linux-kernel - [PATCH 0/1] Fixing the DSI dot clock on Allwinner

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Message-Id: <20230319160704.9858-1-frank@oltmanns.dev>
Date:   Sun, 19 Mar 2023 17:07:03 +0100
From:   Frank Oltmanns <frank@...manns.dev>
To:     jagan@...rulasolutions.com, michael@...rulasolutions.com,
        Maxime Ripard <mripard@...nel.org>,
        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 (open list:DRM DRIVERS FOR ALLWINNER
        A10),
        linux-arm-kernel@...ts.infradead.org (moderated list:ARM/Allwinner
        sunXi SoC support),
        linux-sunxi@...ts.linux.dev (open list:ARM/Allwinner sunXi SoC support),
        linux-kernel@...r.kernel.org (open list)
Cc:     Frank Oltmanns <frank@...manns.dev>
Subject: [PATCH 0/1] Fixing the DSI dot clock on Allwinner

According to the Allwinner A64's BSP code, the PLL rate needs to be set to the following frequency when using DSI:
PLL rate = DCLK * bpp / lanes

Source: [1]
The relevant lines for DSI (ommisions and comments mine):
dclk_rate = lcdp->panel_info.lcd_dclk_freq * 1000000;
lcd_rate = dclk_rate * clk_info.dsi_div; // dsi_div = bpp/lane
pll_rate = lcd_rate * clk_info.lcd_div;  // lcd_div = 1 --> pll_rate = lcd_rate
dsi_rate = pll_rate / clk_info.dsi_div   // --> dsi_rate = dclk_rate
clk_set_rate(lcdp->clk_parent, pll_rate);

This was already discussed by Maxime, Jagan and Michael in the past in the thread following this message: [2]. Unfortunately, there never was a conclusion in the form of code.

The attached patch is a slight variation of a patch that is part of megi's kernel branch that many PinePhone distributions (e.g. postmarketOS) use [3]. It calculates the TCON clock rate by using the formula above and dividing it by SUN6I_DSI_TCON_DIV, in order to force the parent clock to be set to the correct rate.

If I read the thread following this message [2] correctly, this was also what Maxime had in mind.

Please also note that, unfortunately, I only have a single board and panel (namely the PinePhone) to test this on.

Thanks,
  Frank

[1] https://github.com/BPI-SINOVOIP/BPI-M64-bsp/blob/master/linux-sunxi/drivers/video/sunxi/disp2/disp/de/disp_lcd.c#L781
[2] https://lore.kernel.org/lkml/CAMty3ZAsH2iZ+JEqTE3D58aXfGuhMSg9YoO56ZhhOeE4c4yQHQ@mail.gmail.com/
[3] https://github.com/megous/linux/commit/eb5f28fb58727f4a6546f211486aad0d19cdea3f

Frank Oltmanns (1):
  drm/sun4i: tcon: Fix setting PLL rate when using DSI

 drivers/gpu/drm/sun4i/sun4i_tcon.c | 46 ++++++++++++++++++++----------
 1 file changed, 31 insertions(+), 15 deletions(-)

-- 
2.39.2