Subject: Re: [PATCH net-next 2/6] net: qualcomm: rmnet: simplify some byte order logic

From: Bjorn Andersson <[email-masked]>
Date: Thu, 4 Mar 2021 22:07:13 -0600

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

> In rmnet_map_ipv4_ul_csum_header() and rmnet_map_ipv6_ul_csum_header()
> the offset within a packet at which checksumming should commence is
> calculated.  This calculation involves byte swapping and a forced type
> conversion that makes it hard to understand.
> 
> Simplify this by computing the offset in host byte order, then
> converting the result when assigning it into the header field.
> 
> Signed-off-by: Alex Elder <[email-masked]>
> ---
>  .../ethernet/qualcomm/rmnet/rmnet_map_data.c  | 22 ++++++++++---------
>  1 file changed, 12 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/net/ethernet/qualcomm/rmnet/rmnet_map_data.c b/drivers/net/ethernet/qualcomm/rmnet/rmnet_map_data.c
> index 21d38167f9618..bd1aa11c9ce59 100644
> --- a/drivers/net/ethernet/qualcomm/rmnet/rmnet_map_data.c
> +++ b/drivers/net/ethernet/qualcomm/rmnet/rmnet_map_data.c
> @@ -197,12 +197,13 @@ rmnet_map_ipv4_ul_csum_header(void *iphdr,
>  			      struct rmnet_map_ul_csum_header *ul_header,
>  			      struct sk_buff *skb)
>  {
> -	struct iphdr *ip4h = (struct iphdr *)iphdr;
> -	__be16 *hdr = (__be16 *)ul_header, offset;
> +	__be16 *hdr = (__be16 *)ul_header;
> +	struct iphdr *ip4h = iphdr;
> +	u16 offset;
> +
> +	offset = skb_transport_header(skb) - (unsigned char *)iphdr;
> +	ul_header->csum_start_offset = htons(offset);
>  
> -	offset = htons((__force u16)(skb_transport_header(skb) -

Just curious, why does this require a __force, or even a cast?

Regardless, your proposed way of writing it is easier to read.

Reviewed-by: Bjorn Andersson <[email-masked]>

Regards,
Bjorn

> -				     (unsigned char *)iphdr));
> -	ul_header->csum_start_offset = offset;
>  	ul_header->csum_insert_offset = skb->csum_offset;
>  	ul_header->csum_enabled = 1;
>  	if (ip4h->protocol == IPPROTO_UDP)
> @@ -239,12 +240,13 @@ rmnet_map_ipv6_ul_csum_header(void *ip6hdr,
>  			      struct rmnet_map_ul_csum_header *ul_header,
>  			      struct sk_buff *skb)
>  {
> -	struct ipv6hdr *ip6h = (struct ipv6hdr *)ip6hdr;
> -	__be16 *hdr = (__be16 *)ul_header, offset;
> +	__be16 *hdr = (__be16 *)ul_header;
> +	struct ipv6hdr *ip6h = ip6hdr;
> +	u16 offset;
> +
> +	offset = skb_transport_header(skb) - (unsigned char *)ip6hdr;
> +	ul_header->csum_start_offset = htons(offset);
>  
> -	offset = htons((__force u16)(skb_transport_header(skb) -
> -				     (unsigned char *)ip6hdr));
> -	ul_header->csum_start_offset = offset;
>  	ul_header->csum_insert_offset = skb->csum_offset;
>  	ul_header->csum_enabled = 1;
>  
> -- 
> 2.20.1
>