[PATCH 1/3] net: macb: never give hardware a NULL RX buffer

["Théo Lebrun" <theo.lebrun@bootlin.com>]

The refill logic is simple: iterate over all pending rx slots, allocate
SKB (& DMA map) if needed and hand it off the to hardware by clearing
the RX_USED flag.

If the refill operation fails mid-way, it early returns leaving the
following slots untouched. In the normal case that is fine, because all
slots have been properly initialised (and might have been already used
by HW meaning they won't be reused). When slots have not been
initialised however, we are in trouble.

After dma_alloc_coherent() of the rx ring buffer, all slots have NULL
pointers and RX_USED cleared meaning HW will try using them. Ensure
this does not happen by setting the RX_USED flag on all slots before
calling refill at buffer alloc, in gem_init_rx_ring(). That way even if
refill fails on an alloc/dma_map, the HW won't try using NULL pointers
as buffers.

Theoretical bugfix, never encountered in practice. To reproduce,
introduce memory pressure (less than 512 SKBs of free memory) and open
the interface.

Note that this codepath also hits at resume, on HRESP errors and on
set_ringparam (while interface is running).

Fixes: 4df95131ea80 ("net/macb: change RX path for GEM")
Cc: stable@vger.kernel.org
Signed-off-by: Théo Lebrun <theo.lebrun@bootlin.com>
---
 drivers/net/ethernet/cadence/macb_main.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/net/ethernet/cadence/macb_main.c b/drivers/net/ethernet/cadence/macb_main.c
index b8234ac4b602..751fa9e68099 100644
--- a/drivers/net/ethernet/cadence/macb_main.c
+++ b/drivers/net/ethernet/cadence/macb_main.c
@@ -2785,9 +2785,14 @@ static int macb_alloc(struct macb *bp)
 
 static void gem_init_rx_ring(struct macb_queue *queue)
 {
+	unsigned int i;
+
 	queue->rx_tail = 0;
 	queue->rx_prepared_head = 0;
 
+	for (i = 0; i < queue->bp->rx_ring_size; i++)
+		macb_rx_desc(queue, i)->addr |= MACB_BIT(RX_USED);
+
 	gem_rx_refill(queue);
 }
 

-- 
2.55.0