linux-kernel - RE: [PATCH 1/2] drivers: create a pin control subsystem v7

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Message-ID: <74CDBE0F657A3D45AFBB94109FB122FF17395556D5@HQMAIL01.nvidia.com>
Date:	Tue, 27 Sep 2011 17:08:52 -0700
From:	Stephen Warren <swarren@...dia.com>
To:	Linus Walleij <linus.walleij@...aro.org>
CC:	Linus Walleij <linus.walleij@...ricsson.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Grant Likely <grant.likely@...retlab.ca>,
	Barry Song <21cnbao@...il.com>,
	Lee Jones <lee.jones@...aro.org>,
	Joe Perches <joe@...ches.com>,
	Russell King <linux@....linux.org.uk>,
	Linaro Dev <linaro-dev@...ts.linaro.org>,
	Sascha Hauer <s.hauer@...gutronix.de>,
	David Brown <davidb@...eaurora.org>
Subject: RE: [PATCH 1/2] drivers: create a pin control subsystem v7

Linus Walleij wrote at Tuesday, September 27, 2011 1:51 AM:
> On Wed, Sep 21, 2011 at 9:45 PM, Stephen Warren <swarren@...dia.com> wrote:
> > Linus Walleij wrote at Wednesday, September 21, 2011 3:17 AM:
> >> To abstract things the stuff we can do with the group should be
> >> something enumerated too. So:
> >>
> >> pinctrl_config_group(const char *pinctrl_device, const char *group,
> >> const char *mode);
> >> pinctrl_config_pin(const char *pinctrl_device, int pin, const char *mode);
> >>
> >> So the driver need an API to enumerate pin and group modes.
> >>
> >> I might want to save this thing for post-merge of the basic API and
> >> pinmux stuff though so we don't try to push too much upfront
> >> design at once.
> >
> > Yes, adding in a replacement _config API can certainly be a later patch.
> 
> If I'm efficient enough we might get that add-on before the v3.2
> merge window, I'll iterate that patch separately though.
> 
> > One comment on the API above: I think you want "mode" (or "setting"?)
> > /and/ a value parameter; Tegra's pull strength definition for example
> > is:
> >
> > enum tegra_pull_strength {
> >        TEGRA_PULL_0 = 0,
> >        TEGRA_PULL_1,
> > // ... (every value in between)
> >        TEGRA_PULL_30,
> >        TEGRA_PULL_31,
> >        TEGRA_MAX_PULL,
> > };
> >
> > And it seems better to represent that as ("pull", 0) ... ("pull", 31) than
> > "pull0" .. "pull31" such that the value needs to be parsed out of the "mode"
> > string somehow by the driver.
> 
> OK so we might want some public defintion of "things you can do"
> with pins, then an opaque parameter.
> 
> like:
> 
> #define PINCTRL_PULL "pull"
> #define PINCTRL_BIAS "bias"
> #define PINCTRL_LOADCAP "load-capacitance"
> #define PINCTRL_DRIVE "drive"

Yes although Tegra has quite a few more weird options than that; I'm not
sure exactly how many of the options that Tegra supports would be useful
for other SoCs, or what the best way to represent all the device-specific
knobs is. I guess we need another survey of a bunch of SoCs like you did
when designing the pinmux API.

> ...but then it's just simple enumerators, and we might be better off
> with a simple enum after all:
> 
> enum pinctrl_pin_ops {
>     PINCTRL_PULL,
>     PINCTRL_BIAS,
>     PINCTRL_LOADCAP,
>     PINCTRL_DRIVE,
> }
> 
> Surely the device tree will have to translate that enum into strings
> (I guess? Sorry for my low DT competence) but that is more of a
> DT pecularity and can be kept in the DT pin control parser?

DT as a data format can handle strings or integers just fine. However,
there's currently no support for named constants (i.e. #defines) in the
device-tree source format, so strings end up being a lot more self-
documenting. Whatever the DT format ends up being, we can certainly
map from whatever-dt-contains to whatever-Linux-APIs-need inside 
whatever kernel function is written to extract the data from DT.

-- 
nvpublic

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