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

From: Stephen Warren
Date: Wed Apr 01 2015 - 13:28:47 EST

On 03/31/2015 09:46 AM, Andrey Danin wrote:
On 31.03.2015 17:09, Stephen Warren wrote:
On 03/31/2015 12:40 AM, Andrey Danin wrote:

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
for NVEC node.

diff --git a/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

- 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
0 being 0==master,1==slave, cell 1 being the I2C address. The I2C
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?

Device driver registers itself via slave API. Bus driver calls
appropriate callback function when needed.
If device driver decides to access hardware via master API, then it can
do it.

Am I missing something ?

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>;


- "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

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.

If there is one device, then it must be one node. If there is two
devices then it looks incorrect to me to have two devices with the same
address. Does I2C allow two devices with same address ?

One of the nodes is to indicate that the kernel should implement the slave mode device and one is to indicate that the kernel should implement the master mode device. Those two devices/nodes have completely different semantics, so while they share the I2C bus address they don't represent the same thing.

Admittedly it would be uncommon to do this, since it'd be using the I2C bus in loopback mode. However, I don't see why we should set out to prevent that.

I can imagine this:
- we have hardware with I2C device. This device can act as master or as
- we have device driver, that can work in one, other or both modes.

If we want to force master or slave mode, we can use flags (for combined
mode we can use two nodes, but it looks weird).
If we want to let driver decide (preferred mode, arbitration, something
else), we can use current rules.

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

Sorry. I don't understand why you still suggest to use flags. We can use
existing infrastructure in this case. There is already similar case in
arch/arm/boot/dts/r8a7790-lager.dts (see i2c1 and eeprom).

Do we *really* need this extra rules at this moment ?

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