linux-kernel - Re: [PATCH net-next 3/3] net: stmmac: Set CIC bit only for TX queues with COE

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Message-ID: <38d05790-eb4a-482a-89ec-8c17cf2e9680@altera.com>
Date: Thu, 17 Jul 2025 11:50:06 +0530
From: "G Thomas, Rohan" <rohan.g.thomas@...era.com>
To: Simon Horman <horms@...nel.org>
Cc: Andrew Lunn <andrew+netdev@...n.ch>, "David S. Miller"
 <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
 Maxime Coquelin <mcoquelin.stm32@...il.com>,
 Alexandre Torgue <alexandre.torgue@...s.st.com>,
 Serge Semin <fancer.lancer@...il.com>,
 Romain Gantois <romain.gantois@...tlin.com>, netdev@...r.kernel.org,
 linux-stm32@...md-mailman.stormreply.com,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
 Matthew Gerlach <matthew.gerlach@...era.com>
Subject: Re: [PATCH net-next 3/3] net: stmmac: Set CIC bit only for TX queues
 with COE

Hi Simon,

On 7/16/2025 1:52 PM, Simon Horman wrote:
> On Tue, Jul 15, 2025 at 07:14:21PM +0530, G Thomas, Rohan wrote:
>> Hi Simon,
>>
>> Thanks for reviewing the patch.
>>
>> On 7/14/2025 7:10 PM, Simon Horman wrote:
>>> On Mon, Jul 14, 2025 at 03:59:19PM +0800, Rohan G Thomas via B4 Relay wrote:
>>>> From: Rohan G Thomas <rohan.g.thomas@...era.com>
>>>>
>>>> Currently, in the AF_XDP transmit paths, the CIC bit of
>>>> TX Desc3 is set for all packets. Setting this bit for
>>>> packets transmitting through queues that don't support
>>>> checksum offloading causes the TX DMA to get stuck after
>>>> transmitting some packets. This patch ensures the CIC bit
>>>> of TX Desc3 is set only if the TX queue supports checksum
>>>> offloading.
>>>>
>>>> Signed-off-by: Rohan G Thomas <rohan.g.thomas@...era.com>
>>>> Reviewed-by: Matthew Gerlach <matthew.gerlach@...era.com>
>>>
>>> Hi Rohan,
>>>
>>> I notice that stmmac_xmit() handles a few other cases where
>>> checksum offload should not be requested via stmmac_prepare_tx_desc:
>>>
>>>           csum_insertion = (skb->ip_summed == CHECKSUM_PARTIAL);
>>>           /* DWMAC IPs can be synthesized to support tx coe only for a few tx
>>>            * queues. In that case, checksum offloading for those queues that don't
>>>            * support tx coe needs to fallback to software checksum calculation.
>>>            *
>>>            * Packets that won't trigger the COE e.g. most DSA-tagged packets will
>>>            * also have to be checksummed in software.
>>>            */
>>>           if (csum_insertion &&
>>>               (priv->plat->tx_queues_cfg[queue].coe_unsupported ||
>>>                !stmmac_has_ip_ethertype(skb))) {
>>>                   if (unlikely(skb_checksum_help(skb)))
>>>                           goto dma_map_err;
>>>                   csum_insertion = !csum_insertion;
>>>           }
>>>
>>> Do we need to care about them in stmmac_xdp_xmit_zc()
>>> and stmmac_xdp_xmit_xdpf() too?
>>
>> This patch only addresses avoiding the TX DMA hang by ensuring the CIC
>> bit is only set when the queue supports checksum offload. For DSA tagged
>> packets checksum offloading is not supported by the DWMAC IPs but no TX
>> DMA hang. AFAIK, currently AF_XDP paths don't have equivalent handling
>> like skb_checksum_help(), since they operate on xdp buffers. So this
>> patch doesn't attempt to implement a sw fallback but just avoids DMA
>> stall.
> 
> Ok, fair enough.
> 
> As per Andrew's advice elsewhere in this thread.
> This patch also looks like it should be a fix for net,
> and should have a Fixes tag.

Thanks for your comments.

You're right—this patch is a fix for the TX DMA hang issue caused by
setting the CIC bit on queues that don't support checksum offload. But
I couldn’t pinpoint a specific commit that introduced this behavior in
the AF_XDP path. Initially, there was no support for DWMAC IPs with COE
enabled only on specific queues, even though there can be IPs with such
configuration. Commit 8452a05b2c63 ("net: stmmac: Tx coe sw fallback")
added software fallback support for the AF_PACKET path. But the AF_XDP
path has always enabled COE unconditionally even before that. So, do you
think referencing the commit 8452a05b2c63 in the Fixes tag is
appropriate and sufficient?

Best Regards,
Rohan