linux-kernel - [PATCH 5.10 099/131] drm/mcde: Fix off by 10^3 in calculation

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Date:   Mon, 14 Jun 2021 12:27:40 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org,
        Ville Syrjälä 
        <ville.syrjala@...ux.intel.com>,
        Stephan Gerhold <stephan@...hold.net>,
        Linus Walleij <linus.walleij@...aro.org>
Subject: [PATCH 5.10 099/131] drm/mcde: Fix off by 10^3 in calculation

From: Linus Walleij <linus.walleij@...aro.org>

commit c8a570443943304cac2e4186dbce6989b6c2b8b5 upstream.

The calclulation of how many bytes we stuff into the
DSI pipeline for video mode panels is off by three
orders of magnitude because we did not account for the
fact that the DRM mode clock is in kilohertz rather
than hertz.

This used to be:
drm_mode_vrefresh(mode) * mode->htotal * mode->vtotal
which would become for example for s6e63m0:
60 x 514 x 831 = 25628040 Hz, but mode->clock is
25628 as it is in kHz.

This affects only the Samsung GT-I8190 "Golden" phone
right now since it is the only MCDE device with a video
mode display.

Curiously some specimen work with this code and wild
settings in the EOL and empty packets at the end of the
display, but I have noticed an eeire flicker until now.
Others were not so lucky and got black screens.

Cc: Ville Syrjälä <ville.syrjala@...ux.intel.com>
Reported-by: Stephan Gerhold <stephan@...hold.net>
Fixes: 920dd1b1425b ("drm/mcde: Use mode->clock instead of reverse calculating it from the vrefresh")
Signed-off-by: Linus Walleij <linus.walleij@...aro.org>
Tested-by: Stephan Gerhold <stephan@...hold.net>
Reviewed-by: Stephan Gerhold <stephan@...hold.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20210608213318.3897858-1-linus.walleij@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 drivers/gpu/drm/mcde/mcde_dsi.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/drivers/gpu/drm/mcde/mcde_dsi.c
+++ b/drivers/gpu/drm/mcde/mcde_dsi.c
@@ -577,7 +577,7 @@ static void mcde_dsi_setup_video_mode(st
 	 * porches and sync.
 	 */
 	/* (ps/s) / (pixels/s) = ps/pixels */
-	pclk = DIV_ROUND_UP_ULL(1000000000000, mode->clock);
+	pclk = DIV_ROUND_UP_ULL(1000000000000, (mode->clock * 1000));
 	dev_dbg(d->dev, "picoseconds between two pixels: %llu\n",
 		pclk);