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

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Date:   Thu, 4 Mar 2021 22:50:14 -0600
From:   Bjorn Andersson <bjorn.andersson@...aro.org>
To:     Alex Elder <elder@...aro.org>
Cc:     subashab@...eaurora.org, stranche@...eaurora.org,
        davem@...emloft.net, kuba@...nel.org, sharathv@...eaurora.org,
        evgreen@...omium.org, cpratapa@...eaurora.org, elder@...nel.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next 4/6] net: qualcomm: rmnet: use field masks
 instead of C bit-fields

On Thu 04 Mar 16:34 CST 2021, Alex Elder wrote:

> 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.  Instead, define a single-byte
> flags field, and use the functions defined in <linux/bitfield.h>,
> along with field mask constants to extract or assign values within
> that field.
> 

Reviewed-by: Bjorn Andersson <bjorn.andersson@...aro.org>

Regards,
Bjorn

> Signed-off-by: Alex Elder <elder@...aro.org>
> ---
>  .../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..30f8e2f02696b 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 (u8_get_bits(map_header->flags, MAP_CMD_FMASK)) {
>  		/* 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..4824c6328a82c 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_FMASK			GENMASK(7, 7)
> +#define MAP_PAD_LEN_FMASK		GENMASK(5, 0)
> +
>  struct rmnet_map_dl_csum_trailer {
>  	u8  reserved1;
>  #if defined(__LITTLE_ENDIAN_BITFIELD)
> -- 
> 2.20.1
>