Re: [PATCH 4/4] drivers: clk: renesas: enable all clocks which is assinged to non Linux system

[Geert Uytterhoeven <geert@linux-m68k.org>]

Hi Rob,

On Thu, Nov 16, 2023 at 8:23 PM Rob Herring <robh@kernel.org> wrote:
> On Tue, Nov 14, 2023 at 12:01:14AM +0000, Kuninori Morimoto wrote:
> > Some board might use Linux and another OS in the same time. In such
> > case, current Linux will stop necessary module clock when booting
> > which is not used on Linux side, but is used on another OS side.
> >
> > To avoid such situation, renesas-cpg-mssr try to find
> > status = "reserved" devices (A), and add CLK_IGNORE_UNUSED flag to its
> > <&cgp CPG_MOD xxx> clock (B).
>
> See Stephen's presentation from Plumbers this week. The default behavior
> for unused clocks may be changing soon.

Thank you!

ou mean "Make sync_state()/handoff work for the common clk
framework"[1]? IIUIC, that presentation didn't cover the problem we are
facing, except for the big "Kconfig for clk_ignore_unused=true" hammer.

> > Table 2.4: Values for status property
> > https://github.com/devicetree-org/devicetree-specification/releases/download/v0.4/devicetree-specification-v0.4.pdf
> >
> > "reserved"
> >       Indicates that the device is operational, but should not be
> >       used. Typically this is used for devices that are controlled
> >       by another software component, such as platform firmware.
> >
> > ex)
> >       scif5: serial@e6f30000 {
> >               ...
> > (B)           clocks = <&cpg CPG_MOD 202>,
> >                        <&cpg CPG_CORE R8A7795_CLK_S3D1>,
> >                        <&scif_clk>;
> >               ...
> > (A)           status = "reserved";
> >       };
>
> I have some reservations about whether a reserved node should be touched
> at all by Linux. I suppose since it is platform specific, it's okay. I
> don't think we could apply such behavior globally.

That's an interesting comment, as the issue is that currently Linux
does touch (resources belonging to) reserved nodes, and this patch
would prevent doing that for module clock resources;-)

The core issue is that Linux distinguishes only between two cases:
  1. "device is used by Linux" (if a driver is available),
     as indicated by 'status = "okay"' in DT, or
  2. "device is unused by Linux".
On a heterogenous system, the latter actually comprises two cases:
  2a. "device is unused", or
  2b. "device is used by another OS running on another CPU core".

Looking for 'status = "reserved"' allows us to distinguish between 2a
and 2b, and can prevent disabling clocks that are used by another OS.
Probably we need a similar solution for power domains.

Do you have a better or alternative suggestion?
Thanks!

[1] https://lpc.events/event/17/contributions/1432/
    https://www.youtube.com/watch?v=NSSSIVQgsIk?t=164m

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds