There is no need for complex checking between the last consumed index
and current consumed index, a simple subtraction will do.
This also eliminates the possibility of a permanent transmit queue stall
under the following conditions:
- one CPU bursts ring->size worth of traffic (up to 256 buffers), to the
point where we run out of free descriptors, so we stop the transmit
queue at the end of bcm_sysport_xmit()
- because of our locking, we have the transmit process disable
interrupts which means we can be blocking the TX reclamation process
- when TX reclamation finally runs, we will be computing the difference
between ring->c_index (last consumed index by SW) and what the HW
reports through its register
- this register is masked with (ring->size - 1) = 0xff, which will lead
to stripping the upper bits of the index (register is 16-bits wide)
- we will be computing last_tx_cn as 0, which means there is no work to
be done, and we never wake-up the transmit queue, leaving it
permanently disabled
A practical example is e.g: ring->c_index aka last_c_index = 12, we
pushed 256 entries, HW consumer index = 268, we mask it with 0xff = 12,
so last_tx_cn == 0, nothing happens.
Fixes: 80105befdb ("net: systemport: add Broadcom SYSTEMPORT Ethernet MAC driver")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
484d802d0f