linux-kernel - Re: [PATCHv6+ 01/13] of: introduce of_property_for_earch_phandle_with

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Message-ID: <528E577C.2050506@wwwdotorg.org>
Date:	Thu, 21 Nov 2013 11:57:00 -0700
From:	Stephen Warren <swarren@...dotorg.org>
To:	Hiroshi Doyu <hdoyu@...dia.com>,
	"grant.likely@...aro.org" <grant.likely@...aro.org>
CC:	"thierry.reding@...il.com" <thierry.reding@...il.com>,
	"robherring2@...il.com" <robherring2@...il.com>,
	"joro@...tes.org" <joro@...tes.org>,
	Stephen Warren <swarren@...dia.com>,
	"will.deacon@....com" <will.deacon@....com>,
	"mark.rutland@....com" <mark.rutland@....com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"lorenzo.pieralisi@....com" <lorenzo.pieralisi@....com>,
	"galak@...eaurora.org" <galak@...eaurora.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCHv6+ 01/13] of: introduce of_property_for_earch_phandle_with_args()

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 <hdoyu@...dia.com>
> ---
> 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.
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