linux-kernel - Re: [PATCH v2 5/6] drm/panel: st7703: Drive XBD599 panel at higher clock rate

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Message-ID: <poua4bzyciiwt65sqjf2whqfdumvoe4h3bkjpf64px2vwgumrf@sai73byg2iju>
Date: Thu, 8 Feb 2024 20:05:08 +0100
From: Maxime Ripard <mripard@...nel.org>
To: Frank Oltmanns <frank@...manns.dev>
Cc: Michael Turquette <mturquette@...libre.com>, 
	Stephen Boyd <sboyd@...nel.org>, Chen-Yu Tsai <wens@...e.org>, 
	Jernej Skrabec <jernej.skrabec@...il.com>, Samuel Holland <samuel@...lland.org>, 
	Guido Günther <agx@...xcpu.org>, Purism Kernel Team <kernel@...i.sm>, Ondrej Jirman <megi@....cz>, 
	Neil Armstrong <neil.armstrong@...aro.org>, Jessica Zhang <quic_jesszhan@...cinc.com>, 
	Sam Ravnborg <sam@...nborg.org>, Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, 
	Thomas Zimmermann <tzimmermann@...e.de>, David Airlie <airlied@...il.com>, 
	Daniel Vetter <daniel@...ll.ch>, Rob Herring <robh+dt@...nel.org>, 
	Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>, Conor Dooley <conor+dt@...nel.org>, linux-clk@...r.kernel.org, 
	linux-arm-kernel@...ts.infradead.org, linux-sunxi@...ts.linux.dev, linux-kernel@...r.kernel.org, 
	dri-devel@...ts.freedesktop.org, devicetree@...r.kernel.org
Subject: Re: [PATCH v2 5/6] drm/panel: st7703: Drive XBD599 panel at higher
 clock rate

Hi Frank,

On Mon, Feb 05, 2024 at 04:22:28PM +0100, Frank Oltmanns wrote:
> This panel is used in the pinephone that runs on a Allwinner A64 SOC.
> The SOC requires pll-mipi to run at more than 500 MHz.
> 
> This is the relevant clock tree:
>  pll-mipi
>     tcon0
>        tcon-data-clock
> 
> tcon-data-clock has to run at 1/4 the DSI per-lane bit rate. The XBD599
> has 24 bpp and 4 lanes. Therefore, the resulting requested
> tcon-data-clock rate is:
>     crtc_clock * 1000 * (24 / 4) / 4
> 
> tcon-data-clock runs at tcon0 / 4 (fixed divisor), so it requests a
> parent rate of
>     4 * (crtc_clock * 1000 * (24 / 4) / 4)
> 
> Since tcon0 is a ccu_mux, the rate of tcon0 equals the rate of pll-mipi.
> 
> pll-mipi's constraint to run at 500MHz or higher forces us to have a
> crtc_clock >= 83333 kHz if we want a 60 Hz vertical refresh rate.
> 
> Change [hv]sync_(start|end) so that we reach a clock rate of 83502 kHz
> so that it is high enough to align with pll-pipi limits.
> 
> Signed-off-by: Frank Oltmanns <frank@...manns.dev>

That commit log is great, but it's kind of off-topic. It's a panel
driver, it can be used on any MIPI-DSI controller, the only relevant
information there should be the panel timings required in the datasheet.

The PLL setup is something for the MIPI-DSI driver to adjust, not for
the panel to care for.

Maxime

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