Subject: Re: [PATCH net-next 6/6] net: qualcomm: rmnet: don't use C bit-fields in rmnet checksum header

From: Bjorn Andersson <[email-masked]>
Date: Thu, 4 Mar 2021 23:26:24 -0600

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

> Replace the use of C bit-fields in the rmnet_map_ul_csum_header
> structure with a single two-byte (big endian) structure member,
> and use field masks to encode or get values within it.
> 
> Previously rmnet_map_ipv4_ul_csum_header() would update values in
> the host byte-order fields, and then forcibly fix their byte order
> using a combination of byte order operations and types.
> 
> Instead, just compute the value that needs to go into the new
> structure member and save it with a simple byte-order conversion.
> 
> Make similar simplifications in rmnet_map_ipv6_ul_csum_header().
> 
> Finally, in rmnet_map_checksum_uplink_packet() a set of assignments
> zeroes every field in the upload checksum header.  Replace that with
> a single memset() operation.
> 
> Signed-off-by: Alex Elder <[email-masked]>
> ---
>  .../ethernet/qualcomm/rmnet/rmnet_map_data.c  | 34 ++++++-------------
>  include/linux/if_rmnet.h                      | 21 ++++++------
>  2 files changed, 21 insertions(+), 34 deletions(-)
> 
> diff --git a/drivers/net/ethernet/qualcomm/rmnet/rmnet_map_data.c b/drivers/net/ethernet/qualcomm/rmnet/rmnet_map_data.c
> index 29d485b868a65..db76bbf000aa1 100644
> --- a/drivers/net/ethernet/qualcomm/rmnet/rmnet_map_data.c
> +++ b/drivers/net/ethernet/qualcomm/rmnet/rmnet_map_data.c
> @@ -198,23 +198,19 @@ rmnet_map_ipv4_ul_csum_header(void *iphdr,
>  			      struct rmnet_map_ul_csum_header *ul_header,
>  			      struct sk_buff *skb)
>  {
> -	__be16 *hdr = (__be16 *)ul_header;
>  	struct iphdr *ip4h = iphdr;
>  	u16 offset;
> +	u16 val;
>  
>  	offset = skb_transport_header(skb) - (unsigned char *)iphdr;
>  	ul_header->csum_start_offset = htons(offset);
>  
> -	ul_header->csum_insert_offset = skb->csum_offset;
> -	ul_header->csum_enabled = 1;
> +	val = be16_encode_bits(1, MAP_CSUM_UL_ENABLED_FMASK);

Why are you using be16_ here? Won't that cancel the htons() below?

Regards,
Bjorn

>  	if (ip4h->protocol == IPPROTO_UDP)
> -		ul_header->udp_ind = 1;
> -	else
> -		ul_header->udp_ind = 0;
> +		val |= be16_encode_bits(1, MAP_CSUM_UL_UDP_FMASK);
> +	val |= be16_encode_bits(skb->csum_offset, MAP_CSUM_UL_OFFSET_FMASK);
>  
> -	/* Changing remaining fields to network order */
> -	hdr++;
> -	*hdr = htons((__force u16)*hdr);
> +	ul_header->csum_info = htons(val);
>  
>  	skb->ip_summed = CHECKSUM_NONE;
>  
> @@ -241,24 +237,19 @@ rmnet_map_ipv6_ul_csum_header(void *ip6hdr,
>  			      struct rmnet_map_ul_csum_header *ul_header,
>  			      struct sk_buff *skb)
>  {
> -	__be16 *hdr = (__be16 *)ul_header;
>  	struct ipv6hdr *ip6h = ip6hdr;
>  	u16 offset;
> +	u16 val;
>  
>  	offset = skb_transport_header(skb) - (unsigned char *)ip6hdr;
>  	ul_header->csum_start_offset = htons(offset);
>  
> -	ul_header->csum_insert_offset = skb->csum_offset;
> -	ul_header->csum_enabled = 1;
> -
> +	val = be16_encode_bits(1, MAP_CSUM_UL_ENABLED_FMASK);
>  	if (ip6h->nexthdr == IPPROTO_UDP)
> -		ul_header->udp_ind = 1;
> -	else
> -		ul_header->udp_ind = 0;
> +		val |= be16_encode_bits(1, MAP_CSUM_UL_UDP_FMASK);
> +	val |= be16_encode_bits(skb->csum_offset, MAP_CSUM_UL_OFFSET_FMASK);
>  
> -	/* Changing remaining fields to network order */
> -	hdr++;
> -	*hdr = htons((__force u16)*hdr);
> +	ul_header->csum_info = htons(val);
>  
>  	skb->ip_summed = CHECKSUM_NONE;
>  
> @@ -425,10 +416,7 @@ void rmnet_map_checksum_uplink_packet(struct sk_buff *skb,
>  	}
>  
>  sw_csum:
> -	ul_header->csum_start_offset = 0;
> -	ul_header->csum_insert_offset = 0;
> -	ul_header->csum_enabled = 0;
> -	ul_header->udp_ind = 0;
> +	memset(ul_header, 0, sizeof(*ul_header));
>  
>  	priv->stats.csum_sw++;
>  }
> diff --git a/include/linux/if_rmnet.h b/include/linux/if_rmnet.h
> index 1fbb7531238b6..149d696feb520 100644
> --- a/include/linux/if_rmnet.h
> +++ b/include/linux/if_rmnet.h
> @@ -33,17 +33,16 @@ struct rmnet_map_dl_csum_trailer {
>  
>  struct rmnet_map_ul_csum_header {
>  	__be16 csum_start_offset;
> -#if defined(__LITTLE_ENDIAN_BITFIELD)
> -	u16 csum_insert_offset:14;
> -	u16 udp_ind:1;
> -	u16 csum_enabled:1;
> -#elif defined (__BIG_ENDIAN_BITFIELD)
> -	u16 csum_enabled:1;
> -	u16 udp_ind:1;
> -	u16 csum_insert_offset:14;
> -#else
> -#error	"Please fix <asm/byteorder.h>"
> -#endif
> +	__be16 csum_info;		/* MAP_CSUM_UL_*_FMASK */
>  } __aligned(1);
>  
> +/* csum_info field:
> + *  ENABLED:	1 = checksum computation requested
> + *  UDP:	1 = UDP checksum (zero checkum means no checksum)
> + *  OFFSET:	where (offset in bytes) to insert computed checksum
> + */
> +#define MAP_CSUM_UL_OFFSET_FMASK	GENMASK(13, 0)
> +#define MAP_CSUM_UL_UDP_FMASK		GENMASK(14, 14)
> +#define MAP_CSUM_UL_ENABLED_FMASK	GENMASK(15, 15)
> +
>  #endif /* !(_LINUX_IF_RMNET_H_) */
> -- 
> 2.20.1
>