net: enetc: prioritize ability to go down over packet processing

napi_synchronize() from enetc_stop() waits until the softirq has
finished execution and no longer wants to be rescheduled. However under
high traffic load, this will never happen, and the interface can never
be closed.

The problem is the fact that the NAPI poll routine is written to update
the consumer index which makes the device want to put more buffers in
the RX ring, which restarts the madness again.

Browsing around, it seems that some drivers like i40e keep a bit
(__I40E_VSI_DOWN) which they use as communication between the control
path and the data path. But that isn't my first choice, because
complications ensue - since the enetc hardirq may trigger while we are
in a theoretical ENETC_DOWN state, it may happen that enetc_msix() masks
it, but enetc_poll() never unmasks it. To prevent a stall in that case,
one would need to schedule all NAPI instances when ENETC_DOWN gets
cleared, to process what's pending.

I find it more desirable for the control path - enetc_stop() - to just
quiesce the RX ring and let the softirq finish what remains there,
without any explicit communication, just by making hardware not provide
any more packets.

This seems possible with the Enable bit of the RX BD ring (RBaMR[EN]).
I can't seem to find an exact definition of what this bit does, but when
the RX ring is disabled, the port seems to no longer update the producer
index, and not react to software updates of the consumer index.

In fact, the RBaMR[EN] bit is already toggled by the driver, but too
late for what we want:

enetc_close()
-> enetc_stop()
   -> napi_synchronize()
-> enetc_clear_bdrs()
   -> enetc_clear_rxbdr()

The enetc_clear_bdrs() function contains not only logic to disable the
RX and TX rings, but also logic to wait for the TX ring stop being busy.

We split enetc_clear_bdrs() into enetc_disable_bdrs() and
enetc_wait_bdrs(). One needs to run before napi_synchronize() and the
other after (NAPI also processes TX completions, so we maximize our
chances of not waiting for the ENETC_TBSR_BUSY bit - unless a packet is
stuck for some reason, ofc).

We also split off enetc_enable_bdrs() from enetc_setup_bdrs(), and call
this from the mirror position in enetc_start() compared to enetc_stop(),
i.e. right before netif_tx_start_all_queues().

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>