linux-kernel - Re: [PATCH v3 02/15] Driver core: Unified device properties interface for platform firmware

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Message-ID: <7365448.TlsV4zB2It@wuerfel>
Date:	Wed, 01 Oct 2014 09:47:40 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:	linux-kernel@...r.kernel.org,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Mika Westerberg <mika.westerberg@...ux.intel.com>,
	linux-acpi@...r.kernel.org, devicetree@...r.kernel.org,
	Linus Walleij <linus.walleij@...aro.org>,
	Alexandre Courbot <gnurou@...il.com>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Bryan Wu <cooloney@...il.com>,
	Lee Jones <lee.jones@...aro.org>,
	Grant Likely <grant.likely@...aro.org>,
	Aaron Lu <aaron.lu@...el.com>,
	Darren Hart <dvhart@...ux.intel.com>
Subject: Re: [PATCH v3 02/15] Driver core: Unified device properties interface for platform firmware

On Wednesday 01 October 2014 04:10:03 Rafael J. Wysocki wrote:
> From: "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
> 
> Add a uniform interface by which device drivers can request device
> properties from the platform firmware by providing a property name
> and the corresponding data type.  The purpose of it is to help to
> write portable code that won't depend on any particular platform
> firmware interface.
> 
> Three general helper functions, device_get_property(),
> device_read_property() and device_read_property_array() are provided.
> The first one allows the raw value of a given device property to be
> accessed.  The remaining two allow the value of a numeric or string
> property and multiple numeric or string values of one array
> property to be acquired, respectively.  Static inline wrappers are also
> provided for the various property data types that can be passed to
> device_read_property() or device_read_property_array() for extra type
> checking.

These look great!

> In addition to that, new generic routines are provided for retrieving
> properties from device description objects in the platform firmware
> in case a device driver needs/wants to access properties of a child
> object of a given device object.  There are cases in which there is
> no struct device representation of such child objects and this
> additional API is useful then.  Again, three functions are provided,
> device_get_child_property(), device_read_child_property(),
> device_read_child_property_array(), in analogy with device_get_property(),
> device_read_property() and device_read_property_array() described above,
> respectively, along with static inline wrappers for all of the propery
> data types that can be used.  For all of them, the first argument is
> a struct device pointer to the parent device object and the second
> argument is a (void *) pointer to the child description provided by
> the platform firmware (either ACPI or FDT).

I still have my reservations against the child accessors, and would
like to hear what other people think. Passing a void pointer rather
than struct fw_dev_node has both advantages and disadvantages, and
I won't complain about either one if enough other people on the DT
side would like to see the addition of the child functions.

> Finally, device_for_each_child_node() is added for iterating over
> the children of the device description object associated with a
> given device.
> 
> The interface covers both ACPI and Device Trees.
> 
> This change set includes material from Mika Westerberg and Aaron Lu.
> 

Regarding device_for_each_child_node(), the syntax is inconsistent
with what we normally use, which can probably be changed. All of the
DT for_each_* helpers are macros that are used like

	struct device *dev = ...;
	void *child; /* iterator */

	device_for_each_child_node(dev, child) {
		u32 something;
		device_child_property_read_u32(dev, child, "propname", &something);

		do_something(dev, something);
	}

If we get a consensus on having the child interfaces, I'd rather see
them done this way than with a callback pointer, for consistency
reasons.

	Arnd
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