linux-kernel - Re: [PATCH] drm/sun4i: uncouple DSI dotclock divider from TCON0_DCLK

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Date:   Tue, 21 Mar 2023 15:56:46 +0100
From:   Maxime Ripard <maxime@...no.tech>
To:     Roman Beranek <romanberanek@...oud.com>
Cc:     Chen-Yu Tsai <wens@...e.org>, David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        Samuel Holland <samuel@...lland.org>,
        dri-devel@...ts.freedesktop.org,
        linux-arm-kernel@...ts.infradead.org, linux-sunxi@...ts.linux.dev,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/sun4i: uncouple DSI dotclock divider from
 TCON0_DCLK_REG

Hi,

On Mon, Mar 20, 2023 at 05:16:36PM +0100, Roman Beranek wrote:
> In the case of DSI output, the value of SUN4I_TCON0_DCLK_DIV (4) does
> not represent the actual dotclock divider, PLL_MIPI instead runs at
> (bpp / lanes )-multiple [1] of the dotclock. [2] Setting 4 as dotclock
> divder thus leads to reduced frame rate, specifically by 1/3 on 4-lane
> panels, and by 2/3 on 2-lane panels respectively.
> 
> As sun4i_dotclock driver stores its calculated divider directly in
> the register, conditional handling of the DSI output scenario is needed.
> Instead of reading the divider from SUN4I_TCON0_DCLK_REG, retrieve
> the value from tcon->dclk_min_div.
> 
> [1] bits per pixel / number of DSI lanes
> [2] https://github.com/BPI-SINOVOIP/BPI-M64-bsp-4.4/blob/66bef0f2f30b367eb93b1cbad21ce85e0361f7ae/linux-sunxi/drivers/video/fbdev/sunxi/disp2/disp/de/lowlevel_sun50iw1/disp_al.c#L322
> 
> Signed-off-by: Roman Beranek <romanberanek@...oud.com>

This is similar to
https://lore.kernel.org/all/20230319160704.9858-2-frank@oltmanns.dev/

What's the story there? Also, how was it tested/confirmed?

Maxime

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