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

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Date:   Mon, 18 May 2020 12:10:05 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Wolfram Sang <wsa+renesas@...g-engineering.com>
Cc:     "Lad, Prabhakar" <prabhakar.csengg@...il.com>,
        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 11:26 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.

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