Subject: Device tree property names for MDIO bus multiplexer. Was: Re: [PATCH 2/3] netdev/of/phy: Add MDIO bus multiplexer support.

From: David Daney <[email-masked]>
Date: Wed, 14 Sep 2011 14:40:41 -0700

Well, I would really like to get an official maintainer's take on the 
name of the parent MDIO bus property.  Prehaps Grant or Rob could opine 
on the matter.

Sooner would be better than later as I am about to start shipping boards 
with this burnt into the bootloader.  If it needs changing, I could do 
it in the next couple of days, but after that it escapes into the wild.

Thanks in advance,
David Daney

On 09/14/2011 01:42 PM, Kumar Gala wrote:
>
> On Sep 13, 2011, at 6:23 PM, David Daney wrote:
>
>> On 09/13/2011 04:07 PM, Kumar Gala wrote:
>>>
>>>> diff --git a/Documentation/devicetree/bindings/net/mdio-mux.txt b/Documentation/devicetree/bindings/net/mdio-mux.txt
>>>> new file mode 100644
>>>> index 0000000..a908312
>>>> --- /dev/null
>>>> +++ b/Documentation/devicetree/bindings/net/mdio-mux.txt
>>>> @@ -0,0 +1,132 @@
>>>> +Common MDIO bus multiplexer/switch properties.
>>>> +
>>>> +An MDIO bus multiplexer/switch will have several child busses that are
>>>> +numbered uniquely in a device dependent manner.  The nodes for an MDIO
>>>> +bus multiplexer/switch will have one child node for each child bus.
>>>> +
>>>> +Required properties:
>>>> +- parent-bus : phandle to the parent MDIO bus.
>>>
>>> Should probably be mdio-parent-bus
>>
>> Why?  We know it is MDIO.
>>
>> Serial bus multiplexing is not a concept limited to MDIO.  We would want to use "parent-bus" for some I2C multiplexers as well.
>
>> From many years of dealing with device trees.  We typically don't name things overlay generically unless they will be used over and over again as a common idiom (like reg, interrupt, etc.).
>
> We don't really use 'bus' generically today.
>
>>
>>>
>>>> +
>>>> +Optional properties:
>>>> +- Other properties specific to the multiplexer/switch hardware.
>>>> +
>>>> +Required properties for child nodes:
>>>> +- #address-cells =<1>;
>>>> +- #size-cells =<0>;
>>>> +- cell-index : The sub-bus number.
>>>
>>> What does sub-bus number mean?
>>
>> There are N child buses (or sub-buses) coming out of the multiplexer. The cell-index is used as a handle or identifier for each of these.
>>
>> The concrete example in Patch 3/3 is a multiplexer with four child buses.  The happen to have cell-indexes of 0, 1, 2 and 3.
>>
>> In the GPIO case of patch 3/3, these directly correspond the the state of the two GPIO pins controlling the multiplexer.  The driver then uses the cell-index property to determine the state of the GPIO to connect any given child.
>>
>> It is possible that the documentation part of the patch could be made more clear about this.
>>
>>>
>>>> +
>>>> +
>>>> +Example :
>>>
>> [...]
>>>> +
>>>> +int mdio_mux_probe(struct platform_device *pdev,
>>>> +		   int (*switch_fn)(int cur, int desired, void *data),
>>>> +		   void *data)
>>>> +{
>>>> +	struct device_node *parent_bus_node;
>>>> +	struct device_node *child_bus_node;
>>>> +	int r, n, ret_val;
>>>> +	struct mii_bus *parent_bus;
>>>> +	struct mdio_mux_parent_bus *pb;
>>>> +	struct mdio_mux_child_bus *cb;
>>>> +
>>>> +	if (!pdev->dev.of_node)
>>>> +		return -ENODEV;
>>>> +
>>>> +	parent_bus_node = of_parse_phandle(pdev->dev.of_node, "parent-bus", 0);
>>>> +
>>>> +	if (!parent_bus_node)
>>>> +		return -ENODEV;
>>>> +
>>>> +	parent_bus = of_mdio_find_bus(parent_bus_node);
>>>
>>>
>>> So what happens if the parent bus probe happens after the mux probe?
>>>
>>
>> The whole house of cards collapses.
>>
>> Grant Likely has a patch to deal with this by retrying the probing,  but as far as I know, it has not been merged yet.
>
> - k--
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