drivers/net/airoha_eth: fix stalling in package receiving

ARCH_DMA_MINALIGN is 64 for ARMv7a/ARMv8a architectures, but RX/TX
descriptors are 32 bytes long. So they may not be aligned on an
ARCH_DMA_MINALIGN boundary. In case of RX path, this may cause the
following problem

1) Assume that a packet has arrived and the EVEN rx descriptor has been
   updated with the incoming data. The driver will invalidate and check
   the corresponding rx descriptor.

2) Now suppose the next descriptor (ODD) has not yet completed.

   Please note that all even descriptors starts on 64-byte boundary,
   and the odd ones are NOT aligned on 64-byte boundary.

   Inspecting even descriptor, we will read the entire CPU cache line
   (64 bytes). So we read and sore in CPU cache also the next (odd)
   descriptor.

3) Now suppose the next packet (for the odd rx descriptor) arrived
   while the first packet was being processed. So we have new data
   in memory but old data in cache.

4) After packet processing (in arht_eth_free_pkt() function) we will
   cleanup the descriptor and put it back to rx queue.

   This will call flush_dcache_range() function for the even descriptor,
   so the odd one will be flushed as well (it is in the same cache line).
   So the old data will be written to the next rx descriptor.

5) We get a freeze. The next descriptor is empty (so the driver is
   waiting for packets), but the hardware will continue to receive
   packets on other available descriptors. This will continue until
   the last available rx descriptor is full. Then the hardware will
   also freeze.

The problem will be solved if the previous descriptor will be put back
to the queue instead of the current one.

If the current descriptor is even (starts on a 64-byte boundary),
then putting the previous descriptor to the rx queue will affect
the previous cache line. To be 100% ok, we must make sure that the
previous and the one before the previous descriptor cannot be used
for receiving at this moment.

If the current descriptor is odd, then the previous descriptor is on
the same cache line. Both (current and previous) descriptors are not
currently in use, so issue will not arrise.

WARNING: The following restrictions on PKTBUFSRX must be held:
  * PKTBUFSRX is even,
  * PKTBUFSRX >= 4.

The bug appears on 32-bit airoha platform, but should be present on
64-bit as well.

The code was tested both on 32-bit and 64-bit airoha boards.

Signed-off-by: Mikhail Kshevetskiy <mikhail.kshevetskiy@iopsys.eu>

0 files changed, 0 insertions, 0 deletions