Re: [PATCH 3/3] dt: paz00: define nvec as child of i2c bus

[Stephen Warren <swarren@wwwdotorg.org>]

On 03/31/2015 12:40 AM, Andrey Danin wrote:
> Hi,
>
> Thanks for the review.
>
> On 03.02.2015 0:20, Stephen Warren wrote:
>> On 01/29/2015 12:20 AM, Andrey Danin wrote:
>>> NVEC driver was reimplemented to use tegra i2c. Use common i2c bindings
>>> for NVEC node.
>>
>>> diff --git a/Documentation/devicetree/bindings/nvec/nvidia,nvec.txt
>>> b/Documentation/devicetree/bindings/nvec/nvidia,nvec.txt
>>
>> The changes to this file make more sense either as a standalone patch
>> 1/4, or as part of the driver changes.
>>
>>> @@ -2,20 +2,5 @@ NVIDIA compliant embedded controller
>>>
>>>   Required properties:
>>>   - compatible : should be "nvidia,nvec".
>>> -- reg : the iomem of the i2c slave controller
>>> -- interrupts : the interrupt line of the i2c slave controller
>>> -- clock-frequency : the frequency of the i2c bus
>>> -- gpios : the gpio used for ec request
>>> -- slave-addr: the i2c address of the slave controller
>>> -- clocks : Must contain an entry for each entry in clock-names.
>>> -  See ../clocks/clock-bindings.txt for details.
>>> -- clock-names : Must include the following entries:
>>> -  Tegra20/Tegra30:
>>> -  - div-clk
>>> -  - fast-clk
>>> -  Tegra114:
>>> -  - div-clk
>>> -- resets : Must contain an entry for each entry in reset-names.
>>> -  See ../reset/reset.txt for details.
>>> -- reset-names : Must include the following entries:
>>> -  - i2c
>>> +- request-gpios : the gpio used for ec request
>>> +- reg: the i2c address of the slave controller
>>
>> This change breaks ABI.
>>
>> Instead of modifying the definition of the existing compatible value, I
>> think you should introduce a new compatible value to describe the
>> external NVEC chip.
>
> I changed compatible value to nvec-slave in v2.
>>
>>> diff --git a/arch/arm/boot/dts/tegra20-paz00.dts
>>> b/arch/arm/boot/dts/tegra20-paz00.dts
>>
>>> -    nvec@7000c500 {
>>> -        compatible = "nvidia,nvec";
>>> -        reg = <0x7000c500 0x100>;
>>> -        interrupts = <GIC_SPI 92 IRQ_TYPE_LEVEL_HIGH>;
>>> -        #address-cells = <1>;
>>> -        #size-cells = <0>;
>>> +    i2c@7000c500 {
>>> +        status = "okay";
>>>           clock-frequency = <80000>;
>>> -        request-gpios = <&gpio TEGRA_GPIO(V, 2) GPIO_ACTIVE_HIGH>;
>>> -        slave-addr = <138>;
>>> -        clocks = <&tegra_car TEGRA20_CLK_I2C3>,
>>> -                 <&tegra_car TEGRA20_CLK_PLL_P_OUT3>;
>>> -        clock-names = "div-clk", "fast-clk";
>>> -        resets = <&tegra_car 67>;
>>> -        reset-names = "i2c";
>>> +
>>> +        nvec: nvec@45 {
>>
>> This doesn't feel correct. There's nothing here to indicate that this
>> child device is a slave that is implemented by the host SoC rather than
>> something external attached to the I2C bus.
>>
>> Perhaps you can get away with this, since the driver for nvidia,nvec
>> only calls I2C APIs suitable for internal slaves rather than external
>> slaves? Even so though, I think the distinction needs to be clearly
>> marked in the DT so that any generic code outside the NVEC driver that
>> parses the DT can determine the difference.
>>
>> I would recommend the I2C controller having #address-cells=<2> with cell
>> 0 being 0==master,1==slave, cell 1 being the I2C address. The I2C driver
>> would need to support #address-cells=<1> for backwards-compatibility.
>
> Driver (nvec in this case) can decide what mode should it use according
> to compatible value. Is it not enough ?

No, I don't think so.

The I2C binding model is that each child of an I2C controller represents 
a device attached to the bus. which SW will communicate with using the 
I2C controller as master and the device as a slave. If there's no 
explicit representation of child-vs-slave in the DT, how does the I2C 
core know whether a particular node is intended to be accessed as a 
master or slave?

In other words, without an explicit "communicate with this device" or 
"implement this device as a slave" flag, how could DT contain:

i2c-controller {
     ...
     master@1a {
         compatible = "foo,device";
         reg = <0x1a 1>;
     };
     slave@1a {
         compatible = "foo,device-slave";
         reg = <0x1a 1>;
     };
};

where:

- "foo,device" means: instantiate a driver to communicate with a device 
of this type.

- "foo,device-slave" means: instantiate a driver to act as this I2C device.

Sure it's possible for the drivers for those two nodes to simply use the 
I2C subsystem's master or slave APIs, but I suspect DT content would 
confuse the I2C core into thinking that two I2C devices with the same 
address had been represented in DT, and the I2C core would refuse to 
instantiate one of them. The solution here is for the reg value to 
encode a "master" vs. "slave" flag, so the I2C core can allow both a 
master and a slave for each address.

I'm pretty sure this is the nth time I've explained this.