linux-kernel - Re: [PATCH v8 2/6] clk: renesas: rzv2h-cpg: Add support for DSI clocks

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Message-ID: <CA+V-a8tosiUkhaWGoZ9yTBe1Kyy0DLUGreqReH2NOWmVeS5_pw@mail.gmail.com>
Date: Wed, 1 Oct 2025 13:23:17 +0100
From: "Lad, Prabhakar" <prabhakar.csengg@...il.com>
To: Tomi Valkeinen <tomi.valkeinen+renesas@...asonboard.com>, 
	Geert Uytterhoeven <geert+renesas@...der.be>
Cc: dri-devel@...ts.freedesktop.org, devicetree@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-renesas-soc@...r.kernel.org, 
	linux-clk@...r.kernel.org, Fabrizio Castro <fabrizio.castro.jz@...esas.com>, 
	Tommaso Merciai <tommaso.merciai.xr@...renesas.com>, 
	Lad Prabhakar <prabhakar.mahadev-lad.rj@...renesas.com>, 
	Andrzej Hajda <andrzej.hajda@...el.com>, Neil Armstrong <neil.armstrong@...aro.org>, 
	Robert Foss <rfoss@...nel.org>, Laurent Pinchart <Laurent.pinchart@...asonboard.com>, 
	Jonas Karlman <jonas@...boo.se>, Jernej Skrabec <jernej.skrabec@...il.com>, 
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, Maxime Ripard <mripard@...nel.org>, 
	Thomas Zimmermann <tzimmermann@...e.de>, David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>, 
	Rob Herring <robh@...nel.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley <conor+dt@...nel.org>, 
	Michael Turquette <mturquette@...libre.com>, Stephen Boyd <sboyd@...nel.org>, 
	Biju Das <biju.das.jz@...renesas.com>, Magnus Damm <magnus.damm@...il.com>
Subject: Re: [PATCH v8 2/6] clk: renesas: rzv2h-cpg: Add support for DSI clocks

Hi Tomi,

On Thu, Sep 11, 2025 at 3:26 PM Tomi Valkeinen
<tomi.valkeinen+renesas@...asonboard.com> wrote:
>
> Hi,
>
> On 11/09/2025 11:14, Lad, Prabhakar wrote:
> > Hi Tomi,
> >
> > On Wed, Sep 10, 2025 at 1:30 PM Tomi Valkeinen
> > <tomi.valkeinen+renesas@...asonboard.com> wrote:
> >>
> >> Hi,
> >>
> >> On 03/09/2025 19:17, Prabhakar wrote:
> >>> From: Lad Prabhakar <prabhakar.mahadev-lad.rj@...renesas.com>
> >>>
> >>> Add support for PLLDSI and PLLDSI divider clocks.
> >>>
> >>> Introduce the `renesas-rzv2h-cpg-pll.h` header to centralize and share
> >>> PLLDSI related data structures, limits, and algorithms between the
> >>> RZ/V2H(P) CPG and DSI drivers.
> >>>
> >>> The DSI PLL is functionally similar to the CPG's PLLDSI, but has slightly
> >>> different parameter limits and omits the programmable divider present in
> >>> CPG. To ensure precise frequency calculations, especially for milliHz-level
> >>> accuracy needed by the DSI driver, the shared algorithm allows both drivers
> >>> to compute PLL parameters consistently using the same logic and input
> >>> clock.
> >>
> >> Can you elaborate a bit more why a new clock APIs are needed for the DSI
> >> PLL? This is the first time I have heard a DSI TX (well, any IP) require
> >> more precision than Hz. Is that really the case? Are there other reasons?
> >>
> > Im pasting the same reply from Fab
> > (https://lore.kernel.org/all/TYCPR01MB12093A7D99392BC3D6B5E5864C2BC2@TYCPR01MB12093.jpnprd01.prod.outlook.com/#t)
> > for the similar concern.
> >
> > The PLL found inside the DSI IP is very similar to the PLLDSI found in
> > the CPG IP block, although the limits for some of the parameters are
>
> Thanks. As discussed on chat, this confused me: There's a PLLDSI on CPG,
> which doesn't provide a DSI clock, but a pixel clock. And then there's a
> PLL in the DSI D-PHY which provides the DSI clock.
>
> A few comments overall some for this driver but also the dsi driver:
>
> This hardcodes the refclk rate to 24 MHz with RZ_V2H_OSC_CLK_IN_MEGA in
> the header file. That doesn't feel right, shouldn't the refclk rate come
> from the clock framework with clk_get_rate()?
>
>From the CPG perspective we could get the info with clk_get_rate() but
from the DSI part we can't. So to keep it consistent I will keep this
macro as is.

> While not v2h related, I think it would be good to have a comment in the
> dsi driver about how the g2l hs clock rate is derived directly from the
> pixel clock.
>
> I still don't see why all the code here has to be in a header file.
> Usually headers contain only a few lines of inline code. Is there a
> reason why it's not in a .c file?
>
Ok, I will move the functions to rzv2h-cpg.c and export the symbols
and have the declarations in include/linux/clk/renesas.h.

Geert are you OK with the above?

> And last, we discussed the milliHz a bit on chat, you said you'll come
> back to that. I don't think it's a blocker, but I'm very curious why
> exactly it is needed in the DSI. +/- 12 Hz with, say 3.6GHz clock does
> not sound very much to me, and there should be enough time every line
> during blanking period to empty any fifos and "catch up".
>
> In fact, if the DSI is so picky about the rate, I find the HW design
> odd: in g2l the pixel clock and the DSI clock come from a single source,
> which keeps them neatly in sync. If that is required, why change the
> design here so that the DSI PLL is independent of the pixel clock, yet
> still the DSI PLL must be programmed to be exactly matched to the pixel
> clock.
>
As discussed on irc we have to live with mHz solution as the HW is picky.

Cheers,
Prabhakar