Re: [PATCH v1 2/4] ARM: dts: Add i2c-arbitrator bus for exynos5250-snow

From: Doug Anderson
Date: Wed Feb 13 2013 - 19:38:44 EST


On Wed, Feb 13, 2013 at 1:04 PM, Stephen Warren <swarren@xxxxxxxxxxxxx> wrote:
> On 02/13/2013 11:02 AM, Doug Anderson wrote:
>> We need to use the i2c-arbitrator to talk to any of the devices on i2c
>> bus 4 on exynos5250-snow so that we don't confuse the embedded
>> controller (EC). Add the i2c-arbitrator to the device tree. As we
>> add future devices (keyboard, sbs, tps65090) we'll add them on top of
>> this.
>> The arbitrated bus is numbered 104 simply as a convenience to make it
>> easier for people poking around to guess that it might have something
>> to do with the physical bus 4.
>> The addition is split between the cros5250-common and the snow device
>> tree file since not all cros5250-class devices use arbitration.
>> diff --git a/arch/arm/boot/dts/cros5250-common.dtsi b/arch/arm/boot/dts/cros5250-common.dtsi
>> i2c@12CA0000 {
>> - status = "disabled";
>> + samsung,i2c-sda-delay = <100>;
>> + samsung,i2c-max-bus-freq = <66000>;
> Shouldn't that use the standard clock-frequency property?

At the moment this is the way that you specify it for the i2c
controller we're using...

>> diff --git a/arch/arm/boot/dts/exynos5250-snow.dts b/arch/arm/boot/dts/exynos5250-snow.dts
>> + i2c-arbitrator {
>> + compatible = "i2c-arbitrator";
>> + #address-cells = <1>;
>> + #size-cells = <0>;
>> + /* Use ID 104 as a hint that we're on physical bus 4 */
>> + i2c_104: i2c@0 {
> Does something use that hint? It sounds a little odd.

The i2c bus numbering patches will end up creating "/dev/i2c-104". the hint is really for developers who are poking around. It's
definitely a strange number and that's part of the point. Right now
there's nothing preventing someone from using 'i2cdetect' or 'i2cget'
on bus 4 and bypassing arbitration. My hope was that having something
crazy like "104" would help someone realize that it's a bad idea.

...besides, if I don't come up with _some_ number then this will get
assigned the first unused ID which could be extra confusing.

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