Currently we try to clear EDRRR and EDTRR and immediately continue to
free buffers.  This is unsafe because:

- In general, register writes are not serialised with DMA, so we still
  have to wait for DMA to complete somehow
- The R8A7790 (R-Car H2) manual states that the TX running flag cannot
  be cleared by writing to EDTRR
- The same manual states that clearing the RX running flag only stops
  RX DMA at the next packet boundary

I applied this patch to the driver to detect DMA writes to freed
buffers:

> --- a/drivers/net/ethernet/renesas/sh_eth.c
> +++ b/drivers/net/ethernet/renesas/sh_eth.c
> @@ -1098,7 +1098,14 @@ static void sh_eth_ring_free(struct net_device *ndev)
>  	/* Free Rx skb ringbuffer */
>  	if (mdp->rx_skbuff) {
>  		for (i = 0; i < mdp->num_rx_ring; i++)
> +			memcpy(mdp->rx_skbuff[i]->data,
> +			       "Hello, world", 12);
> +		msleep(100);
> +		for (i = 0; i < mdp->num_rx_ring; i++) {
> +			WARN_ON(memcmp(mdp->rx_skbuff[i]->data,
> +				       "Hello, world", 12));
>  			dev_kfree_skb(mdp->rx_skbuff[i]);
> +		}
>  	}
>  	kfree(mdp->rx_skbuff);
>  	mdp->rx_skbuff = NULL;

then ran the loop:

    while ethtool -G eth0 rx 128 ; ethtool -G eth0 rx 64; do echo -n .; done

and 'ping -f' toward the sh_eth port from another machine.  The
warning fired several times a minute.

To fix these issues:

- Deactivate all TX descriptors rather than writing to EDTRR
- As there seems to be no way of telling when RX DMA is stopped,
  perform a soft reset to ensure that both DMA enginess are stopped
- To reduce the possibility of the reset truncating a transmitted
  frame, disable egress and wait a reasonable time to reach a
  packet boundary before resetting
- Update statistics before resetting

(The 'reasonable time' does not allow for CS/CD in half-duplex
mode, but half-duplex no longer seems reasonable!)

Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>