[PATCH RFC v4 3/6] iio: osf: add protocol decoding

[Jinseob Kim <kimjinseob88@gmail.com>]

Use a FourCC-style wire magic comparison.

Document signed little-endian sample handling.

Signed-off-by: Jinseob Kim <kimjinseob88@gmail.com>
---
 drivers/iio/opensensorfusion/osf_protocol.c | 4 +++-
 drivers/iio/opensensorfusion/osf_protocol.h | 4 +++-
 2 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/drivers/iio/opensensorfusion/osf_protocol.c b/drivers/iio/opensensorfusion/osf_protocol.c
index ed91d3dd5..5bee545f3 100644
--- a/drivers/iio/opensensorfusion/osf_protocol.c
+++ b/drivers/iio/opensensorfusion/osf_protocol.c
@@ -11,6 +11,7 @@
 
 #define OSF_CRC32_INIT		GENMASK(31, 0)
 #define OSF_CRC32_XOROUT	GENMASK(31, 0)
+#define OSF_FRAME_MAGIC		0x3046534f /* "OSF0" little-endian */
 
 static bool osf_sensor_type_valid(u16 sensor_type)
 {
@@ -38,7 +39,7 @@ int osf_protocol_decode_frame(const u8 *buf, size_t len,
 	if (len < OSF_FRAME_MIN_LEN)
 		return -EMSGSIZE;
 
-	if (buf[0] != 'O' || buf[1] != 'S' || buf[2] != 'F' || buf[3] != '0')
+	if (get_unaligned_le32(buf) != OSF_FRAME_MAGIC)
 		return -EPROTO;
 
 	major = buf[4];
@@ -136,6 +137,7 @@ int osf_protocol_sensor_sample_value(const struct osf_sensor_sample *sample,
 	if (index >= sample->channel_count)
 		return -ERANGE;
 
+	/* Samples are little-endian two's-complement signed values. */
 	*value = (s32)get_unaligned_le32(sample->samples + index * sizeof(s32));
 
 	return 0;
diff --git a/drivers/iio/opensensorfusion/osf_protocol.h b/drivers/iio/opensensorfusion/osf_protocol.h
index 4b6fb131a..c62c2c254 100644
--- a/drivers/iio/opensensorfusion/osf_protocol.h
+++ b/drivers/iio/opensensorfusion/osf_protocol.h
@@ -2,6 +2,7 @@
 #ifndef _OSF_PROTOCOL_H
 #define _OSF_PROTOCOL_H
 
+#include <linux/bits.h>
 #include <linux/types.h>
 
 #define OSF_PROTOCOL_MAJOR		0
@@ -14,7 +15,7 @@
 #define OSF_DEVICE_STATUS_LEN		20
 #define OSF_CAP_REPORT_BASE_LEN		4
 #define OSF_CAP_SENSOR_ENTRY_LEN		20
-#define OSF_CAPABILITY_FLAGS_MASK	0x00000003U
+#define OSF_CAPABILITY_FLAGS_MASK	GENMASK(1, 0)
 
 enum osf_message_type {
 	OSF_MSG_SENSOR_SAMPLE		= 0x0001,
@@ -44,6 +45,7 @@ struct osf_frame {
 	u64 sequence;
 	u64 timestamp_us;
 	u32 flags;
+	/* payload points into the caller-owned frame buffer. */
 	const u8 *payload;
 	u32 crc;
 };
-- 
2.43.0