BestComm engine has some data cached internally and that it detects a write
to the address from where those data comes, he will invalidate his cache.
But when the kernel writes data to the skb buffer, they may partially
stay in cache so there won't be any transaction at all on the xlb bus.
It's when
bestcomm will read the skb, that the core will snoop the bus, detects
there is
a read request for some data he has in cache, force a retry of the
bestcomm read,
write the data to memory (via xlb), and finally let bestcomm retry the
transaction to fetch the good data.
So I guess what "could" happen is that :
- The kernel allocate a skb, but it ends up being as the same memory
location
as a "previous" one. (or maybe in a directly following position
because of
prefetch).
- You submit it to bestcomm
- When bestcomm does the read, since the skb was used "just before", the
line is still in cache but with the wrong data. Since the kernel just
wrote the
data, there was not yet a xlb transaction because the data are still in
cpu cache.
Bestcomm think he has the data (no xlb write so it's cache was not
invalidated),
so he doesn't generate a xlb read. But if there is no xlb read the core
doesn't get
a chance to snoop it and doesn't flush it's cache ...
Although that doesn't explain why setting BSDIS high solve the problem, nor
why there is only 1 byte wrong ...
Have you checked your XLB snoop window setting ? And that core snooping
is enabled ? Also that you don't use the "nap" power saving feature of the
core ? (it disables snooping altogether ...).
Sylvain
