Re: [PATCH 2/3] dt-bindings: allow child nodes inside the Tegra BPMP

[Thierry Reding <thierry.reding-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

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On Thu, Jul 28, 2016 at 03:24:22PM -0600, Stephen Warren wrote:
> On 07/28/2016 01:07 PM, Rob Herring wrote:
> > On Tue, Jul 26, 2016 at 11:21 AM, Stephen Warren <swarren-3lzwWm7+Weoh9ZMKESR00Q@public.gmane.org> wrote:
> > > On 07/26/2016 04:03 AM, Thierry Reding wrote:
> > > > 
> > > > On Tue, Jul 19, 2016 at 01:14:41PM -0600, Stephen Warren wrote:
> > > > > 
> > > > > From: Stephen Warren <swarren-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org>
> > > > > 
> > > > > The BPMP implements some services which must be represented by separate
> > > > > nodes. For example, it can provide access to certain I2C controllers, and
> > > > > the I2C bindings represent each I2C controller as a device tree node.
> > > > > Update the binding to describe how the BPMP supports this.
> > > > > 
> > > > > Signed-off-by: Stephen Warren <swarren-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org>
> > > > > ---
> > > > >  .../bindings/firmware/nvidia,tegra186-bpmp.txt     | 23
> > > > > ++++++++++++++++++++++
> > > > >  1 file changed, 23 insertions(+)
> > > > > 
> > > > > diff --git
> > > > > a/Documentation/devicetree/bindings/firmware/nvidia,tegra186-bpmp.txt
> > > > > b/Documentation/devicetree/bindings/firmware/nvidia,tegra186-bpmp.txt
> > > > > index 9a3864f56955..142d363406f6 100644
> > > > > --- a/Documentation/devicetree/bindings/firmware/nvidia,tegra186-bpmp.txt
> > > > > +++ b/Documentation/devicetree/bindings/firmware/nvidia,tegra186-bpmp.txt
> > > > > @@ -38,6 +38,24 @@ implemented by this node:
> > > > >  - .../reset/reset.txt
> > > > >  - <dt-bindings/reset/tegra186-reset.h>
> > > > > 
> > > > > +The BPMP implements some services which must be represented by separate
> > > > > nodes.
> > > > > +For example, it can provide access to certain I2C controllers, and the
> > > > > I2C
> > > > > +bindings represent each I2C controller as a device tree node. Such nodes
> > > > > should
> > > > > +be nested directly inside the main BPMP node.
> > > > > +
> > > > > +Software can determine whether a child node of the BPMP node represents
> > > > > a device
> > > > > +by checking for a compatible property. Any node with a compatible
> > > > > property
> > > > > +represents a device that can be instantiated. Nodes without a compatible
> > > > > +property may be used to provide configuration information regarding the
> > > > > BPMP
> > > > > +itself, although no such configuration nodes are currently defined by
> > > > > this
> > > > > +binding.
> > > > > +
> > > > > +The BPMP firmware defines no single global name-/numbering-space for
> > > > > such
> > > > > +services. Put another way, the numbering scheme for I2C buses is
> > > > > distinct from
> > > > > +the numbering scheme for any other service the BPMP may provide (e.g. a
> > > > > future
> > > > > +hypothetical SPI bus service). As such, child device nodes will have no
> > > > > reg
> > > > > +property, and the BPMP node will have no #address-cells or #size-cells
> > > > > property.
> > > > 
> > > > 
> > > > My understanding is that the I2C bus number is passed as part of the
> > > > request to the BPMP firmware. Does that not count as addressing? Could
> > > > we not represent that generically using a device tree hierarchy? I'm
> > > > thinking something along these lines:
> > > > 
> > > >         bpmp {
> > > >                 ...
> > > > 
> > > >                 services {
> > > >                         i2c {
> > > >                                 i2c@0 {
> > > >                                         reg = <0>;
> > > 
> > > 
> > > Technically, that is possible. However, Rob Herring rejected the idea of
> > > multiple levels of sub-nodes.
> > 
> > I think I questioned the need, not rejected. What about the above, but
> > remove serivces level:
> > 
> >          bpmp {
> >                  ...
> > 
> >                  i2c {
> >                          i2c@0 {
> >                                  reg = <0>;
> 
> Sigh. Can you please talk to Thierry and work out what the binding would be
> (perhaps on IRC to expedite things?) and I'll just implement whatever you
> two agree upon. I don't really care much what the binding looks like any
> more; I just need something that will pass review. Thanks.

Like I said, I'm fine going with what you proposed. I'm sure by now the
BPMP has long been frozen and if the proposed binding works fine that's
good. No need to over-engineer.

If we ever need more than one I2C (or expose other busses) I'm sure we
can find a way to switch to something like the above later on. Like I
said, that's very unlikely to happen on Tegra186, so we'll have a new
compatible string (and hence an easy way to differentiate between these
bindings) anyway.

Thierry

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