RE: Pinmux bindings proposal V2

From: Stephen Warren
Date: Thu Feb 02 2012 - 13:37:25 EST

Shawn Guo wrote at Wednesday, February 01, 2012 7:36 AM:
> I had a talk with Dong about this binding, and we think that it should
> work well for imx if we have a couple of small pieces added.
> On Fri, Jan 20, 2012 at 02:22:20PM -0800, Stephen Warren wrote:
> ...
> > pmx_sdhci: pinconfig-sdhci {
> > /*
> > * The mux property is a list of muxable entities
> > * and the mux function to select for it. The number
> > * of cells in each entry is the pin controller's
> > * #pinmux-cells property. The pin controller's
> > * binding defines what the cells mean. The pinctrl
> > * driver is responsible for mapping this data to
> > * the (group, function) pair required to fill in
> > * the pinctrl subsystem's pinmux mapping table.
> > */
> > mux =
> We need a property like 'mux-unit' whose value can be either 'pin' or
> 'pingroup' to reflect something you mentioned as muxable entity.

I'm not sure I agree; see below.

> The reason behind this is the DT logic inside pinctrl core needs to
> know how the pinmux_map should be constructed from device tree.

As a general statement, yes.

> In tegra case, the 'mux-unit' is 'pingroup', the core should construct
> pinmux_map entry for each row/element of 'mux'.


> In imx case, the 'mux-unit' will be 'pin',


Just a note: Tegra30 also has per-pin muxability. Only Tegra20 muxes pins
in groups. (although Tegra30 does some if its pin configuration in groups)

> and we would expect core construct only
> one pinmux_map entry there, with all the pins listed in 'mux' composing
> the group that pinmux_map needs.

This is where I disagree.

If the pinmux_map should only contain a single entry, wouldn't the DT
mux property only contain a single entry?

The reason being that if there's a single entry in the pinmux_map, the
group name used in that entry must be a group that's supported directly
by the pinctrl driver (that's just the way pinctrl works). As such, why
not just write the device tree in terms of those groups?

The only way I can see this not being true is if your pinctrl driver is
also parsing these mux properties, and dynamically creating the groups
that it exposes based on the list of pins in the mux property. However,
that seems like the wrong approach; If you're dynamically defining groups
in DT, I'd expect separate explicit driver-specific properties/nodes to
define those groups, such that the pinctrl core's processing of the mux
property to be identical in all cases.

i.e. instead of what your "mux-unit" proposal:

pmx_sdhci: pinconfig-sdhci {
mux =
mux-unit = "pingroup";
mux-name = "sdio-config-1";

I'd expect something more like:

/* Standardized pinctrl properties */
pmx_sdhci: pinconfig-sdhci {

* Driver-specific properties which tell the driver which potentially
* board-specific pin-groups to implement.
imx-pingroup-sdio-cfg-1 {
id = <IMX_PG_SDIO_CFG_1>;

Does that make sense?

> And in case of 'mux-unit' is 'pin', we would need one more property
> 'mux-name' to present the group name. Then we have all the pieces to
> construct pinmux_map for cases like imx, where we do not define all
> those functions, groups in pinctrl driver at all.


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