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

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Message-ID: <87sf1zxb0s.fsf@oltmanns.dev>
Date: Sun, 11 Feb 2024 16:42:43 +0100
From: Frank Oltmanns <frank@...manns.dev>
To: Maxime Ripard <mripard@...nel.org>
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


On 2024-02-08 at 20:05:08 +0100, Maxime Ripard <mripard@...nel.org> wrote:
> [[PGP Signed Part:Undecided]]
> 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.
>

I absolutely agree. It even was the reason for my submission of a
sunxi-ng patch series last year that was accepted, to make pll-mipi more
flexible. :)

The only remaining option I currently see for adjusting the sunxi-ng
driver to further accomodate the panel, is trying to use a higher
divisor than 4 for calculating tcon-data-clock from tcon0. I remember
reading a discussion about this, but as far as I remember that proposal
was rejected (by you, IIRC).

While I appreciate other suggestion as well, I'll look into options for
using a different divisor than 4.

Best regards,
  Frank

>
> Maxime
>
> [[End of PGP Signed Part]]