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From:   Rob Herring <robh+dt@kernel.org>
Date:   Thu, 2 Sep 2021 09:24:29 -0500
Message-ID: <CAL_JsqJOv7D5nHteGPDKC2+ns1caVNs-NFFJppLuK0OEB8dztQ@mail.gmail.com>
Subject: Re: [PATCH v1 2/2] of: platform: Mark bus devices nodes with FWNODE_FLAG_NEVER_PROBES
To:     Saravana Kannan <saravanak@google.com>
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        Frank Rowand <frowand.list@gmail.com>,
        Len Brown <lenb@kernel.org>,
        Ulf Hansson <ulf.hansson@linaro.org>,
        Android Kernel Team <kernel-team@android.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        devicetree@vger.kernel.org,
        "open list:ACPI FOR ARM64 (ACPI/arm64)" <linux-acpi@vger.kernel.org>
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On Wed, Sep 1, 2021 at 9:55 PM Saravana Kannan <saravanak@google.com> wrote:
>
> We don't want fw_devlink creating device links for bus devices as
> they'll never probe. So mark those device node with this flag.
>
> Signed-off-by: Saravana Kannan <saravanak@google.com>
> ---
>  drivers/of/platform.c | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
>
> diff --git a/drivers/of/platform.c b/drivers/of/platform.c
> index 74afbb7a4f5e..42b3936d204a 100644
> --- a/drivers/of/platform.c
> +++ b/drivers/of/platform.c
> @@ -392,6 +392,22 @@ static int of_platform_bus_create(struct device_node *bus,
>         if (!dev || !of_match_node(matches, bus))
>                 return 0;
>
> +       /*
> +        * If the bus node has only one compatible string value and it has
> +        * matched as a bus node, it's never going to get probed by a device
> +        * driver. So flag it as such so that fw_devlink knows not to create
> +        * device links with this device.
> +        *
> +        * This doesn't catch all devices that'll never probe, but this is good
> +        * enough for now.
> +        *
> +        * This doesn't really work for PPC because of how it uses
> +        * of_platform_bus_probe() to add normal devices. So ignore PPC cases.
> +        */
> +       if (!IS_ENABLED(CONFIG_PPC) &&
> +           of_property_count_strings(bus, "compatible") == 1)
> +               bus->fwnode.flags |= FWNODE_FLAG_NOT_DEVICE;

This looks fragile relying on 1 compatible string, and the DT flags in
this code have been fragile too. I'm pretty sure we have cases of
simple-bus or simple-mfd that also have another compatible.

Couldn't we solve this with a simple driver? Make 'simple-pm-bus'
driver work for other cases? BTW, this patch doesn't even work for
simple-pm-bus. A driver for simple-bus may cause issues if there's a
more specific driver to bind to as we don't handle that. It's simply
whichever matches first.

Rob