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

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Date:   Fri, 25 Mar 2022 08:57:12 +0000
From:   <Claudiu.Beznea@...rochip.com>
To:     <tomas.melin@...sala.com>, <netdev@...r.kernel.org>
CC:     <Nicolas.Ferre@...rochip.com>, <davem@...emloft.net>,
        <kuba@...nel.org>, <pabeni@...hat.com>
Subject: Re: [PATCH] net: macb: Restart tx only if queue pointer is lagging

On 25.03.2022 08:50, Tomas Melin wrote:
> [Some people who received this message don't often get email from tomas.melin@...sala.com. Learn why this is important at http://aka.ms/LearnAboutSenderIdentification.]
> 
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> 
> commit 5ea9c08a8692 ("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 TXUBR interrupt. As more data gets pushed to the queue,
> transmission will resume.
> 
> This issue was observed on a Xilinx Zynq 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>
> ---
>  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;
> +

This looks like TBQP is not advancing though there are packets in the
software queues (head != tail). Packets are added in the software queues on
TX path and removed when TX was done for them.

Maybe TX_WRAP is missing on one TX descriptor? Few months ago while
investigating some other issues on this I found that this might be missed
on one descriptor [1] but haven't managed to make it break at that point
anyhow.

Could you check on your side if this is solving your issue?

	/* Set 'TX_USED' bit in buffer descriptor at tx_head position
	 * to set the end of TX queue
	 */
	i = tx_head;
	entry = macb_tx_ring_wrap(bp, i);
	ctrl = MACB_BIT(TX_USED);
+	if (entry == bp->tx_ring_size - 1)
+		ctrl |= MACB_BIT(TX_WRAP);
	desc = macb_tx_desc(queue, entry);
	desc->ctrl = ctrl;

[1]
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/net/ethernet/cadence/macb_main.c#n1958

>         macb_writel(bp, NCR, macb_readl(bp, NCR) | MACB_BIT(TSTART));
>  }
> 
> --
> 2.35.1
>