[PATCH v5 11/15] drm/bridge: cdns-dsi: Fix REG_WAKEUP_TIME value

From: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
To: Jyri Sarha <jyri.sarha@iki.fi>,
	 Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
	 Maxime Ripard <mripard@kernel.org>,
	Thomas Zimmermann <tzimmermann@suse.de>,
	 David Airlie <airlied@gmail.com>,
	Simona Vetter <simona@ffwll.ch>,  Vinod Koul <vkoul@kernel.org>,
	Kishon Vijay Abraham I <kishon@kernel.org>,
	 Andrzej Hajda <andrzej.hajda@intel.com>,
	 Neil Armstrong <neil.armstrong@linaro.org>,
	Robert Foss <rfoss@kernel.org>,
	 Laurent Pinchart <Laurent.pinchart@ideasonboard.com>,
	 Jonas Karlman <jonas@kwiboo.se>,
	Jernej Skrabec <jernej.skrabec@gmail.com>,
	 Jayesh Choudhary <j-choudhary@ti.com>,
	Dmitry Baryshkov <lumag@kernel.org>
Cc: dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org,
	 linux-phy@lists.infradead.org,
	Francesco Dolcini <francesco@dolcini.it>,
	 Aradhya Bhatia <aradhya.bhatia@linux.dev>,
	 Devarsh Thakkar <devarsht@ti.com>,
	 Parth Pancholi <parth.pancholi@toradex.com>,
	 Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Subject: [PATCH v5 11/15] drm/bridge: cdns-dsi: Fix REG_WAKEUP_TIME value
Date: Wed, 23 Jul 2025 13:05:17 +0300	[thread overview]
Message-ID: <20250723-cdns-dsi-impro-v5-11-e61cc06074c2@ideasonboard.com> (raw)
In-Reply-To: <20250723-cdns-dsi-impro-v5-0-e61cc06074c2@ideasonboard.com>

The driver tries to calculate the value for REG_WAKEUP_TIME. However,
the calculation itself is not correct, and to add on it, the resulting
value is almost always larger than the field's size, so the actual
result is more or less random.

According to the docs, figuring out the value for REG_WAKEUP_TIME
requires HW characterization and there's no way to have a generic
algorithm to come up with the value. That doesn't help at all...

However, we know that the value must be smaller than the line time, and,
at least in my understanding, the proper value for it is quite small.
Testing shows that setting it to 1/10 of the line time seems to work
well. All video modes from my HDMI monitor work with this algorithm.

Hopefully we'll get more information on how to calculate the value, and
we can then update this.

Tested-by: Parth Pancholi <parth.pancholi@toradex.com>
Tested-by: Jayesh Choudhary <j-choudhary@ti.com>
Reviewed-by: Devarsh Thakkar <devarsht@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
---
 drivers/gpu/drm/bridge/cadence/cdns-dsi-core.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/bridge/cadence/cdns-dsi-core.c b/drivers/gpu/drm/bridge/cadence/cdns-dsi-core.c
index d49b4789a074..6bc0a0d00d69 100644
--- a/drivers/gpu/drm/bridge/cadence/cdns-dsi-core.c
+++ b/drivers/gpu/drm/bridge/cadence/cdns-dsi-core.c
@@ -793,7 +793,13 @@ static void cdns_dsi_bridge_atomic_pre_enable(struct drm_bridge *bridge,
 
 	tx_byte_period = DIV_ROUND_DOWN_ULL((u64)NSEC_PER_SEC * 8,
 					    phy_cfg->hs_clk_rate);
-	reg_wakeup = (phy_cfg->hs_prepare + phy_cfg->hs_zero) / tx_byte_period;
+
+	/*
+	 * Estimated time [in clock cycles] to perform LP->HS on D-PHY.
+	 * It is not clear how to calculate this, so for now,
+	 * set it to 1/10 of the total number of clocks in a line.
+	 */
+	reg_wakeup = dsi_cfg.htotal / nlanes / 10;
 	writel(REG_WAKEUP_TIME(reg_wakeup) | REG_LINE_DURATION(tmp),
 	       dsi->regs + VID_DPHY_TIME);
 

-- 
2.43.0


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