linux-kernel - Re: Pinmux bindings proposal V2

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Date:	Mon, 30 Jan 2012 09:49:10 -0800
From:	Tony Lindgren <tony@...mide.com>
To:	Shawn Guo <shawn.guo@...aro.org>
Cc:	Simon Glass <sjg@...omium.org>,
	Stephen Warren <swarren@...dia.com>,
	Dong Aisheng-B29396 <B29396@...escale.com>,
	"Linus Walleij (linus.walleij@...aro.org)" <linus.walleij@...aro.org>,
	"Sascha Hauer (s.hauer@...gutronix.de)" <s.hauer@...gutronix.de>,
	"rob.herring@...xeda.com" <rob.herring@...xeda.com>,
	"kernel@...gutronix.de" <kernel@...gutronix.de>,
	"cjb@...top.org" <cjb@...top.org>,
	Dong Aisheng <dongas86@...il.com>,
	Thomas Abraham <thomas.abraham@...aro.org>,
	"Grant Likely (grant.likely@...retlab.ca)" 
	<grant.likely@...retlab.ca>,
	"devicetree-discuss@...ts.ozlabs.org" 
	<devicetree-discuss@...ts.ozlabs.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: Pinmux bindings proposal V2

* Shawn Guo <shawn.guo@...aro.org> [120129 18:30]:
> On Fri, Jan 27, 2012 at 07:43:36AM -0800, Simon Glass wrote:
> ...
> > The cost of the pmx@dta node is about 12 bytes for the header (it
> > depends on the length of the name), and each of the properties above
> > is 16 bytes. So in total this node is 76 bytes. If we have 250 pins
> > being muxed as Tegra3 then this is about 20KB (including a bit of
> > slack for longer names). My point about being able to 'optimise out'
> > some of these remains, though, but probably not for the kernel.
> > 
> > Stephen's 'mux' property uses 12 bytes plus 8 bytes per pin/group (I
> > am removing the prefixes):
> > 
> >                        mux =
> >                                <PG_DTA   MUX_SDIO1>
> >                                <PG_DTD   MUX_SDIO1>;
> > 
> > so 28 bytes. What I proposed would use (12 + 2 * 16) per pin/group, or
> > 44 bytes (60% bigger):
> > 
> It's not only about size but also run-time tree travelling efficiency.
> Your proposal requires every single pin show as a node in device tree.
> Looking at these for_each_node_by_*() APIs in include/linux/of.h, you
> might agree we should avoid bloating device tree with so many nodes.

And that's why I'm suggesting two bindings: A minimal pinctrl-static
binding and more verbose pinctrl-dynamic binding.

AFAIK the number of pinctrl-dynamic bindings needed are just a fraction
of the pinctrl-static bindings. So the extra parsing needed for a few
pinctrl-dynamic bindings should not matter.

Sure it would be nice to have it all in a single binding, but these
bindings have conflicting requirements. So it may not be possible to
do it in a single binding in an efficient way.

Regards,

Tony
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