Subject: [PATCHv6+ 01/13] of: introduce of_property_for_earch_phandle_with_args()

From: [email-masked] (Stephen Warren)
Date: Thu, 21 Nov 2013 11:57:00 -0700

On 11/21/2013 10:17 AM, Hiroshi Doyu wrote:
> Iterating over a property containing a list of phandles with arguments
> is a common operation for device drivers. This patch adds a new
> of_property_for_each_phandle_with_args() macro to make the iteration
> simpler.
> 
> Signed-off-by: Hiroshi Doyu <[email-masked]>
> ---
> v6+:
> Use the description, which Grant Likely proposed, to be full enough
> that a future reader can figure out why a patch was written.
>   http://lists.linuxfoundation.org/pipermail/iommu/2013-November/007062.html

This new version only addresses one of the concerns that Grant had,
namely the commit message.

> diff --git a/include/linux/of.h b/include/linux/of.h

> +#define of_property_for_each_phandle_with_args(np, list, cells, i, args) \
> +	for (i = 0; !of_parse_phandle_with_args(np, list, cells, i, args); i++)
> +

Grant also wanted the actual implementation fixed so that it wasn't so
inefficient.

What this current patch does is basically:

for every entry in the property:
    for every entry in the property before the current index:
        parse the phandle+specifier

That's roughly O(n^2). (n is # entries in the property)

Instead, what should happen is:

for every entry in the property:
    parse the phandle+specifier
    yield the result

That's roughly O(n).

In other words, an implementation more along the lines of
include/linux/of.h's:

#define of_property_for_each_u32(np, propname, prop, p, u)      \
        for (prop = of_find_property(np, propname, NULL),       \
                p = of_prop_next_u32(prop, NULL, &u);           \
                p;                                              \
                p = of_prop_next_u32(prop, p, &u))

... so you'd need functions like of_prop_first_specifier() and
of_prop_next_specifier(), and perhaps some associated set of state
variables, perhaps with all the state wrapped into a single struct for
simplicity.