linux-kernel - Re: [PATCH v3 4/6] clk: renesas: rzv2h: Switch MSTOP configuration to per-bit basis

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Message-ID: <CAMuHMdXqF6SbU3ScXLUt7e5LGckouX6fQqg0sJ3+3cYeFtpN4A@mail.gmail.com>
Date: Tue, 7 Jan 2025 13:49:52 +0100
From: Geert Uytterhoeven <geert@...ux-m68k.org>
To: Biju Das <biju.das.jz@...renesas.com>
Cc: "Lad, Prabhakar" <prabhakar.csengg@...il.com>, Michael Turquette <mturquette@...libre.com>, 
	Stephen Boyd <sboyd@...nel.org>, 
	"linux-renesas-soc@...r.kernel.org" <linux-renesas-soc@...r.kernel.org>, 
	"linux-clk@...r.kernel.org" <linux-clk@...r.kernel.org>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, 
	Fabrizio Castro <fabrizio.castro.jz@...esas.com>, 
	Prabhakar Mahadev Lad <prabhakar.mahadev-lad.rj@...renesas.com>
Subject: Re: [PATCH v3 4/6] clk: renesas: rzv2h: Switch MSTOP configuration to
 per-bit basis

Hi Biju,

On Tue, Jan 7, 2025 at 1:38 PM Biju Das <biju.das.jz@...renesas.com> wrote:
> > From: Lad, Prabhakar <prabhakar.csengg@...il.com>
> > On Tue, Jan 7, 2025 at 12:25 PM Biju Das <biju.das.jz@...renesas.com> wrote:
> > > > From: Lad, Prabhakar <prabhakar.csengg@...il.com>
> > > > On Tue, Jan 7, 2025 at 11:24 AM Biju Das <biju.das.jz@...renesas.com> wrote:
> > > > > > From: Lad Prabhakar <prabhakar.mahadev-lad.rj@...renesas.com>
> > > > > >
> > > > > > Switch MSTOP handling from group-based to per-bit configuration
> > > > > > to address issues with shared dependencies between module
> > > > > > clocks. In the current group-based configuration, multiple
> > > > > > module clocks may rely on a single MSTOP bit. When both clocks
> > > > > > are turned ON and one is subsequently turned OFF, the shared
> > > > > > MSTOP bit will still be set, which is incorrect since the
> > > > other dependent module clock remains ON.
> > > > >
> > > > > I guess this statement is incorrect. Still in group-based, mstop
> > > > > bit is controlled by usage
> > > > count(ref_cnt).
> > > > >
> > > > It is valid, consider an example say IP-A reuiqres MSTOP bits 8 | 9
> > > > |
> > > > 10 and consider IP-B requires MSTOP bits 10 | 11 | 12 (of the same
> > > > MSTOP register say MSTOP1). Now this will be seperate groups having
> > > > separate count(ref_cnt). Say you turn ON IP-A module clock and
> > > > correspondingly clear the MSTOP bits and similarly now lets turn ON
> > > > module clocks for IP-B and clear the MSTOP bits. Now let's say you
> > > > want to turn OFF IP-A so you turn OFF module clock and set the MSTOP bits 8 | 9 | 10. In this case
> > you will now see issues with IP-B as MSTOP BIT(10) has been set when we turned OFF IP-A block.  This
> > case is handled by switching refcount on per mstop bit by this patch.
> > >
> > > I agree, Do we have such use case?
> > >
> > Yes, for USB2.0 on RZ/V2H.
>
> OK, then it make sense for per-bit configuration.
>
> > > Consider another use case, index 0, bit 8| index 0, bit9| index0, bit10 and index 0, bit8 | index1,
> > bit 0 | index1 10 is addressed in current patch series?
> > >
> > Can you please elaborate, the above isn't clear to me.
>
> I just provide a random example for a future IP, where
>
> IP_A requires mstop1 {8,9,10}
> And
> IP_B requires mstop1 {8} and mstop2 {9, 10}
>
> Note: I haven't seen this scenario in hardware manual.

That case is indeed not handled, and I had already checked before it is
not needed for the current SoCs (until we discover e.g. a dependency
between different GTM channels ;-)  If it is ever needed for future SoCs,
the system has to be adapted...

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-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