linux-kernel - Re: [PATCH 10/22] dt-bindings: display: renesas,rzg2l-du: Add support for RZ/G3E SoC

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Message-ID: <CAMuHMdUhke83ZVXxDQE_Dt1HRwyGeoMq1pYmEP47WOgR_vYNtA@mail.gmail.com>
Date: Thu, 15 Jan 2026 09:24:26 +0100
From: Geert Uytterhoeven <geert@...ux-m68k.org>
To: Biju Das <biju.das.jz@...renesas.com>
Cc: Tommaso Merciai <tommaso.merciai.xr@...renesas.com>, 
	Krzysztof Kozlowski <krzk@...nel.org>, Tommaso Merciai <tomm.merciai@...il.com>, 
	"linux-renesas-soc@...r.kernel.org" <linux-renesas-soc@...r.kernel.org>, 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>, David Airlie <airlied@...il.com>, 
	Simona Vetter <simona@...ll.ch>, Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, 
	Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>, 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>, 
	"magnus.damm" <magnus.damm@...il.com>, 
	"dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>, 
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, 
	"linux-clk@...r.kernel.org" <linux-clk@...r.kernel.org>
Subject: Re: [PATCH 10/22] dt-bindings: display: renesas,rzg2l-du: Add support
 for RZ/G3E SoC

Hi Biju,

On Thu, 15 Jan 2026 at 08:48, Biju Das <biju.das.jz@...renesas.com> wrote:
> > From: Geert Uytterhoeven <geert@...ux-m68k.org>
> > On Wed, 3 Dec 2025 at 14:42, Tommaso Merciai <tommaso.merciai.xr@...renesas.com> wrote:
> > > On Wed, Dec 03, 2025 at 09:23:53AM +0100, Krzysztof Kozlowski wrote:
> > > > On Wed, Nov 26, 2025 at 03:07:22PM +0100, Tommaso Merciai wrote:
> > > > > The RZ/G3E Soc has 2 LCD controller (LCDC), contain a Frame
> > > > > Compression Processor (FCPVD), a Video Signal Processor (VSPD),
> > > > > Video Signal Processor (VSPD), and Display Unit (DU).
> > > > >
> > > > >  - LCDC0 supports DSI and LVDS (single or dual-channel) outputs.
> > > > >  - LCDC1 supports DSI, LVDS (single-channel), and RGB outputs.
> > > > >
> > > > > Add then two new SoC-specific compatible strings 'renesas,r9a09g047-du0'
> > > > > and 'renesas,r9a09g047-du1'.
> > > >
> > > > LCDC0/1 but compatibles du0/du1...
> > > >
> > > > What are the differences between DU0 and DU1? Just different
> > > > outputs? Is the programming model the same?
> > >
> > > The hardware configurations are different: these are two distinct hardware blocks.
> > >
> > > Based on the block diagrams shown in Figures 9.4-2 (LCDC1) and 9.4-1
> > > (LCDC0), the only difference concerns the output, but this variation
> > > is internal to the hardware blocks themselves.
> > > Therefore, LCDC0 and LCDC1 are not identical blocks, and their
> > > programming models differ as a result.
> > >
> > > In summary, although most of the internal functions are the same, the
> > > two blocks have output signals connected to different components within the SoC.
> > > This requires different hardware configurations and inevitably leads
> > > to different programming models for LCDC0 and LCDC1.
> >
> > Isn't that merely an SoC integration issue?
> > Are there any differences in programming LCDC0 or LCDC1 for the common output types supported by both
> > (single channel LVDS and 4-lane MIPI-DSI)?
>
> Dual LVDS case, dot clock from LCDC0 is used in both LCDC's.

For the single dual-channel LVDS output on LCDC0, or for using two
independent LVDS outputs on both instances? How is this handled?
Don't you need a companion property to link them together?

Is this similar to dual-channel LVDS on R-Car E3 and RZ/G2E?
On these SoCs we have a single combined device node for all DU instances
(which comes with its own set of issues, e.g. Runtime PM and Clock
Domain handling).

> Standalone LVDS and DSI the programming flow is same.

OK.

> > Of there are no such differences, both instances should use the same compatible value.
>
> Then we need to use a property called display-id, to describe the supported
> output types in bindings, right??
>
> Display-id=0 {LVDS, DSI)

LVDS twice?

> Display-id=1 {LVDS, DSI, DPI)

Not necessarily: if this is purely different SoC integration per
instance, describing all possible options is fine.

But I'd like to defer to Laurent for the details...

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