From: Oliver Freyermuth <o.freyerm...@googlemail.com>
Date: Mon, 22 Jan 2018 23:55:58 +0100
> Checking through the driver sources, I find rtnl_link_stats64 can
> not be the culprit, since it has rx_packets and only after
> tx_packets. However, struct rtl8169_counters looks like:
>
> struct rtl8169_counters {
> __le64 tx_packets;
> __le64 rx_packets;
> __le64 tx_errors;
> __le32 rx_errors;
> __le16 rx_missed;
> __le16 align_errors;
> __le32 tx_one_collision;
> __le32 tx_multi_collision;
> __le64 rx_unicast;
> __le64 rx_broadcast;
> __le32 rx_multicast;
> __le16 tx_aborted;
> __le16 tx_underun;
> };
>
> This looks like it could very well match the structure found in
> memory, so something would be broken related to rtl8169_do_counters,
> in the DMA transfer.
>
> Does this help - can I provide more info? I get the feeling this
> affects many tens of thousands of systems and just has been hidden
> due to network stats being read rarely...
Looking at how these DMA counters are handled, there appears to be a
requirement that the memory buffer is 64-byte aligned.
This is because the low bits in the counter address register are used
for various commands, for example:
/* ResetCounterCommand */
CounterReset = 0x1,
/* DumpCounterCommand */
CounterDump = 0x8,
Looking at the FreeBSD driver, the requirement seems to be 64-bytes of
alignment. (see RL_DUMP_ALIGN define)
However, nothing is being done in r8169.c to enforce this alignment at
counter allocation time:
tp->counters = dmam_alloc_coherent (&pdev->dev, sizeof(*tp->counters),
&tp->counters_phys_addr,
There is no alignment guaranteed by this allocation interface. On a
lot of platforms you get PAGE_SIZE aligned buffers, but this is not
a universal thing at all.
Therefore the driver needs to allocate "size + (64 - 1)" bytes and do
the 64-byte alignment of the CPU pointer and the DMA address by hand.
