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

[Stephen Warren]

On 07/29/2016 02:28 PM, Rob Herring wrote:
> On Fri, Jul 29, 2016 at 11:06 AM, Stephen Warren <swarren at wwwdotorg.org> wrote:
>> On 07/29/2016 07:48 AM, Thierry Reding wrote:
>>>
>>> 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 at wwwdotorg.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 at nvidia.com>
>>>>>>>>
>>>>>>>> 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 at nvidia.com>
>>>>>>>> ---
>>>>>>>>  .../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 at 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 at 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.
>>
>>
>> OK, great. For the record, I'm going to send U-Boot patches today that apply
>> this binding patch (the original one I sent, unmodified) to the U-Boot tree,
>> and implement it in the U-Boot driver.
>
> That's fine, I did ack that.
>
> I really don't want to ask, but why are you documenting in u-boot? Is
> it some how different for u-boot?

There's a copy of the DT binding documents in U-Boot as well. At least 
for the bindings I deal with, they should be the same, although the 
U-Boot copies might get stale as hopefully-backwards-compatible 
enhancements are made to the kernel versions.

I'm implementing a U-Boot driver for this new feature, so am copying 
these new binding docs into the U-Boot DT binding docs directory too.

I'd be happy personally not to have another copy of the bindings, and 
just tell people to refer to the docs in the kernel, or any other 
canonical source, but that's not my call.