Re: [PATCH 1/2] Documentation: Add sbs-manager device tree node documentation

[Phil Reid <preid@electromag.com.au>]

G'day Rob

Couple of thoughts / question below.

On 25/06/2016 01:50, Rob Herring wrote:
> On Wed, Jun 22, 2016 at 09:07:15PM +0200, Karl-Heinz Schneider wrote:
>> This patch adds device tree documentation for the sbs-manager
>>
>> Reviewed-by: Phil Reid <preid@electromag.com.au>
>> Signed-off-by: Karl-Heinz Schneider <karl-heinz@schneider-inet.de>
>> ---
>>  .../devicetree/bindings/power/sbs,sbs-manager.txt  | 58 ++++++++++++++++++++++
>>  1 file changed, 58 insertions(+)
>>  create mode 100644 Documentation/devicetree/bindings/power/sbs,sbs-manager.txt
>>
>> diff --git a/Documentation/devicetree/bindings/power/sbs,sbs-manager.txt b/Documentation/devicetree/bindings/power/sbs,sbs-manager.txt
>> new file mode 100644
>> index 0000000..d52b466
>> --- /dev/null
>> +++ b/Documentation/devicetree/bindings/power/sbs,sbs-manager.txt
>> @@ -0,0 +1,58 @@
>> +Binding for sbs-manager
>> +
>> +Required properties:
>> +- compatible: should be "sbs,sbs-manager" or "lltc,ltc1760" if device is a
>> +    ltc1760.
>
> sbs is not a vendor. What chip is sbs-manager? I suspect you should drop
> it and only list specific chips.
This follows the interface to the existing paired sbs,sbs-battery driver defined in power/sbs-battery.c
It implements a generic driver for the Smart Battery System Manager Specification.
Spec available here: http://sbs-forum.org/specs/sbsm100b.pdf

In addition the ltc1760 extends the spec.


>
>> +- reg: integer, i2c address of the device. Should be <0xa>.
>> +
>> +Optional properties:
>> +- sbsm,i2c-retry-count: integer, number of retries for trying to read or write
>> +    to registers. Default: 1
>
> Seems like a driver setting. Is having a retry in the driver a problem
> if the h/w works and never actually needs it?
Similarly the sbs-battery driver specifies the same same retry behaviour. And is a model for this implementation.

I've found the ltc1760 and sbs batteries to be problematic when communicating to them.
A lot of drivers (and the associated hardware) don't handle multiple bus masters well.
The bus arbitation doesn't seem to work correctly.
Retries where the only thing I could do to to get things to work reliably.
Mostly means the driver needs fixing, but in one case the designware core hardware seemed to be the problem for me.


>
>> +
>> +From OS view the device is basically an i2c-mux used to communicate with up to
>> +four smart battery devices at address 0xb. The driver actually implements this
>> +behaviour. So standard i2c-mux nodes can be used to register up to four slave
>> +batteries. Channels will be numerated as 1, 2, 4 and 8.
>> +
>> +Example:
>> +
>> +batman@0a {
>> +    compatible = "sbs,sbs-manager";
>> +    reg = <0x0a>;
>> +    sbsm,i2c-retry-count = <3>;
>> +    #address-cells = <1>;
>> +    #size-cells = <0>;
>> +
>> +    channel1@1 {
>
> channel@1
>
> Do we have a standard node name for mux nodes? If not, we should.
>
>> +        #address-cells = <1>;
>> +        #size-cells = <0>;
>> +        reg = <1>;
>> +
>> +        battery1@0b {
>
> battery@b
>
>> +            compatible = "sbs-battery";
>
> This should be an actual battery model. Or all this information is
> generic, you don't really need it in DT.
Do we really want to restrict to battery model?
I have hardware where complete different sbs compliant batteries can be plugged in.
Without the compatible flag here how do you get the sbs-battery driver to load and manage the battery itself?
Or are you suggesting the manager should do this somehow?
I'm still learning...


>
>> +            reg = <0x0b>;
>> +        };
>> +    };
>> +
>> +    channel2@2 {
>> +        #address-cells = <1>;
>> +        #size-cells = <0>;
>> +        reg = <2>;
>> +
>> +        battery2@0b {
>> +            compatible = "sbs-battery";
>> +            reg = <0x0b>;
>> +        };
>> +    };
>> +
>> +    channel3@4 {
>> +        #address-cells = <1>;
>> +        #size-cells = <0>;
>> +        reg = <4>;
>> +
>> +        battery3@0b {
>> +            compatible = "sbs-battery";
>> +            reg = <0x0b>;
>> +        };
>> +    };
>> +};
>> --
>> 1.9.1
>>


-- 
Regards
Phil Reid