linux-kernel - Re: [PATCH net-next v4 2/6] net: qualcomm: rmnet: simplify some byte order logic

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Date:   Mon, 15 Mar 2021 12:12:01 -0500
From:   Alex Elder <elder@...e.org>
To:     Alexander Duyck <alexander.duyck@...il.com>,
        Alex Elder <elder@...aro.org>
Cc:     Subash Abhinov Kasiviswanathan <subashab@...eaurora.org>,
        stranche@...eaurora.org, David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, sharathv@...eaurora.org,
        bjorn.andersson@...aro.org, evgreen@...omium.org,
        cpratapa@...eaurora.org, David Laight <David.Laight@...lab.com>,
        Vladimir Oltean <olteanv@...il.com>, elder@...nel.org,
        Netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next v4 2/6] net: qualcomm: rmnet: simplify some byte
 order logic

On 3/15/21 11:02 AM, Alexander Duyck wrote:
> On Mon, Mar 15, 2021 at 6:36 AM Alex Elder <elder@...aro.org> 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@...aro.org>
>> Reviewed-by: Bjorn Andersson <bjorn.andersson@...aro.org>
>> ---
>>   .../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);
> 
> Rather than using skb_transport_header the correct pointer to use is
> probably skb_checksum_start. The two are essentially synonymous but
> the checksumming code is supposed to use skb_checksum_start.

That's a great suggestion.  I was mimicking the existing
code but it would be much better to use that.

> Alternatively you could look at possibly using skb_network_header_len
> as that would be the same value assuming that both headers are the
> outer headers. Then you could avoid the extra pointer overhead.

Actually, I think this is better still, because the purpose
here is to tell the hardware where to start checksumming.
I.e., it's unrelated to the SKB, and hides the calculation.

Thank you.  I'll put together version 5, incorporating your
suggestion.

					-Alex

>> -       offset = htons((__force u16)(skb_transport_header(skb) -
>> -                                    (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);
> 
> Same here.
> 
>>
>> -       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;
>>
>> --
>> 2.27.0
>>