+Mark Rutland, +Rob Herring


Alexandre, Boris, have a look at https://www.spinics.net/lists/arm-kernel/msg572652.html

That will tell you the story.

On Thu, Jun 08, 2017 at 07:42:36AM +0200, Boris Brezillon wrote:

Le Thu, 8 Jun 2017 01:17:15 +0200,
Alexandre Belloni [off-list ref] a écrit :

quoted

On 07/06/2017 at 23:08:48 +0200, Daniel Lezcano wrote:

quoted

quoted

I was going to agree but this is not flexible enough because the
quadrature decoder always uses the first two channels. So on some
products, we may have:
 - TCB0:
   o channels 0,1: qdec
   o channel 2: clocksource

 - TCB1:
   o channels 0,1: qdec
   o channel 2: clockevent

This avoids wasting TCB channels.  

Ok. In this case you can check if the interrupt is specified for the node, if
yes, then it is a clockevent.
  

But currently it is always specified in the SoC's dtsi. I don't find
that too practical to push that to the board's dts. Also, lying by
omission (the IRQ is always wired) in the DT is not different from
having a property selecting which timer is the clocksource and which is
the clockevent.

I agree with Alexandre here. Really, there's not much we can do to
detect which timer should be used as a clockevent and which one should
be used as a clocksource except explicitly specifying it in the DT.
Having an interrupt defined in one case (clockevent) and undefined in
the other case (clocksource), is just as hack-ish as the detection logic
Alexandre developed to avoid explicitly specifying the function
assigned to a specific timer.

Can we please find a solution that makes everyone happy (DT,
clocksoure/clockevent and at91 maintainers)?

How about adding a linux,timer-function property to specify which
function this timer is providing?

Something like that for example:

	tcb0: timer@fff7c000 {
		compatible = "atmel,at91rm9200-tcb", "simple-mfd", "syscon";
		#address-cells = <1>;
		#size-cells = <0>;
		reg = <0xfff7c000 0x100>;
		interrupts = <18 4>;
		clocks = <&tcb0_clk>, <&clk32k>;
		clock-names = "t0_clk", "slow_clk";

		timer@0 {
			compatible = "atmel,tcb-timer";
			reg = <0>, <1>;
			linux,timer-function = "clocksource";
		};

		timer@2 {
			compatible = "atmel,tcb-timer";
			reg = <2>;
			linux,timer-function = "clockevent";
		};
	};

Alternatively, we could have a property or a node in chosen describing which
timer should be used:

	chosen {
		clockevent {
			timer = <&timer2>;
		};

		clocksource {
			timer = <&timer0>;
		};

		/*
		 * or
		 *
		 * clockevent = <&timer2>;
		 * clocksource = <&timer0>;
		 *
		 * but I think the clocksource/clockevent node approach
		 * is more future proof in case we need to add extra
		 * information like the expected resolution/precision or
		 * anything that could be tweakable.
		 */
	};

	tcb0: timer@fff7c000 {
		compatible = "atmel,at91rm9200-tcb", "simple-mfd", "syscon";
		#address-cells = <1>;
		#size-cells = <0>;
		reg = <0xfff7c000 0x100>;
		interrupts = <18 4>;
		clocks = <&tcb0_clk>, <&clk32k>;
		clock-names = "t0_clk", "slow_clk";

		timer0: timer@0 {
			compatible = "atmel,tcb-timer";
			reg = <0>, <1>;
		};

		timer2: timer@2 {
			compatible = "atmel,tcb-timer";
			reg = <2>;
		};
	};

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