linux-kernel - [RFC PATCH 1/1] drm/bridge: ti-sn65dsi83: Remove burst mode

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Date:   Tue,  4 Oct 2022 13:57:10 +0200
From:   estl@....net
To:     dri-devel@...ts.freedesktop.org,
        Andrzej Hajda <a.hajda@...sung.com>,
        Neil Armstrong <narmstrong@...libre.com>,
        Robert Foss <robert.foss@...aro.org>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        Jonas Karlman <jonas@...boo.se>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>
Cc:     Marek Vasut <marex@...x.de>,
        Eberhard Stoll <eberhard.stoll@...tron.de>,
        linux-kernel@...r.kernel.org
Subject: [RFC PATCH 1/1] drm/bridge: ti-sn65dsi83: Remove burst mode

From: Eberhard Stoll <eberhard.stoll@...tron.de>

Remove LVDS panel overclocking for some configurations by disabling
burst mode for this chip

With burst mode enabled, some DSI controllers increase their DSI lane
clock beyond the clock which is needed to transfer all pixel data.

But this driver operates with a pixel clock derived from the DSI lane
clock by a fixed prescaler. So, every increase of the DSI clock also
increases the LVDS pixel clock. In this case, the LVDS pixel clock is
overclocked.

This is the case e.g. for synopsys stm DSI bridge
(drm/stm/dw_mipi_dsi-stm.c). With burst mode the DSI clock is
increased by 20% and therefore also for the LVDS panel.

Signed-off-by: Eberhard Stoll <eberhard.stoll@...tron.de>
---
 drivers/gpu/drm/bridge/ti-sn65dsi83.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/bridge/ti-sn65dsi83.c b/drivers/gpu/drm/bridge/ti-sn65dsi83.c
index c901c0e1a3b0..ffc39208536e 100644
--- a/drivers/gpu/drm/bridge/ti-sn65dsi83.c
+++ b/drivers/gpu/drm/bridge/ti-sn65dsi83.c
@@ -272,7 +272,7 @@ static int sn65dsi83_attach(struct drm_bridge *bridge,

 	dsi->lanes = ctx->dsi_lanes;
 	dsi->format = MIPI_DSI_FMT_RGB888;
-	dsi->mode_flags = MIPI_DSI_MODE_VIDEO | MIPI_DSI_MODE_VIDEO_BURST;
+	dsi->mode_flags = MIPI_DSI_MODE_VIDEO;

 	ret = mipi_dsi_attach(dsi);
 	if (ret < 0) {
--
2.25.1