Prior to this commit the driver was running into tx_timeouts when a
queue was stressed enough. This was happening because the HW tail
and SW tail (NTU) were incorrectly out of sync. Consequently this was
causing the HW head to collide with the HW tail, which to the hardware
means that all descriptors posted for Tx have been processed.

Due to the Tx logic used in the driver SW tail and HW tail are allowed
to be out of sync. This is done as an optimization because it allows the
driver to write HW tail as infrequently as possible, while still
updating the SW tail index to keep track. However, there are situations
where this results in the tail never getting updated, resulting in Tx
timeouts.

Tx HW tail write condition:
	if (netif_xmit_stopped(txring_txq(tx_ring) || !skb->xmit_more)
		writel(sw_tail, tx_ring->tail);

An issue was found in the Tx logic that was causing the afore mentioned
condition for updating HW tail to never happen, causing tx_timeouts.

In ice_xmit_frame_ring we calculate how many descriptors we need for the
Tx transaction based on the skb the kernel hands us. This is then passed
into ice_maybe_stop_tx along with some extra padding to determine if we
have enough descriptors available for this transaction. If we don't then
we return -EBUSY to the stack, otherwise we move on and eventually
prepare the Tx descriptors accordingly in ice_tx_map and set
next_to_watch. In ice_tx_map we make another call to ice_maybe_stop_tx
with a value of MAX_SKB_FRAGS + 4. The key here is that this value is
possibly less than the value we sent in the first call to
ice_maybe_stop_tx in ice_xmit_frame_ring. Now, if the number of unused
descriptors is between MAX_SKB_FRAGS + 4 and the value used in the first
call to ice_maybe_stop_tx in ice_xmit_frame_ring then we do not update
the HW tail because of the "Tx HW tail write condition" above. This is
because in ice_maybe_stop_tx we return success from ice_maybe_stop_tx
instead of calling __ice_maybe_stop_tx and subsequently calling
netif_stop_subqueue, which sets the __QUEUE_STATE_DEV_XOFF bit. This
bit is then checked in the "Tx HW tail write condition" by calling
netif_xmit_stopped and subsequently updating HW tail if the
afore mentioned bit is set.

In ice_clean_tx_irq, if next_to_watch is not NULL, we end up cleaning
the descriptors that HW sets the DD bit on and we have the budget. The
HW head will eventually run into the HW tail in response to the
description in the paragraph above.

The next time through ice_xmit_frame_ring we make the initial call to
ice_maybe_stop_tx with another skb from the stack. This time we do not
have enough descriptors available and we return NETDEV_TX_BUSY to the
stack and end up setting next_to_watch to NULL.

This is where we are stuck. In ice_clean_tx_irq we never clean anything
because next_to_watch is always NULL and in ice_xmit_frame_ring we never
update HW tail because we already return NETDEV_TX_BUSY to the stack and
eventually we hit a tx_timeout.

This issue was fixed by making sure that the second call to
ice_maybe_stop_tx in ice_tx_map is passed a value that is >= the value
that was used on the initial call to ice_maybe_stop_tx in
ice_xmit_frame_ring. This was done by adding the following defines to
make the logic more clear and to reduce the chance of mucking this up
again:

ICE_CACHE_LINE_BYTES		64
ICE_DESCS_PER_CACHE_LINE	(ICE_CACHE_LINE_BYTES / \
				 sizeof(struct ice_tx_desc))
ICE_DESCS_FOR_CTX_DESC		1
ICE_DESCS_FOR_SKB_DATA_PTR	1

The ICE_CACHE_LINE_BYTES being 64 is an assumption being made so we
don't have to figure this out on every pass through the Tx path. Instead
I added a sanity check in ice_probe to verify cache line size and print
a message if it's not 64 Bytes. This will make it easier to file issues
if they are seen when the cache line size is not 64 Bytes when reading
from the GLPCI_CNF2 register.

Signed-off-by: Brett Creeley <brett.creeley@intel.com>
Signed-off-by: Anirudh Venkataramanan <anirudh.venkataramanan@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>