Date:   Fri, 25 Mar 2022 16:41:20 +0200
From:   Tomas Melin <tomas.melin@...sala.com>
To:     Claudiu.Beznea@...rochip.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

Hi,

On 25/03/2022 15:41, Claudiu.Beznea@...rochip.com wrote:
> On 25.03.2022 11:35, Tomas Melin wrote:
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>>
>> Hi,
>>
>> On 25/03/2022 10:57, Claudiu.Beznea@...rochip.com wrote:
>>> On 25.03.2022 08:50, Tomas Melin wrote:
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>>>>
>>>> EXTERNAL EMAIL: Do not click links or open attachments unless you know
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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.
>>
>> TBQP is at the end of the queue, and that matches with tx_head
>> maintained by driver. So seems controller is happily at end marker,
>> and when restarted immediately sees that end marker used tag and
>> triggers an interrupt again.
>>
>> Also when looking at the buffer descriptor memory it shows that all
>> frames between tx_tail and tx_head have been marked as used.
> 
> I see. Controller sets TX_USED on the 1st descriptor of the transmitted
> frame. If there were packets with one descriptor enqueued that should mean
> controller did its job >
> head != tail on software data structures when receiving TXUBR interrupt and
> all descriptors in queue have TX_USED bit set might signal that  a
> descriptor is not updated to CPU on TCOMP interrupt when CPU uses it and
> thus driver doesn't treat a TCOMP interrupt. See the above code on

Both TX_USED and last buffer (bit 15) indicator looks ok from
memory, so controller seems to be up to date. If we were to get a TCOMP
interrupt things would be rolling again.
Ofcourse this is speculation, but perhaps there could also be some
corner cases where the controller fails to generate TCOMP as expected?

> macb_tx_interrupt():
> 
> desc = macb_tx_desc(queue, tail);
> 
> /* Make hw descriptor updates visible to CPU */
> rmb();
> 
> ctrl = desc->ctrl;
> 
> /* TX_USED bit is only set by hardware on the very first buffer
> * descriptor of the transmitted frame.
> */
> 
> if (!(ctrl & MACB_BIT(TX_USED)))
> 	break;
> 
> 
>>
>> GEM documentation says "transmission is restarted from
>> the first buffer descriptor of the frame being transmitted when the
>> transmit start bit is rewritten" but since all frames are already marked
>> as transmitted, restarting wont help. Adding this additional check will
>> help for the issue we have.
>>
> 
> I see but according to your description (all descriptors treated by
> controller) if no packets are enqueued for TX after:
> 
> +       if (tbqp == head_idx)
> +               return;
> +
> 
> there are some SKBs that were correctly treated by controller but not freed
> by software (they are freed on macb_tx_unmap() called from
> macb_tx_interrupt()). They will be freed on next TCOMP interrupt for other
> packets being transmitted.
Yes, that is idea. We cannot restart since it triggers new irq,
but instead need to break out. When more data arrives, the controller
continues operation again.


> 
>>
>>>
>>> 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?
>>
>> I have seen that we can get stuck at any location in the ring buffer, so
>> this does not seem to be the case here. I can try though if it would
>> have any effect.
> 
> I was thinking that having small packets there is high chance that TBQP to
> not reach a descriptor with wrap bit set due to the code pointed in my
> previous email.

I tested with the additions suggested below, but with no change.

Thanks,
Tomas


> 
> Thank you,
> Claudiu Beznea
> 
>>
>> thanks,
>> Tomas
>>
>>
>>>
>>>        /* 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://eur03.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgit.kernel.org%2Fpub%2Fscm%2Flinux%2Fkernel%2Fgit%2Ftorvalds%2Flinux.git%2Ftree%2Fdrivers%2Fnet%2Fethernet%2Fcadence%2Fmacb_main.c%23n1958&amp;data=04%7C01%7Ctomas.melin%40vaisala.com%7C4d5ef5a2a27247697b0b08da0e6539ff%7C6d7393e041f54c2e9b124c2be5da5c57%7C0%7C0%7C637838125183624670%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&amp;sdata=v%2F%2FdE1Y8BxHWsmtn3nX70OFN5oCb%2Bzlb881UXuYtoMs%3D&amp;reserved=0
>>>
>>>
>>>>           macb_writel(bp, NCR, macb_readl(bp, NCR) | MACB_BIT(TSTART));
>>>>    }
>>>>
>>>> -- 
>>>> 2.35.1
>>>>
>>>
> 

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