Re: [PATCH v2] ARM: at91: Document new TCB bindings

[Boris Brezillon]

On Thu, 30 Jun 2016 20:27:43 -0500
Rob Herring <robh@xxxxxxxxxx> wrote:

> On Mon, Jun 27, 2016 at 05:05:01PM +0200, Alexandre Belloni wrote:
> > The current binding for the TCB is not flexible enough for some use cases
> > and prevents proper utilization of all the channels.
> > 
> > Cc: Daniel Lezcano <daniel.lezcano@xxxxxxxxxx>
> > Cc: Thierry Reding <thierry.reding@xxxxxxxxx>
> > Cc: linux-pwm@xxxxxxxxxxxxxxx
> > Cc: Rob Herring <robh+dt@xxxxxxxxxx>
> > Cc: devicetree@xxxxxxxxxxxxxxx
> > Signed-off-by: Alexandre Belloni <alexandre.belloni@xxxxxxxxxxxxxxxxxx>
> > ---
> > 
> > Changes in v2:
> >  - added slow_clk in the examples
> >  - removed linuxisms
> > 
> > Rob, does that fit what you wanted? I prefer getting your ack on that one before
> > resending a 48 patches series.
> > 
> > 
> >  .../devicetree/bindings/arm/atmel-at91.txt         | 32 -----------
> >  .../devicetree/bindings/mfd/atmel-tcb.txt          | 62 ++++++++++++++++++++++
> >  .../devicetree/bindings/pwm/atmel-tcb-pwm.txt      | 12 +++--
> >  3 files changed, 69 insertions(+), 37 deletions(-)
> >  create mode 100644 Documentation/devicetree/bindings/mfd/atmel-tcb.txt
> > 
> > diff --git a/Documentation/devicetree/bindings/arm/atmel-at91.txt b/Documentation/devicetree/bindings/arm/atmel-at91.txt
> > index e1f5ad855f14..3dc758403d03 100644
> > --- a/Documentation/devicetree/bindings/arm/atmel-at91.txt
> > +++ b/Documentation/devicetree/bindings/arm/atmel-at91.txt
> > @@ -60,38 +60,6 @@ System Timer (ST) required properties:
> >  Its subnodes can be:
> >  - watchdog: compatible should be "atmel,at91rm9200-wdt"
> >  
> > -TC/TCLIB Timer required properties:
> > -- compatible: Should be "atmel,<chip>-tcb".
> > -  <chip> can be "at91rm9200" or "at91sam9x5"
> > -- reg: Should contain registers location and length
> > -- interrupts: Should contain all interrupts for the TC block
> > -  Note that you can specify several interrupt cells if the TC
> > -  block has one interrupt per channel.
> > -- clock-names: tuple listing input clock names.
> > -	Required elements: "t0_clk", "slow_clk"
> > -	Optional elements: "t1_clk", "t2_clk"
> > -- clocks: phandles to input clocks.
> > -
> > -Examples:
> > -
> > -One interrupt per TC block:
> > -	tcb0: timer@fff7c000 {
> > -		compatible = "atmel,at91rm9200-tcb";
> > -		reg = <0xfff7c000 0x100>;
> > -		interrupts = <18 4>;
> > -		clocks = <&tcb0_clk>;
> > -		clock-names = "t0_clk";
> > -	};
> > -
> > -One interrupt per TC channel in a TC block:
> > -	tcb1: timer@fffdc000 {
> > -		compatible = "atmel,at91rm9200-tcb";
> > -		reg = <0xfffdc000 0x100>;
> > -		interrupts = <26 4 27 4 28 4>;
> > -		clocks = <&tcb1_clk>;
> > -		clock-names = "t0_clk";
> > -	};
> > -
> >  RSTC Reset Controller required properties:
> >  - compatible: Should be "atmel,<chip>-rstc".
> >    <chip> can be "at91sam9260" or "at91sam9g45" or "sama5d3"
> > diff --git a/Documentation/devicetree/bindings/mfd/atmel-tcb.txt b/Documentation/devicetree/bindings/mfd/atmel-tcb.txt
> > new file mode 100644
> > index 000000000000..71c8d83c01a7
> > --- /dev/null
> > +++ b/Documentation/devicetree/bindings/mfd/atmel-tcb.txt
> > @@ -0,0 +1,62 @@
> > +* Device tree bindings for Atmel Timer Counter Blocks
> > +- compatible: Should be "atmel,<chip>-tcb", "simple-mfd", "syscon".
> > +  <chip> can be "at91rm9200" or "at91sam9x5"
> > +- reg: Should contain registers location and length
> > +- #address-cells: has to be 1
> > +- #size-cells: has to be 0
> > +- interrupts: Should contain all interrupts for the TC block
> > +  Note that you can specify several interrupt cells if the TC
> > +  block has one interrupt per channel.
> > +- clock-names: tuple listing input clock names.
> > +	Required elements: "t0_clk", "slow_clk"
> > +	Optional elements: "t1_clk", "t2_clk"
> > +- clocks: phandles to input clocks.
> > +
> > +The TCB can expose multiple subdevices:
> > + * a clocksource and clockevent device  
> 
> This still uses Linux terminology.
> 
> > +   - compatible: Should be "atmel,tcb-free-running-timer"
> > +   - reg: Should contain the TCB channels to be used. If the
> > +     counter width is 16 bits (at91rm9200-tcb), two consecutive
> > +     channels are needed. Else, only one channel will be used.
> > +
> > + * a clockevent device
> > +   - compatible: Should be "atmel,tcb-programmable-timer"  
> 
> This still looks like assigning usage in DT. As I'm willing to accept 
> that for PWM, either timer channels should be whatever channels are not 
> assigned to PWM (i.e. not in DT) or they should just be "timer" and let 
> the kernel decide their usage.

I just reviewed Alexandre's new binding, and it makes the whole thing
a lot more obscure: on older SoCs, we have to chain 2 channels to
create an acceptable wraparound time (16 bits at 5MHz is generating too
much interrupts to be acceptable).

If we don't assign the mode from the DT, how should we know which
channels should be chained to create the free-running timer? Note that
not all channels can be chained together: they have to be part of the
same timer counter block and have to be consecutive (0+1, 1+2 or 3+0).

Honestly, I don't see the difference between assigning the channel to a
PWM mode and assigning it to a free-running or oneshot timer mode. Why
is it more acceptable?