[PATCH] media: i2c: imx471: Fix pixel rate and line length

[Christian Murphy <christian@themurphys.eu>]

The pixel rate is derived from the CSI-2 link frequency, 200 MHz * 2 *
4 lanes / 10 bits = 160 MHz, and the line length is the binned array
width, 2328. Neither describes the pixel array.

The VT PLL gives 19.2 MHz / PREPLLCK_VT_DIV 2 * PLL_VT_MPY 121 /
VTPXCK_DIV 6 = 193.6 MHz and the array reads two pixels per clock, so
the pixel rate is 387.2 MHz. LINE_LENGTH_PCK is never written and its
power-on default reads back 5120. Timing frames at several written
line lengths confirms 387.2 MHz at every value.

With the declared values the line duration comes out 14.55 us instead
of 13.22 us and exposure is overstated by 10%: libcamera reports a
maximum ExposureTime longer than the frame.

Set the pixel rate to 387.2 MHz and the line length to 5120, which
changes HBLANK from 400 to 3192. No register write is added.

Fixes: be1589e567ae ("media: i2c: imx471: Add Sony IMX471 image sensor driver")
Link: https://lore.kernel.org/linux-media/20260728042013.23707-1-hpa@redhat.com/
Assisted-by: Claude-Code:claude-fable-5
Signed-off-by: Christian Murphy <christian@themurphys.eu>
---

Notes:
    LINE_LENGTH_PCK reads back 0x1400 (5120) from the streaming sensor.
    Written to 5632, 6144, 5008 and 5120 in one stream, the line period is
    llp / 387.2 MHz at every value (14.5457, 15.8680, 12.9341, 13.2234 us).
    
    Tested on a ThinkPad X1 Carbon Gen 14 (Debian linux 7.1.8-2, libcamera
    0.7.2): controls read back 387200000 / 3192, 17.296 ms frames, maximum
    ExposureTime 17.057 ms. v4l2-compliance 46/46; W=1 and sparse clean.
    The driver is in no release (7.3 merge window), so no Cc: stable.

 drivers/media/i2c/imx471.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/media/i2c/imx471.c b/drivers/media/i2c/imx471.c
index 4053aed84340..7a1335873270 100644
--- a/drivers/media/i2c/imx471.c
+++ b/drivers/media/i2c/imx471.c
@@ -282,7 +282,7 @@ static const struct imx471_mode imx471_modes[] = {
 		.height = 1088,
 		.fll_def = 1308,
 		.fll_min = 1308,
-		.llp = 2328,
+		.llp = 5120,
 		.default_mode_regs = mode_1928x1088_regs,
 		.default_mode_regs_length = ARRAY_SIZE(mode_1928x1088_regs),
 	},
@@ -691,8 +691,8 @@ static int imx471_init_controls(struct imx471 *sensor)
 					   0,
 					   link_freq_menu_items);
 
-	/* pixel_rate = link_freq * 2 * nr_of_lanes / bits_per_sample */
-	pixel_rate = div_u64(IMX471_LINK_FREQ_DEFAULT * 2 * 4, 10);
+	/* pixel_rate = 2 * vt_pix_clk, with vt_pix_clk = 19.2 MHz / 2 * 121 / 6 */
+	pixel_rate = 387200000;
 
 	v4l2_ctrl_new_std(ctrl_hdlr, &imx471_ctrl_ops,
 			  V4L2_CID_PIXEL_RATE, pixel_rate,

base-commit: 4900cad020c0580dfb1be27776ff10a4ef110cfa
-- 
2.53.0