[PATCH/RFC 06/11] ARM: shmobile: r8a7790: Link CPG to RST using "renesas, modemr"

[laurent.pinchart@ideasonboard.com (Laurent Pinchart)]

Hi Geert,

On Wednesday 08 July 2015 10:29:56 Geert Uytterhoeven wrote:
> On Tue, Jul 7, 2015 at 6:20 PM, Laurent Pinchart wrote:
> >>>> --- a/arch/arm/boot/dts/r8a7790.dtsi
> >>>> +++ b/arch/arm/boot/dts/r8a7790.dtsi
> >>>> @@ -1115,6 +1115,7 @@
> >>>>                                            "lb", "qspi", "sdh", "sd0",
> >>>>                                            "sd1",
> >>>>                                            "z", "rcan", "adsp";
> >>>>                       #power-domain-cells = <0>;
> >>>> +                     renesas,modemr = <&rst 0x60>;
> >>> 
> >>> I have mixed feelings about this as I don't think it really describes
> >>> the hardware.
> >> 
> >> From the R-Car Gen2 manual:
> >> 
> >> 8. Reset (RST)
> >> 8.1 Features
> >> The following functions are implemented by RST.
> >> [...]
> >> Latching of the levels on mode pins when PRESET# is negated
> >> Mode monitoring register
> >> 
> >> 7. Clock Pulse Generator (CPG)
> >> 7.2 Input/Output Pins
> >> Table 7.1 lists the CPG pin configuration.
> >> Table 7.1 Pin Configuration and Functions of CPG
> >> Pin Name Function I/O Description
> >> [...]
> >> MD0 Mode 0 ...
> >> 
> >> Hence there definitely is a link between the (latched) values in the RST
> >> module and CPG configuration. This link is expressed using the
> >> "renesas,modemr" property, where the phandle provides the link to the RST
> >> block, and the register offset provides a way for software to read the
> >> configuration.
> > 
> > The mode bits of course influence the CPG (otherwise we wouldn't be having
> > this discussion :-)), but to me it looks more like a configuration
> > broadcast through the whole SoC than a real link between two IP cores.
> > It's obviously subject to interpretation.
> 
> Yep.
> 
> Broadcast? Like a bus clock? For that we also have properties with
> phandles...

I knew someone would raise that topic ;-)

> Syscon is the Hot New Abstraction for a module that provides a bunch of
> registers needed by drivers for other modules. Those other modules need
> some way to refer to the appropriate syscon register.
> 
> The RST module definitely falls in that category: CPG needs the MODEMR
> register, watchdog (RWDT/SWDT) needs the Watchdog Timer Reset Control
> Register, and APMU probably needs the CA15/7 reset control registers.

syscon is a quick hack to be used when no clean solution exists. Sometimes 
implementing a proper API is just overkill, especially when the "system 
controller" aggregates registers that really belong to individual IP cores.

Proper abstract kernel APIs should of course be used wherever possible. Some 
system controllers are already supported in such a way, using the MFD 
subsystem and exposing proper APIs for part of their features, and letting 
drivers access registers directly for other features.

In the boot mode case I believe adding a new API would be both useful and 
quite simple, so I'd like to explore that option.

By the way, regarding the CA15/7 reset control registers, shouldn't they be 
exposed through the reset controller API ?

-- 
Regards,

Laurent Pinchart