[PATCH] media: i2c: ov13858: cap analogue gain at 15.5x

[Sergey Lebedev]

V4L2_CID_ANALOGUE_GAIN is advertised as 0..0x1fff, but the control is not
monotonic over that range. Measured on raw Bayer from the CSI-2 receiver
at fixed exposure, on a Surface Pro 11, taking signal as the frame mean
minus the black level:

  register    requested    measured
      1024          8x        7.55
      1536         12x       11.09
      1984       15.5x       14.23
      2047      15.99x       13.71
      2048         16x        7.54
      4096         32x        7.58
      8191         64x       14.28

Up to 0x7c0 the control tracks. At 0x800 the gain halves, and above that it
wanders: 0x1000 behaves as 0x400 did, 0x1800 as 0x600, 0x1fff as 0x7ff. An
autoexposure loop that raises its request and gets back less signal cannot
converge, and 0x800 in particular costs a full stop against 0x7ff.

0x07c0 is what ov13b10 and ov08x40 use, and 1984/128 is 15.5x, the usual
analogue ceiling for these parts. I have no datasheet for the ov13858; the
value is the one its siblings use and the one the measurements agree with.

This does not make pictures brighter, since 0x7c0 and 0x1fff reach the same
gain. It stops the driver claiming range that is not there.

Signed-off-by: Sergey Lebedev <lsa.uz@xxxxx>
---
Tested on a Surface Pro 11 (Intel Lunar Lake, IPU7) with libcamera's
software ISP. Image quality is unchanged: over three runs, mean luma
85.6/85.6/86.2 against 87.9 before, matching 14.23x against 14.28x.

 drivers/media/i2c/ov13858.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/media/i2c/ov13858.c b/drivers/media/i2c/ov13858.c
--- a/drivers/media/i2c/ov13858.c
+++ b/drivers/media/i2c/ov13858.c
@@ -63,7 +63,7 @@
 /* Analog gain control */
 #define OV13858_REG_ANALOG_GAIN		0x3508
 #define OV13858_ANA_GAIN_MIN		0
-#define OV13858_ANA_GAIN_MAX		0x1fff
+#define OV13858_ANA_GAIN_MAX		0x07c0
 #define OV13858_ANA_GAIN_STEP		1
 #define OV13858_ANA_GAIN_DEFAULT	0x80