Date:	Mon, 23 Jan 2012 14:43:59 -0800
From:	Stephen Warren <swarren@...dia.com>
To:	Thomas Abraham <thomas.abraham@...aro.org>
CC:	Dong Aisheng-B29396 <B29396@...escale.com>,
	"linus.walleij@...ricsson.com" <linus.walleij@...ricsson.com>,
	"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>,
	"Simon Glass (sjg@...omium.org)" <sjg@...omium.org>,
	Dong Aisheng <dongas86@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"devicetree-discuss@...ts.ozlabs.org" 
	<devicetree-discuss@...ts.ozlabs.org>
Subject: RE: Pinmux bindings proposal

Thomas Abraham wrote at Friday, January 20, 2012 6:28 PM:
> On 21 January 2012 02:41, Stephen Warren <swarren@...dia.com> wrote:
> > Thomas Abraham wrote at Thursday, January 19, 2012 6:10 AM:
> >> On 14 January 2012 02:09, Stephen Warren <swarren@...dia.com> wrote:
...
> >> * Specifying the pinmux/pinconfig settings in dts files:
> >>
> >> Device nodes which require specific pinmux/pinconfig settings should
> >> include information about the required settings. For example, a i2c
> >> controller node in dts file is listed below.
> >>
> >> i2c0: i2c@...00000 {
> >>         [... other properties ...]
> >>         pinctrl-active = <&pctrl0 5 0 2 3 0>,
> >>                              <&pctrl0 5 1 2 3 0>;
> >>         pinctrl-suspend = <&pctrl0 5 0 2 0 0>,
> >>                                 <&pctrl0 5 1 2 0 0>;
> >> };
> >
> > The idea of encoding the state names into the property names came up
> > before in this thread.
> 
> Yes, I did borrow the idea of states 'active' and 'suspend' from the
> dt binding discussions here. The current Samsung mainline dt support
> for gpio and pinmux is using this type of encoding but did not have
> the states.
> 
> > The problem is that it's hard for core code
> > to know which properties are actually related to pinctrl and which
> > aren't. reserving one name such as "pinctrl" seems reasonable, whereas
> > reserving a whole class of names; everything prefixed "pinctrl-foo" is
> > a little less so.
> 
> The basic premise with which I proposed about the dt support for pinctrl was
> 
> - The pinctrl core code does not have to know anything about these
> bindings inside device nodes (of say i2c).
> 
> - Device drivers (such as i2c) retrieve the value of pinctrl-*
> property and pass on two parameters to the pinctrl code.
>   (a) The 'struct device_node' pointer of the intended pinctrl_dev
> instance (obtained from the phandle above).
>   (b) The encoding that specifies the hardware register values.

That seems like a bunch of extra work for device drivers, and also
something that's only required for device tree. By having a standardized
representation (or framework for one) of the pinctrl properties within
each device node, then drivers can:

a) Drivers don't have to do a/b above, instead the pinctrl core will
do it.

b) Drivers can use the exact same pinctrl APIs when the system booted
using DT or not.

> - Pinctrl core scans its list for all registered pinctrl_dev, matches
> it against the 'device node' specified and selects one of the
> pinctrl_dev. It then passes on the second parameter to the pinctrl
> driver which decodes it and writes appropriate values to the hardware
> registers.
> 
> - pinmux/pinconfig/pindesc tables are not built from DT. There are no
> static tables built into the SoC specific pinctrl driver.

Not having any pin/group/function tables in either the driver or DT
would be a pretty radical departure from the way the pinctrl subsystem
works today. It'd basically make pinctrl irrelevant on DT, I think, if
I'm understanding what you're saying correctly. I don't know if we
really want to go down that path.

Not having the pinctrl pinmux mapping table parsed up front, but instead
deferring it until a driver called pinmux_get() might be reasonable. I'm
not totally opposed to that, but others have expressed a desire to parse
up front, so that the pinctrl sysfs files that contain the pinmux mapping
table are identical for a non-DT and DT boot.

...
> >> In the example above, the specifier of pinctrl-* is specific for
> >> Samsung io-pad controllers. Its format/syntax would be mainly
> >> dependent on the io-pad controller used, the above is only an example
> >> for Samsung io-pad controller. In the above node, the format of the
> >> pinctrl-* specifier is
> >>
> >> property-name = <phandle of the pin-controller
> >>                            pin bank within the pin-controller
> >>                            pin number within the pin-bank
> >>                            pin-mux function number
> >>                            pull up/down config
> >>                            drive strength config>;
> >
> > Yes, that seems reasonable.
> >
> > The only thing different between that example and my proposal is that:
> >
> > a) In my proposal, the property in the device nodes doesn't actually
> >   contain all those fields, but a phandle to a child node of the pin
> >   controller where that data is kept. This keeps all the pin mux data
> >   stored under the pin controller's node for neatness.
> 
> I am not sure if that is required. In case dt support for interrupt
> controller such as gic, the interrupt number, type of interrupt and
> edge/level type flags are all listed in the device node itself and not
> under the gic device node.

Yes, there's no specific need to place those nodes inside the
pin controller's node. I intended to put them inside the individual
device nodes for my boards. However, others have expressed a strong
desire to only allow these nodes to be inside the pin controller node,
and I've gone along with that in order to move the binding forward.
While this is a little different to GPIOs and IRQs, I don't see a
significant disadvantage to this requirement; when I started looking
at pinmux DT months ago, I had planned on simply putting a single big
table inside the pin controller node anyway, so this isn't so different.

...
> > The pinctrl subsystem already has APIs such as pinmux_get() and
> > pinmux_enable() that initiate pin mux programming, so I've been
> > assuming they'd be used identically for both systems that use board
> > files and systems that use DT.
> 
> In case of DT based boot, the code that handles the device node
> containing pinctrl-* (either device driver or platform code) need not
> call the pinmux_get(). The encoding in the pinctrl-* (as listed above)
> is sufficient to setup the pinmux/pinconfig as required.

I think drivers should call pinmux_get() in all cases. We don't want
some drivers using pinmux_get() and only working without DT, and others
using some different API and only working with DT. We need to hide all
the DT/non-DT details behind the existing APIs so that drivers are as
portable as possible.

-- 
nvpublic

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