[PATCH RFC 1/2] media: i2c: ov8865: fix horizontal flip control polarity

[Jakob Berg Jespersen]

The ov8865's native readout is horizontally mirrored and the
FORMAT2 FLIP_HORZ bits (reg 0x3821) un-mirror it: with the bits
cleared the image is mirrored, with them set it is not. The driver
maps V4L2_CID_HFLIP=1 to setting the bits, so requesting a flip
produces an unflipped image and vice versa.

This is user-visible on the Surface Pro 7+ rear camera (mounted with a
180 degree rotation, SSDB degree=180): libcamera requests HFLIP=1+VFLIP=1
to undo the mount rotation and gets a horizontally flipped image.

Verified by a live 4-state flip/image matrix on the streaming sensor:

  hflip=0 vflip=0 -> 180 degree rotation (both flips)
  hflip=1 vflip=0 -> vertical flip only
  hflip=0 vflip=1 -> correct image
  hflip=1 vflip=1 -> horizontal flip only

(vertical flip = reflection over a horizontal mirror line, as on water;
horizontal flip = reflection over a vertical mirror line, as in a mirror)

which is only consistent with an inverted HFLIP and a correct VFLIP.

Invert the polarity so HFLIP=0 yields the unflipped image.

Signed-off-by: Jakob Berg Jespersen <dev@xxxxxxx>
---
 drivers/media/i2c/ov8865.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/drivers/media/i2c/ov8865.c b/drivers/media/i2c/ov8865.c
index c6d53c3d55ca..901ea7c16395 100644
--- a/drivers/media/i2c/ov8865.c
+++ b/drivers/media/i2c/ov8865.c
@@ -2204,8 +2204,14 @@ static int ov8865_flip_horz_configure(struct ov8865_sensor *sensor, bool enable)
 	u8 bits = OV8865_FORMAT2_FLIP_HORZ_ISP_EN |
 		  OV8865_FORMAT2_FLIP_HORZ_SENSOR_EN;
 
+	/*
+	 * The sensor's native readout is horizontally mirrored; the
+	 * FLIP_HORZ bits un-mirror it. Map the control so that HFLIP=0
+	 * yields an unmirrored image (verified on Surface Pro 7+ rear
+	 * camera by a live flip-control/image matrix test).
+	 */
 	return ov8865_update_bits(sensor, OV8865_FORMAT2_REG, bits,
-				  enable ? bits : 0);
+				  enable ? 0 : bits);
 }
 
 /* Test Pattern */

-- 
2.55.0