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From:   Geert Uytterhoeven <geert@linux-m68k.org>
Date:   Fri, 21 Feb 2020 10:45:57 +0100
Message-ID: <CAMuHMdWaPfc050dZiRr+gAFzsdjSo9Vo70ztWgrMGPJxLUqupw@mail.gmail.com>
Subject: Re: [RFC PATCH 3/7] i2c: allow DT nodes without 'compatible'
To:     Wolfram Sang <wsa+renesas@sang-engineering.com>
Cc:     Linux I2C <linux-i2c@vger.kernel.org>,
        Linux-Renesas <linux-renesas-soc@vger.kernel.org>,
        linux-i3c@lists.infradead.org,
        Kieran Bingham <kieran@ksquared.org.uk>,
        =?UTF-8?Q?Niklas_S=C3=B6derlund?= <niklas.soderlund@ragnatech.se>,
        Luca Ceresoli <luca@lucaceresoli.net>,
        Jacopo Mondi <jacopo@jmondi.org>,
        Laurent Pinchart <laurent.pinchart@ideasonboard.com>,
        Vladimir Zapolskiy <vz@mleia.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@vger.kernel.org>
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Hi Wolfram,

On Thu, Feb 20, 2020 at 6:26 PM Wolfram Sang
<wsa+renesas@sang-engineering.com> wrote:
> Sometimes, we have unknown devices in a system and still want to block
> their address. For that, we allow DT nodes with only a 'reg' property.
> These devices will be bound to the "dummy" driver but with the name
> "reserved". That way, we can distinguish them and even hand them over to
> the "dummy" driver later when they are really requested using
> i2c_new_ancillary_device().
>
> Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>

Thanks for your patch!

Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
but one question below.

> --- a/Documentation/devicetree/bindings/i2c/i2c-ocores.txt
> +++ b/Documentation/devicetree/bindings/i2c/i2c-ocores.txt
> @@ -50,7 +50,6 @@ Examples:
>                 reg-io-width = <1>;     /* 8 bit read/write */
>
>                 dummy@60 {
> -                       compatible = "dummy";
>                         reg = <0x60>;
>                 };
>         };

There's a second instance to remove 18 lines below.

> --- a/drivers/i2c/i2c-core-of.c
> +++ b/drivers/i2c/i2c-core-of.c
> @@ -27,17 +27,15 @@ int of_i2c_get_board_info(struct device *dev, struct device_node *node,
>
>         memset(info, 0, sizeof(*info));
>
> -       if (of_modalias_node(node, info->type, sizeof(info->type)) < 0) {
> -               dev_err(dev, "of_i2c: modalias failure on %pOF\n", node);
> -               return -EINVAL;
> -       }
> -
>         ret = of_property_read_u32(node, "reg", &addr);
>         if (ret) {
>                 dev_err(dev, "of_i2c: invalid reg on %pOF\n", node);
>                 return ret;
>         }
>
> +       if (of_modalias_node(node, info->type, sizeof(info->type)) < 0)
> +               strlcpy(info->type, I2C_RESERVED_DRV_NAME, sizeof(I2C_RESERVED_DRV_NAME));

Could this cause a regression, e.g. if people already have such dummy
nodes in their DTS, and use sysfs new_device from userspace to
instantiate the device later?

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