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

[Alex Elder]

On 3/4/21 10:07 PM, Bjorn Andersson wrote:
> 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 <elder@xxxxxxxxxx>
> > ---
> >   .../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?

The argument to htons() has type __u16.  In this case it
is passed the difference between pointers, which will
have type ptrdiff_t, which is certainly bigger than
16 bits.  I don't think the __force is needed, but the
cast to u16 might just be making the conversion to the
smaller type explicit.  Here too though, I don't think
it's necessary.

					-Alex

> Regardless, your proposed way of writing it is easier to read.
>
> Reviewed-by: Bjorn Andersson <bjorn.andersson@xxxxxxxxxx>
>
> 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