linux-kernel - RE: [PATCH 1/4] pinctrl: add a driver for NVIDIA Tegra

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Message-ID: <74CDBE0F657A3D45AFBB94109FB122FF178CB81F1A@HQMAIL01.nvidia.com>
Date:	Tue, 24 Jan 2012 15:36:06 -0800
From:	Stephen Warren <swarren@...dia.com>
To:	Linus Walleij <linus.walleij@...aro.org>
CC:	Olof Johansson <olof@...om.net>, Colin Cross <ccross@...roid.com>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH 1/4] pinctrl: add a driver for NVIDIA Tegra

Linus Walleij wrote at Tuesday, January 24, 2012 4:28 PM:
> On Wed, Jan 25, 2012 at 12:09 AM, Stephen Warren <swarren@...dia.com> wrote:
> > [Me]
> >> Why are all of these things (reg bank bit ets) signed?
> >
> > The basic issue is that all of these features are optional for any
> > given pin group. I used -1 to indicate an unsupported feature.
> 
> Ahyeah. Makes sense. Well do it any way that makes sense
> to you. Plus note in the kerneldoc that this is the reason these
> are signed.
> 
> >> Also, things named  *_bit are a bit (no pun intended) binary,
> >> are they containing a single bit? In that case say
> >
> > They're the bit number/shift within a register. Range 0..31
> 
> Can that thing really be negative then?
> Or is it really: u8 foo_bit:5 ?

Only reg needs to be signed/negative; the rest I just did for consistency
in the struct definition; I think an old version of the code wrote -1 to
foo_reg, foo_bank and foo_bit too.

> >> u8 foo:1
> >>
> >> To mark that it's only one bit wide, or u8 foo:4 for four bits
> >> etc.
> >
> > I guess I could be explicit about the max range for each value. It might
> > save up to about 8 bytes per pin group, perhaps less based on how well
> > things pack to u32 boundaries.
> 
> Nah I think you would have to pack the struct with #pragmas for that
> and I don't even know if it'd pack below byte limits. Probably not.

u32 x:4;
u32 y:5;
u32 z:4;

should pack them all together, and adjacent u32s in the struct would be
packed into the struct without padding.

...
> >> func_safe doesn't look like an int either when I look at
> >> the code. It's something else, u8?
> >
> > It's a "enum tegra_mux", which IIRC is technically an int, but unsigned
> > is more consistent with the rest of pinctrl.
> 
> Hm "enum tegra_mux foo" should work fine too then,
> what am I missing here...

Good point. It's because each chip has a different set definition of
enum tegra_mux, and pinctrl-tegra.h is shared across both chips.

-- 
nvpublic

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