net: macb: Add better comment for RXUBR handling

Describe the handler for RXUBR better with a new comment.

Signed-off-by: Nathan Sullivan <nathan.sullivan@ni.com>
Reviewied-by: Josh Cartwright <joshc@ni.com>
Reviewied-by: Ben Shelton <ben.shelton@ni.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/drivers/net/ethernet/cadence/macb.c b/drivers/net/ethernet/cadence/macb.c
index 61aa570..5f10dfc 100644 (file)
--- a/drivers/net/ethernet/cadence/macb.c
+++ b/drivers/net/ethernet/cadence/macb.c
@@ -1037,6 +1037,12 @@ static irqreturn_t macb_interrupt(int irq, void *dev_id)
                 * add that if/when we get our hands on a full-blown MII PHY.
                 */
 
+               /* There is a hardware issue under heavy load where DMA can
+                * stop, this causes endless "used buffer descriptor read"
+                * interrupts but it can be cleared by re-enabling RX. See
+                * the at91 manual, section 41.3.1 or the Zynq manual
+                * section 16.7.4 for details.
+                */
                if (status & MACB_BIT(RXUBR)) {
                        ctrl = macb_readl(bp, NCR);
                        macb_writel(bp, NCR, ctrl & ~MACB_BIT(RE));