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>

1 files changed, 6 insertions, 0 deletions

diff --git a/drivers/net/ethernet/cadence/macb.c b/drivers/net/ethernet/cadence/macb.c
index 61aa570aad9a..5f10dfc631d7 100644
--- 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));