linux-kernel - [PATCH 2/4] media: tc358743: Increase FIFO trigger level to 374

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Message-Id: <20250611-media-tc358743-v1-2-708556026f9d@raspberrypi.com>
Date: Wed, 11 Jun 2025 19:37:14 +0100
From: Dave Stevenson <dave.stevenson@...pberrypi.com>
To: Hans Verkuil <hverkuil@...all.nl>, 
 Mauro Carvalho Chehab <mchehab@...nel.org>
Cc: Maxime Ripard <mripard@...hat.com>, 
 Philipp Zabel <p.zabel@...gutronix.de>, linux-media@...r.kernel.org, 
 linux-kernel@...r.kernel.org, 
 Dave Stevenson <dave.stevenson@...pberrypi.com>
Subject: [PATCH 2/4] media: tc358743: Increase FIFO trigger level to 374

The existing fixed value of 16 worked for UYVY 720P60 over
2 lanes at 594MHz, or UYVY 1080P60 over 4 lanes. (RGB888
1080P60 needs 6 lanes at 594MHz).
It doesn't allow for lower resolutions to work as the FIFO
underflows.

374 is required for 1080P24 or 1080P30 UYVY over 2 lanes @
972Mbit/s, but >374 means that the FIFO underflows on 1080P50
UYVY over 2 lanes @ 972Mbit/s.

Whilst it would be nice to compute it, the required information
isn't published by Toshiba.

Signed-off-by: Dave Stevenson <dave.stevenson@...pberrypi.com>
---
 drivers/media/i2c/tc358743.c | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/drivers/media/i2c/tc358743.c b/drivers/media/i2c/tc358743.c
index b87db3de20db..ddba8c392ead 100644
--- a/drivers/media/i2c/tc358743.c
+++ b/drivers/media/i2c/tc358743.c
@@ -1972,8 +1972,19 @@ static int tc358743_probe_of(struct tc358743_state *state)
 	state->pdata.refclk_hz = clk_get_rate(refclk);
 	state->pdata.ddc5v_delay = DDC5V_DELAY_100_MS;
 	state->pdata.enable_hdcp = false;
-	/* A FIFO level of 16 should be enough for 2-lane 720p60 at 594 MHz. */
-	state->pdata.fifo_level = 16;
+	/*
+	 * Ideally the FIFO trigger level should be set based on the input and
+	 * output data rates, but the calculations required are buried in
+	 * Toshiba's register settings spreadsheet.
+	 * A value of 16 works with a 594Mbps data rate for 720p60 (using 2
+	 * lanes) and 1080p60 (using 4 lanes), but fails when the data rate
+	 * is increased, or a lower pixel clock is used that result in CSI
+	 * reading out faster than the data is arriving.
+	 *
+	 * A value of 374 works with both those modes at 594Mbps, and with most
+	 * modes on 972Mbps.
+	 */
+	state->pdata.fifo_level = 374;
 	/*
 	 * The PLL input clock is obtained by dividing refclk by pll_prd.
 	 * It must be between 6 MHz and 40 MHz, lower frequency is better.

-- 
2.34.1