[RFC PATCH v2 06/13] docs/dt: Document nvidia,tegra20-pinmux binding

[Shawn Guo]

On Tue, Aug 16, 2011 at 10:32:05AM -0700, Stephen Warren wrote:
> Arnd Bergmann wrote at Tuesday, August 16, 2011 7:52 AM:
> > On Monday 15 August 2011, Stephen Warren wrote:
> > > Signed-off-by: Stephen Warren <swarren at nvidia.com>
> > > ---
> > >  .../devicetree/bindings/pinmux/pinmux_nvidia.txt   |  294 ++++++++++++++++++++
> > >  1 files changed, 294 insertions(+), 0 deletions(-)
> > >  create mode 100644 Documentation/devicetree/bindings/pinmux/pinmux_nvidia.txt
> > >
> > > diff --git a/Documentation/devicetree/bindings/pinmux/pinmux_nvidia.txt b/Documentation/devicetree/bindings/pinmux/pinmux_nvidia.txt
> > > new file mode 100644
> > > index 0000000..744e1b7
> > > --- /dev/null
> > > +++ b/Documentation/devicetree/bindings/pinmux/pinmux_nvidia.txt
> > > @@ -0,0 +1,294 @@
> > > +NVIDIA Tegra 2 pinmux controller
> > > +
> > > +Required properties:
> > > +- compatible : "nvidia,tegra20-pinmux"
> > 
> > Hmm, I think it would be much better in general if we could define
> > pinmux bindings in a way that is less specific to just one soc.
> 
> > The contents of this file seem to be specific to even just one
> > version of the tegra chip, and might be completely different for
> > tegra30 and later, right?
> 
> The general concepts are the same between tegra20 and tegra30.
> 
> Tegra30 has a different set of mux and drive pingroups.
> 
> Tegra30 has more functions that can be assigned to pins.
> 
> Tegra30 has three more options per mux pingroup, although I haven't
> Investigated whether any of those would need to be represented in DT;
> I suspect at least one will, possibly all.
> 
> > Maybe Linus W can comment on this and say whether he thinks it can
> > be generalized enough to apply to other pinmux drivers.
> > 
> > > +Optional sub-nodes:
> > > +- nvidia,mux-groups : Mux group settings; see below.
> > > +- nvidia,drive-groups : Drive group settings; see below.
> > > +
> > > +nvidia,mux-groups sub-node:
> > 
> > These concepts seem general enough to me that they can apply to
> > other chips, and I would consequently drop the nvidia, prefix.
> 
> Two things to note about Tegra that might not apply to all SoCs:
> 
> * There are separate groups of pins for Muxing (plus some config) vs.
> drive-strength (and related config). Some SoCs might use the same set
> of groups for both. Perhaps some SoC might even have three or more types
> of groups! I expect this to have some impact on the binding; I created
> explicit mux-groups and drive-groups sub-nodes to represent this.
> 
> * Tegra's pinmux apply settings to groups of pins instead of individual
> pins. Some SoCs might allow each setting to be configurable per-pin.
> I don't expect this to have any impact as far as the bindings go though;
> it'll simply impact the names of the available pin "groups"; some SoCs
> will name groups, others pins.
> 
+1

imx's pinmux applies settings to individual pins.

> > > +Each mux pin group is represented as a sub-node of the nvidia,mux-groups node.
> > > +The name of the sub-node should be the name of the mux pingroup. The following
> > > +names are valid:
> > > +
> > > +	ata
> > > +	atb
> > > +	atc
> > > +	atd
> > > +	ate
> > > +	cdev1
> > > +	cdev2
> > > ...
> > 
> > I noticed that each board you define has a complete list of these. Would
> > it be possible to move a generic list into a tegra20-pinmux.dtsi file and
> > just override the pins in the per-board .dts file that require some special
> > setup?
> 

It sounds sane for imx though.  I'm going to take this suggestion to
have imx53-pinmux.dtsi holding register offset for each pin, which is
common across all i.mx53 boards.

> I'm not sure how much commonality there will really be.
> 
> Certainly, many boards are based off our reference designs and so will
> have many similarities that could be shared.
> 
> That said, comparing even tegra-harmony.dts and tegra-seaboard.dts shows
> a lot of differences. It seems a lot less error-prone to just completely
> describe the entire pinmux state in each board's .dts file, rather than
> trying to represent half the information as default in the SoC .dtsi file,
> and just the diff in the board .dts file. I suppose perhaps if we put the
> hardware's own default settings in tegra20.dtsi, that'd make sense, since
> people are presumably reasonably aware of the delta relative to that.
> 
> We'd need to add new properties to override defaults, like:
> 
> nvidia,tristate --> nvidia,drive
> nvidia,pull-*   --> nvidia,no-pull
> 
> > > +
> > > +optional subnode-properties:
> > > +- nvidia,pull-up : Boolean, apply Tegra's internal pull-up to the pin.
> > > +- nvidia,pull-down : Boolean, apply Tegra's internal pull-down to the pin.
> > > +- nvidia,tristate : Boolean, tristate the pin. Otherwise, drive it.
> > > +
> > > +If both nvidia,pull-up and nvidia,pull-down are specified, nvidia,pull-up
> > > +takes precedence.
> > 
> > These again seem generic enough to go into a general pinmux binding, without
> > the nvidia, prefix.
> 
> Yes, I believe Jamie Iles was going to try cooking up a generic core binding
> that could be shared across different SoCs, and extended with custom stuff.
> 
I would image that whatever common binding for pin/pad configuration
comes can hardly work for imx out of box.  For each pin/pad, imx have
4 configurations for pull:

  00: 100KOhm Pull Down
  01: 47KOhm Pull Up
  10: 100KOhm Pull Up
  11: 22KOhm Pull Up

4 configurations for drive:

  00: 100KOhm Pull Down
  01: 47KOhm Pull Up
  10: 100KOhm Pull Up
  11: 22KOhm Pull Up

some other 1 bit configuration fields:

  low/high output voltage Field
  Hyst. Enable Field
  Pull / Keep Enable Field
  Pull / Keep Select Field
  Slew Rate Field

All these are all in one register <pad_name>_PAD_CTL (mux configuration
is in another register <pad_name>_PAD_MUX).  The best binding for
PAD_CTL I can think of so far is to encode the register value in DT
directly.

-- 
Regards,
Shawn