Re: [PATCH 46/58] clocksource/drivers: Add a new driver for the Atmel ARM TC blocks

[Boris Brezillon <boris.brezillon@free-electrons.com>]

On Thu, 8 Jun 2017 09:44:46 +0200
Daniel Lezcano <daniel.lezcano@linaro.org> wrote:

> +Mark Rutland, +Rob Herring

Mark doesn't seem to be CCed.

> 
> 
> Alexandre, Boris, have a look at https://www.spinics.net/lists/arm-kernel/msg572652.html
> 
> That will tell you the story.

Then we're in a deadlock situation here. I'm tired of hearing this kind
of argument "DT is only supposed to describe HW, not configuration, bla
bla". The truth is, we already have plenty of bindings that do not
strictly describe HW.

A simple example: ECC configuration on NAND devices. This is clearly a
configuration thing, the NAND controller is usually able to support
several kind of strength+ECC-block-size config, but we are able to
overload this with the nand-ecc-xxx properties. Another example, still
MTD related: MTD partitions, this is purely a software configuration,
still we allow users to pass this information in the DT. You want
another one? What about the linux,code and linux,input-type properties
described here [1]?

So please, let's not use these "this is not decribing HW" or "this is
linux specific" arguments every time someone tries to encode something
that can be considered a configuration detail.

Let's be pragmatic. How you want to use your timer counter blocks (I'm
talking about atmel TCBs) is clearly board specific. Whether you want
to use the PIT for your clocksource or use one or 2 channels of a TCB
at a specific resolution is again board specific. We need a solution to
assign timer channels to a linux function, and I'm not convinced
passing this information through the command line makes much more sense
than specifying it in the DT (and it's definitely less intuitive, since
you have to reference something defined in the DT from the cmdline).

Now, in his review, Mark says:

"
To me it sounds like what we need is Linux infrastructure that allows
one to register a device as having both clockevent/clocksource
functionality.

That way, we can choose to do something sane at boot time, and if the
user really wants specific devices used in specific ways, they can
request that.
"

Does that mean that, after adding this "HW timer" infrastructure, we
would have a standard way to assign a function to a specific timer
block from the DT? How is this different from what I suggest below
(note the linux, prefix on my linux,timer-function property, which
clearly shows that this is Linux specific)?

> 
> On Thu, Jun 08, 2017 at 07:42:36AM +0200, Boris Brezillon wrote:
> > Le Thu, 8 Jun 2017 01:17:15 +0200,
> > Alexandre Belloni <alexandre.belloni@free-electrons.com> a écrit :
> >   
> > > On 07/06/2017 at 23:08:48 +0200, Daniel Lezcano wrote:  
> > > > > 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>;
> > 		};
> > 	};  
> 

[1]http://elixir.free-electrons.com/linux/latest/source/Documentation/devicetree/bindings/input/gpio-keys.txt