Re: [PATCH v8 14/16] ARM: dts: Introduce STM32F429 MCU

[Arnd Bergmann <arnd@arndb.de>]

On Wednesday 13 May 2015 16:20:34 Daniel Thompson wrote:
> For the all reset bits:
> 
>    clock idx = reset idx + 256
> 
> The opposite is not true; the clock bits are a superset of the reset 
> bits (the reset bits act on cells but some cells have >1 clock).

Ok, in that case, I would strongly recommend subtracting that 256
offset keeping the numbers the same, to remove the function-type
macros.

> >> However there are a couple of clocks without gating just before the
> >> clock reaches the peripheral:
> >>
> >> 1. A hard coded /8. I think this will have to be given a synthetic
> >>      number.
> >
> > If this is just a divider, why not use a separate DT node for that,
> > like this:
> >
> >          clock {
> >                  compatible = "fixed-factor-clock";
> >                  clocks = <&parentclk>;
> >                  #clock-cells = <0>;
> >                  clock-div = <8>;
> >                  clock-mult = <1>;
> >          };
> >
> > No need to assign a number for this.
> 
> I'd wondered about doing that.
> 
> It will certainly work but it seemed a bit odd to me to have one (really 
> tiny) part of the RCC cell included seperately in the platform 
> description whilst all the complicated bits end up aggregated into the 
> RCC cell.
> 
> Is there much prior art that uses this type of trick to avoid having 
> magic numbers into the bindings?

Are you sure that divider is actually part of the RCC?

> >> 2. Ungated dividers. For these I am using the bit offset of the LSB of
> >>      the mux field.
> >
> > Do these ones also come with resets?
> 
> No. They mostly run to the core and its intimate peripherals (i.e. only 
> reset line comes from WDT).

Ok.

> >> So I think there is only one value that is completely unrelated to the
> >> hardware and will use a magic constant instead.
> >>
> >> I had planned to macros similar to the STM32F4_AxB_RESET() family of
> >> macros in both clk driver and DT in order to reuse the bit layouts from
> >> dt-bindings/mfd/stm32f4-rcc.h .
> >>
> >> Normal case would have looked like this:
> >>
> >>                  timer3: timer@40000000 {
> >>                          compatible = "st,stm32-timer";
> >>                          reg = <0x40000000 0x400>;
> >>                          interrupts = <28>;
> >>                          resets = <&rcc STM32F4_APB1_RESET(TIM3)>;
> >>                          clocks = <&rcc STM32F4_APB1_CLK(TIM3)>;
> >>                          status = "disabled";
> >>                  };
> >>
> >> Without the macros it looks like this:
> >>
> >>                  timer3: timer@40000000 {
> >>                          compatible = "st,stm32-timer";
> >>                          reg = <0x40000000 0x400>;
> >>                          interrupts = <28>;
> >>                          resets = <&rcc 257>;
> >>                          clocks = <&rcc 513>;
> >>                          status = "disabled";
> >>                  };
> >>
> >> However we could perhaps be more literate even if we don't use the macros?
> >>
> >>                  timer3: timer@40000000 {
> >>                          compatible = "st,stm32-timer";
> >>                          reg = <0x40000000 0x400>;
> >>                          interrupts = <28>;
> >>                          resets = <&rcc ((0x20*8) + 1)>;
> >>                          clocks = <&rcc ((0x40*8) + 1)>;
> >>                          status = "disabled";
> >>                  };
> >
> > How about #address-cells = <2>, so you can do
> >
> > 		resets = <&rcc 8 1>;
> > 		clocks = <&rcc 8 1>;
> >
> > with the first cell being an index for the block and the second cell the
> > bit number within that block.
> 
> That would suit me very well (although is the 0x20/0x40 not the 8 that 
> we would need in the middle column).

We don't normally use register offsets in DT. The number 8 here instead
would indicate block 8, where each block is four bytes wide. Using the
same index here for reset and clock would also help readability.

	Arnd