[PATCH] iio: proximity: sx9324: Correct proximity channel resolution

The proximity channels were previously defined with 12 realbits.
However, PROXDIFF is read from RegDiffMsb (0x65) and RegDiffLsb
(0x66). The SX9324 datasheet assigns bits 7:0 of each register to
PROXDIFF and documents it as a signed two's-complement value
(Revision 3, Section 8, Table 8, page 43). In contrast, RegOffsetMsb
explicitly marks bits 7:6 as reserved. Thus, PROXDIFF is a 16-bit
signed value.

With realbits = 12, sx_common_read_proximity() uses bit 11 as the
sign bit in sign_extend32(), causing samples outside the 12-bit
signed range to wrap into the [-2048, 2047] range.

Correct the realbits value to 16 to accurately reflect the hardware.

Tested on an SX9324-based device: a phase 0 DIFF readback of 0x7fff
was reported as -1 before this change and as 32767 afterward.

Fixes: 4c18a890dff8 ("iio:proximity:sx9324: Add SX9324 support")
Cc: [email protected]
Signed-off-by: Zhang Jie <[email protected]>
---
 drivers/iio/proximity/sx9324.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/iio/proximity/sx9324.c b/drivers/iio/proximity/sx9324.c
index 36c45d101336..de6176fd8d47 100644
--- a/drivers/iio/proximity/sx9324.c
+++ b/drivers/iio/proximity/sx9324.c
@@ -223,7 +223,7 @@ static const struct iio_chan_spec_ext_info sx9324_channel_ext_info[] = {
 	.scan_index = idx,					 \
 	.scan_type = {						 \
 		.sign = 's',					 \
-		.realbits = 12,					 \
+		.realbits = 16,					 \
 		.storagebits = 16,				 \
 		.endianness = IIO_BE,				 \
 	},							 \
-- 
2.53.0