[PATCH net-next v4 4/6] net: qualcomm: rmnet: use masks instead of C bit-fields

[Alex Elder]

The actual layout of bits defined in C bit-fields (e.g. int foo : 3)
is implementation-defined.  Structures defined in <linux/if_rmnet.h>
address this by specifying all bit-fields twice, to cover two
possible layouts.

I think this pattern is repetitive and noisy, and I find the whole
notion of compiler "bitfield endianness" to be non-intuitive.

Stop using C bit-fields for the command/data flag and the pad length
fields in the rmnet_map structure, and define a single-byte flags
field instead.  Define a mask for the single bit "command" flag,
and another mask for the encoded pad length.  The content of both
fields can be accessed using a simple bitwise AND operation.

Signed-off-by: Alex Elder <elder@xxxxxxxxxx>
Reviewed-by: Bjorn Andersson <bjorn.andersson@xxxxxxxxxx>
---
v4: - Don't use u8_get_bits() to access the pad length
    - Added BUILD_BUG_ON() to ensure field width is adequate
v3: - Use BIT(x) and don't use u8_get_bits() for the command flag

 .../ethernet/qualcomm/rmnet/rmnet_handlers.c  |  4 ++--
 .../ethernet/qualcomm/rmnet/rmnet_map_data.c  |  4 +++-
 include/linux/if_rmnet.h                      | 23 ++++++++-----------
 3 files changed, 15 insertions(+), 16 deletions(-)

diff --git a/drivers/net/ethernet/qualcomm/rmnet/rmnet_handlers.c b/drivers/net/ethernet/qualcomm/rmnet/rmnet_handlers.c
index 2a6b2a609884c..0be5ac7ab2617 100644
--- a/drivers/net/ethernet/qualcomm/rmnet/rmnet_handlers.c
+++ b/drivers/net/ethernet/qualcomm/rmnet/rmnet_handlers.c
@@ -61,7 +61,7 @@ __rmnet_map_ingress_handler(struct sk_buff *skb,
 	u16 len, pad;
 	u8 mux_id;
 
-	if (map_header->cd_bit) {
+	if (map_header->flags & MAP_CMD_FLAG) {
 		/* Packet contains a MAP command (not data) */
 		if (port->data_format & RMNET_FLAGS_INGRESS_MAP_COMMANDS)
 			return rmnet_map_command(skb, port);
@@ -70,7 +70,7 @@ __rmnet_map_ingress_handler(struct sk_buff *skb,
 	}
 
 	mux_id = map_header->mux_id;
-	pad = map_header->pad_len;
+	pad = map_header->flags & MAP_PAD_LEN_MASK;
 	len = ntohs(map_header->pkt_len) - pad;
 
 	if (mux_id >= RMNET_MAX_LOGICAL_EP)
diff --git a/drivers/net/ethernet/qualcomm/rmnet/rmnet_map_data.c b/drivers/net/ethernet/qualcomm/rmnet/rmnet_map_data.c
index fd55269c2ce3c..3c3307949db00 100644
--- a/drivers/net/ethernet/qualcomm/rmnet/rmnet_map_data.c
+++ b/drivers/net/ethernet/qualcomm/rmnet/rmnet_map_data.c
@@ -286,6 +286,7 @@ struct rmnet_map_header *rmnet_map_add_map_header(struct sk_buff *skb,
 		return map_header;
 	}
 
+	BUILD_BUG_ON(MAP_PAD_LEN_MASK < 3);
 	padding = ALIGN(map_datalen, 4) - map_datalen;
 
 	if (padding == 0)
@@ -299,7 +300,8 @@ struct rmnet_map_header *rmnet_map_add_map_header(struct sk_buff *skb,
 
 done:
 	map_header->pkt_len = htons(map_datalen + padding);
-	map_header->pad_len = padding & 0x3F;
+	/* This is a data packet, so the CMD bit is 0 */
+	map_header->flags = padding & MAP_PAD_LEN_MASK;
 
 	return map_header;
 }
diff --git a/include/linux/if_rmnet.h b/include/linux/if_rmnet.h
index 8c7845baf3837..a02f0a3df1d9a 100644
--- a/include/linux/if_rmnet.h
+++ b/include/linux/if_rmnet.h
@@ -6,21 +6,18 @@
 #define _LINUX_IF_RMNET_H_
 
 struct rmnet_map_header {
-#if defined(__LITTLE_ENDIAN_BITFIELD)
-	u8  pad_len:6;
-	u8  reserved_bit:1;
-	u8  cd_bit:1;
-#elif defined (__BIG_ENDIAN_BITFIELD)
-	u8  cd_bit:1;
-	u8  reserved_bit:1;
-	u8  pad_len:6;
-#else
-#error	"Please fix <asm/byteorder.h>"
-#endif
-	u8  mux_id;
-	__be16 pkt_len;
+	u8 flags;			/* MAP_CMD_FLAG, MAP_PAD_LEN_MASK */
+	u8 mux_id;
+	__be16 pkt_len;			/* Length of packet, including pad */
 }  __aligned(1);
 
+/* rmnet_map_header flags field:
+ *  PAD_LEN:	number of pad bytes following packet data
+ *  CMD:	1 = packet contains a MAP command; 0 = packet contains data
+ */
+#define MAP_PAD_LEN_MASK		GENMASK(5, 0)
+#define MAP_CMD_FLAG			BIT(7)
+
 struct rmnet_map_dl_csum_trailer {
 	u8  reserved1;
 #if defined(__LITTLE_ENDIAN_BITFIELD)
-- 
2.27.0