linux-kernel - Re: [PATCH 2/2] device property: Add fwnode_property_get_reference_optional

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Message-ID: <2d674a0e-9a54-4315-bd81-0cb3a2fb1602@linux.dev>
Date: Tue, 8 Apr 2025 11:28:40 -0400
From: Sean Anderson <sean.anderson@...ux.dev>
To: Rob Herring <robh@...nel.org>
Cc: Saravana Kannan <saravanak@...gle.com>, devicetree@...r.kernel.org,
 Sakari Ailus <sakari.ailus@...ux.intel.com>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 "Rafael J . Wysocki" <rafael@...nel.org>,
 Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
 Len Brown <lenb@...nel.org>,
 Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
 Daniel Scally <djrscally@...il.com>, linux-kernel@...r.kernel.org,
 Danilo Krummrich <dakr@...nel.org>, linux-acpi@...r.kernel.org
Subject: Re: [PATCH 2/2] device property: Add
 fwnode_property_get_reference_optional_args

On 4/8/25 11:19, Rob Herring wrote:
> On Tue, Apr 8, 2025 at 10:12 AM Sean Anderson <sean.anderson@...ux.dev> wrote:
>>
>> On 4/8/25 09:00, Rob Herring wrote:
>> > On Mon, Apr 7, 2025 at 5:37 PM Sean Anderson <sean.anderson@...ux.dev> wrote:
>> >>
>> >> Add a fwnode variant of of_parse_phandle_with_optional_args to allow
>> >> nargs_prop to be absent from the referenced node. This improves
>> >> compatibility for references where the devicetree might not always have
>> >> nargs_prop.
>> >
>> > Can't we just make fwnode_property_get_reference_args() handle this
>> > case? Or why is it not just a 1 line wrapper function?
>>
>> fwnode_property_get_reference_args ignores nargs when nargs_prop is
>> non-NULL. So all the existing callers just pass 0 to nargs. Rather than
>> convert them, I chose to add another function with different defaults.
>> There are only four callers that pass nargs_prop, so I could just as
>> easily change the callers instead.
> 
> Why do you have to change the callers? nargs value won't matter
> because they obviously have nargs_prop present or they would not have
> worked in the first place. If behavior changes because there's an
> error in their DT, who cares. That's their problem for not validating
> the DT.

Because the change would be to make nargs matter even when nargs_prop is
present. For the sake of example, consider something like

  foo: foo {
    #my-cells = <1>;
  };

  bar: bar {
  };

  baz {
    my-prop = <&bar>, <&foo 5>, ;
    my-prop-names = "bar", "foo";
  };

Before we would have

fwnode_property_get_reference_args(baz, "my-prop", NULL, 0, "bar", args) <bar>
fwnode_property_get_reference_args(baz, "my-prop", NULL, 0, "foo", args) <foo>
fwnode_property_get_reference_args(baz, "my-prop", "#my-cells", -1, "bar", args) ERROR
fwnode_property_get_reference_args(baz, "my-prop", "#my-cells", -1, "foo", args) ERROR
fwnode_property_get_reference_args(baz, "my-prop", "#my-cells", 0, "bar", args) ERROR
fwnode_property_get_reference_args(baz, "my-prop", "#my-cells", 0, "foo", args) ERROR

and after we would have

fwnode_property_get_reference_args(baz, "my-prop", NULL, 0, "bar", args) <bar>
fwnode_property_get_reference_args(baz, "my-prop", NULL, 0, "foo", args) <foo>
fwnode_property_get_reference_args(baz, "my-prop", "#my-cells", -1, "bar", args) ERROR
fwnode_property_get_reference_args(baz, "my-prop", "#my-cells", -1, "foo", args) ERROR
fwnode_property_get_reference_args(baz, "my-prop", "#my-cells", 0, "bar", args) <bar>
fwnode_property_get_reference_args(baz, "my-prop", "#my-cells", 0, "foo", args) <foo 5>

The problem is that all existing callers pass nargs=0 when
nargs_prop="#my-cells" so they will get the new behavior even when they
shouldn't. So if we change the behavior we have to change the callers
too. If we make a new function with new behavior the callers stay the
same.

--Sean