linux-kernel - [PATCH v3 2/2] gpu/drm: tegra: dsi: re-add clear enable register if DSI was powered by bootloader

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Message-ID: <20260125131323.45108-3-clamor95@gmail.com>
Date: Sun, 25 Jan 2026 15:13:23 +0200
From: Svyatoslav Ryhel <clamor95@...il.com>
To: Thierry Reding <thierry.reding@...il.com>,
	Mikko Perttunen <mperttunen@...dia.com>,
	David Airlie <airlied@...il.com>,
	Simona Vetter <simona@...ll.ch>,
	Jonathan Hunter <jonathanh@...dia.com>,
	Diogo Ivo <diogo.ivo@...nico.ulisboa.pt>,
	Svyatoslav Ryhel <clamor95@...il.com>
Cc: dri-devel@...ts.freedesktop.org,
	linux-tegra@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH v3 2/2] gpu/drm: tegra: dsi: re-add clear enable register if DSI was powered by bootloader

Original commit b22fd0b9639e ("drm/tegra: dsi: Clear enable register if
powered by bootloader") was added to address the issue of DSI being in an
unknown state after the bootloader, ensuring correct panel configuration.
This worked fairly well under the assumption that the bootloader had set
up DSI; however, in cases where it did not, the device would hang because
a DSI read was called before the DSI hardware was ready.

Removing this workaround results in the issue described in the original
fix: the panel initialization sequence fails and the panel gets stuck in
an undefined state. This is especially noticeable with command mode panels

In order to properly address this issue, the original workaround is
restored and placed after the DSI hardware is prepared for R/W operations.
This fixes behavior for both cases: where DSI is set by the bootloader and
where DSI is untouched.

I have tested this change on Tegra20 (Motorola Atrix 4G),
Tegra114 (NVIDIA Tegra Note 7 and ASUS Transformer Pad TF701T), and
Tegra124 (Xiaomi Mi Pad) with U-Boot, using both bootloader-initialized
DSI and untouched DSI.

Fixes: b22fd0b9639e ("drm/tegra: dsi: Clear enable register if powered by bootloader")
Fixes: 660b299bed2a ("Revert "drm/tegra: dsi: Clear enable register if powered by bootloader"")

Signed-off-by: Svyatoslav Ryhel <clamor95@...il.com>
---
 drivers/gpu/drm/tegra/dsi.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/drivers/gpu/drm/tegra/dsi.c b/drivers/gpu/drm/tegra/dsi.c
index ebc78dceaee6..e499822b6030 100644
--- a/drivers/gpu/drm/tegra/dsi.c
+++ b/drivers/gpu/drm/tegra/dsi.c
@@ -934,6 +934,15 @@ static void tegra_dsi_encoder_enable(struct drm_encoder *encoder)
 		return;
 	}

+	/* If the bootloader enabled DSI it needs to be disabled
+	 * in order for the panel initialization commands to be
+	 * properly sent.
+	 */
+	value = tegra_dsi_readl(dsi, DSI_POWER_CONTROL);
+
+	if (value & DSI_POWER_CONTROL_ENABLE)
+		tegra_dsi_disable(dsi);
+
 	state = tegra_dsi_get_state(dsi);

 	tegra_dsi_set_timeout(dsi, state->bclk, state->vrefresh);
-- 
2.51.0