linux-kernel - [PATCH net-next v3 4/6] net: qualcomm: rmnet: use field masks instead of C bit-fields

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Message-Id: <20210309124848.238327-5-elder@linaro.org>
Date:   Tue,  9 Mar 2021 06:48:46 -0600
From:   Alex Elder <elder@...aro.org>
To:     subashab@...eaurora.org, stranche@...eaurora.org,
        davem@...emloft.net, kuba@...nel.org
Cc:     sharathv@...eaurora.org, bjorn.andersson@...aro.org,
        evgreen@...omium.org, cpratapa@...eaurora.org,
        David.Laight@...LAB.COM, elder@...nel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH net-next v3 4/6] net: qualcomm: rmnet: use field masks instead of C bit-fields

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
encoded in the flags field.  Define a field mask for the encoded pad
length, and access it using functions defined in <linux/bitfield.h>.

Signed-off-by: Alex Elder <elder@...aro.org>
---
v3: Use BIT(x) and don't use u8_get_bits() for the command flag

 .../ethernet/qualcomm/rmnet/rmnet_handlers.c  |  5 ++--
 .../ethernet/qualcomm/rmnet/rmnet_map_data.c  |  4 +++-
 include/linux/if_rmnet.h                      | 23 ++++++++-----------
 3 files changed, 16 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..28d355b094683 100644
--- a/drivers/net/ethernet/qualcomm/rmnet/rmnet_handlers.c
+++ b/drivers/net/ethernet/qualcomm/rmnet/rmnet_handlers.c
@@ -4,6 +4,7 @@
  * RMNET Data ingress/egress handler
  */
 
+#include <linux/bitfield.h>
 #include <linux/netdevice.h>
 #include <linux/netdev_features.h>
 #include <linux/if_arp.h>
@@ -61,7 +62,7 @@ __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 +71,7 @@ __rmnet_map_ingress_handler(struct sk_buff *skb,
 	}
 
 	mux_id = map_header->mux_id;
-	pad = map_header->pad_len;
+	pad = u8_get_bits(map_header->flags, MAP_PAD_LEN_FMASK);
 	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..3291f252d81b0 100644
--- a/drivers/net/ethernet/qualcomm/rmnet/rmnet_map_data.c
+++ b/drivers/net/ethernet/qualcomm/rmnet/rmnet_map_data.c
@@ -4,6 +4,7 @@
  * RMNET Data MAP protocol
  */
 
+#include <linux/bitfield.h>
 #include <linux/netdevice.h>
 #include <linux/ip.h>
 #include <linux/ipv6.h>
@@ -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 = u8_encode_bits(padding, MAP_PAD_LEN_FMASK);
 
 	return map_header;
 }
diff --git a/include/linux/if_rmnet.h b/include/linux/if_rmnet.h
index 8c7845baf3837..22ccc89bb5d8e 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_*_FMASK */
+	u8 mux_id;
+	__be16 pkt_len;			/* Length of packet, including pad */
 }  __aligned(1);
 
+/* rmnet_map_header flags field:
+ *  CMD:	1 = packet contains a MAP command; 0 = packet contains data
+ *  PAD_LEN:	number of pad bytes following packet data
+ */
+#define MAP_CMD_FLAG			BIT(7)
+#define MAP_PAD_LEN_FMASK		GENMASK(5, 0)
+
 struct rmnet_map_dl_csum_trailer {
 	u8  reserved1;
 #if defined(__LITTLE_ENDIAN_BITFIELD)
-- 
2.27.0