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

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Message-Id: <20131211132845.5461FC4061A@trevor.secretlab.ca>
Date:	Wed, 11 Dec 2013 13:28:45 +0000
From:	Grant Likely <grant.likely@...aro.org>
To:	Stephen Warren <swarren@...dotorg.org>,
	Hiroshi Doyu <hdoyu@...dia.com>
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 Thu, 21 Nov 2013 11:57:00 -0700, Stephen Warren <swarren@...dotorg.org> wrote:
> 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.

That's right, I forgot I said that. Yes please fix the implementation.

g.

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