Frieder Schrempf reported a TX throuthput issue [1], it happens quite often
that the measured bandwidth in TX direction drops from its expected/nominal
value to something like ~50% (for 100M) or ~67% (for 1G) connections.

[1] https://lore.kernel.org/linux-arm-kernel/421cc86c-b66f-b372-32f7-21e59f9a98bc@kontron.de/

The issue becomes clear after digging into it, Net core would select
queues when transmitting packets. Since FEC have not impletemented
ndo_select_queue callback yet, so it will call netdev_pick_tx to select
queues randomly.

For i.MX6SX ENET IP with AVB support, driver default enables this
feature. According to the setting of QOS/RCMRn/DMAnCFG registers, AVB
configured to Credit-based scheme, 50% bandwidth of each queue 1&2.

With below tests let me think more:
1) With FEC_QUIRK_HAS_AVB quirk, can reproduce TX bandwidth fluctuations issue.
2) Without FEC_QUIRK_HAS_AVB quirk, can't reproduce TX bandwidth fluctuations issue.

The related difference with or w/o FEC_QUIRK_HAS_AVB quirk is that, whether we
program FTYPE field of TxBD or not. As I describe above, AVB feature is
enabled by default. With FEC_QUIRK_HAS_AVB quirk, frames in queue 0
marked as non-AVB, and frames in queue 1&2 marked as AVB Class A&B. It's
unreasonable if frames in queue 1&2 are not required to be time-sensitive.
So when Net core select tx queues ramdomly, Credit-based scheme would work
and lead to TX bandwidth fluctuated. On the other hand, w/o
FEC_QUIRK_HAS_AVB quirk, frames in queue 1&2 are all marked as non-AVB, so
Credit-based scheme would not work.

Till now, how can we fix this TX throughput issue? Yes, please remove
FEC_QUIRK_HAS_AVB quirk if you suffer it from time-nonsensitive networking.
However, this quirk is used to indicate i.MX6SX, other setting depends
on it. So this patch adds a new quirk FEC_QUIRK_HAS_MULTI_QUEUES to
represent i.MX6SX, it is safe for us remove FEC_QUIRK_HAS_AVB quirk
now.

FEC_QUIRK_HAS_AVB quirk is set by default in the driver, and users may
not know much about driver details, they would waste effort to find the
root cause, that is not we want. The following patch is a implementation
to fix it and users don't need to modify the driver.
Tested-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Reported-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Signed-off-by: Joakim Zhang <qiangqing.zhang@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>