-Car E2 and RZ/G1E, we forgot, and used both SoC-specific and
> family-specific compatible values.
>
What are your thoughts on the above.

Cheers,
--Prabhakar

> 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