netdev - [PATCH v2] net: macb: Restart tx only if queue pointer is lagging

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Message-Id: <20220407161659.14532-1-tomas.melin@vaisala.com>
Date:   Thu,  7 Apr 2022 19:16:59 +0300
From:   Tomas Melin <tomas.melin@...sala.com>
To:     netdev@...r.kernel.org
Cc:     nicolas.ferre@...rochip.com, claudiu.beznea@...rochip.com,
        davem@...emloft.net, kuba@...nel.org, pabeni@...hat.com,
        Tomas Melin <tomas.melin@...sala.com>
Subject: [PATCH v2] net: macb: Restart tx only if queue pointer is lagging

commit 4298388574da ("net: macb: restart tx after tx used bit read")
added support for restarting transmission. Restarting tx does not work
in case controller asserts TXUBR interrupt and TQBP is already at the end
of the tx queue. In that situation, restarting tx will immediately cause
assertion of another TXUBR interrupt. The driver will end up in an infinite
interrupt loop which it cannot break out of.

For cases where TQBP is at the end of the tx queue, instead
only clear TX_USED interrupt. As more data gets pushed to the queue,
transmission will resume.

This issue was observed on a Xilinx Zynq-7000 based board.
During stress test of the network interface,
driver would get stuck on interrupt loop within seconds or minutes
causing CPU to stall.

Signed-off-by: Tomas Melin <tomas.melin@...sala.com>
---
Changes v2:
- change referenced commit to use original commit ID instead of stable branch ID

 drivers/net/ethernet/cadence/macb_main.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/net/ethernet/cadence/macb_main.c b/drivers/net/ethernet/cadence/macb_main.c
index 800d5ced5800..e475be29845c 100644
--- a/drivers/net/ethernet/cadence/macb_main.c
+++ b/drivers/net/ethernet/cadence/macb_main.c
@@ -1658,6 +1658,7 @@ static void macb_tx_restart(struct macb_queue *queue)
 	unsigned int head = queue->tx_head;
 	unsigned int tail = queue->tx_tail;
 	struct macb *bp = queue->bp;
+	unsigned int head_idx, tbqp;

 	if (bp->caps & MACB_CAPS_ISR_CLEAR_ON_WRITE)
 		queue_writel(queue, ISR, MACB_BIT(TXUBR));
@@ -1665,6 +1666,13 @@ static void macb_tx_restart(struct macb_queue *queue)
 	if (head == tail)
 		return;

+	tbqp = queue_readl(queue, TBQP) / macb_dma_desc_get_size(bp);
+	tbqp = macb_adj_dma_desc_idx(bp, macb_tx_ring_wrap(bp, tbqp));
+	head_idx = macb_adj_dma_desc_idx(bp, macb_tx_ring_wrap(bp, head));
+
+	if (tbqp == head_idx)
+		return;
+
 	macb_writel(bp, NCR, macb_readl(bp, NCR) | MACB_BIT(TSTART));
 }

-- 
2.35.1