Alignment issues with freescale FEC driver

[Eric Nelson]

Thanks Eric,

On 09/23/2016 09:54 AM, Eric Dumazet wrote:
> On Fri, Sep 23, 2016 at 9:43 AM, Eric Nelson <eric at nelint.com> wrote:
>>
>> Hello all,
>>
>> We're seeing alignment issues from the ethernet stack on an i.MX6UL board:
>>
>>

<snip>

>>
>> - id = ntohl(*(__be32 *)&iph->id);
>> - flush = (u16)((ntohl(*(__be32 *)iph) ^ skb_gro_len(skb)) | (id & ~IP_DF));
>> - id >>= 16;
>> + id = ntohs(*(__be16 *)&iph->id);
>> + frag = ntohs(*(__be16 *)&iph->frag_off);
>> + flush = (u16)((ntohl(*(__be32 *)iph) ^ skb_gro_len(skb)) | (frag &
>> ~IP_DF));
>>
>> for (p = *head; p; p = p->next) {
>> struct iphdr *iph2;
>>
> 
> This solves nothing, because a few lines after you'll have yet another
> unaligned access :
> 

Oddly, it does prevent the vast majority (90%+) of the alignment errors.

I believe this is because the compiler is generating an ldm instruction
when the ntohl() call is used, but I'm stumped about why these aren't
generating faults:

> ((__force u32)iph->saddr ^ (__force u32)iph2->saddr) |
> ((__force u32)iph->daddr ^ (__force u32)iph2->daddr)) {
> 
> So you might have one less problematic access, out of hundreds of them
> all over the places.
> 
> Really the problem is that whole stack depends on the assumption that
> IP headers are aligned on arches that care
> (ie where NET_IP_ALIGN == 2)
> 
> If your build does have NET_IP_ALIGN = 2 and you get a fault here, it
> might be because of a buggy driver.
> 

NET_IP_ALIGN is set to 2.

> The other known case is some GRE encapsulations that break the
> assumption, and this is discussed somewhere else.
> 
I don't think that's the case.

# CONFIG_IPV6_GRE is not set

Hmm... Instrumenting the kernel, it seems that iphdr **is** aligned on
a 4-byte boundary.

Does the ldm instruction require 8-byte alignment?

There's definitely a compiler-version dependency involved here,
since using gcc 4.9 also reduced the number of faults dramatically.