Re: [PATCH net-next 7/7] net: bcmgenet: reassemble jumbo frames from status block fragments

[Nicolai Buchwitz <nb@tipi-net.de>]

On 5.10.2026 17:04, netdev-bot+sashiko@kernel.org wrote:
> Thank you for your contribution! Sashiko AI review found 3 potential
> issue(s) to consider.
> 
> Critical: 0 · High: 1 (1) · Medium: 2 · Low: 0
> 
> If you believe none of the issues are legitimate, please reply to each
> issue of Medium severity or higher and explain why the AI is wrong.
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> reply
> (one such reply per series is enough).
> 
> - [High] bcmgenet: skb_trim() panics on non-linear jumbo frame SKBs
> - [Medium] This patch adds a TX checksum fallback in bcmgenet_xmit().
> - [Medium] The patch raises dev->max_mtu from the old ENET_MAX_MTU 
> (3564
>   on 4K pages, 3820 otherwise) to ENET_MAX_JUMBO_MTU = GENMASK(13,0) -…
> 
> Pre-existing issues:
> - [High] This problem wasn't introduced by this patch, but
>   bcmgenet_timeout() updates ring->stats64 without holding ring->lock.
> 
> --
> 
> net: bcmgenet: reassemble jumbo frames from status block fragments
> 
> This commit strips the per-descriptor status blocks from RX frames 
> longer
> than the packet ready threshold and reassembles them into one skb. It 
> also
> adds a software checksum fallback in bcmgenet_xmit() for 
> CHECKSUM_PARTIAL
> frames longer than tx_thld_len, and raises max_mtu to 
> ENET_MAX_JUMBO_MTU
> (16347).
> 
>> diff --git a/drivers/net/ethernet/broadcom/genet/bcmgenet.c 
>> b/drivers/net/ethernet/broadcom/genet/bcmgenet.c
>> index 0cd56ec1c5ccf..62edbe51fe076 100644
>> --- a/drivers/net/ethernet/broadcom/genet/bcmgenet.c
>> +++ b/drivers/net/ethernet/broadcom/genet/bcmgenet.c
> 
> [ ... ]
> 
>> @@ -2176,6 +2173,19 @@ static netdev_tx_t bcmgenet_xmit(struct sk_buff 
>> *skb, struct net_device *dev)
>>  		goto out;
>>  	}
>> 
>> +	/* The MAC holds a frame to insert its checksum, but only as much as
>> +	 * its FIFO takes. Longer frames are dropped silently.
>> +	 */
>> +	if (unlikely(skb->len > priv->tx_thld_len) &&
>> +	    skb->ip_summed == CHECKSUM_PARTIAL) {
>> +		if (skb_checksum_help(skb)) {
>> +			BCMGENET_STATS64_INC((&ring->stats64), dropped);
>> +			dev_kfree_skb_any(skb);
>> +			ret = NETDEV_TX_OK;
>> +			goto out;
>> +		}
>> +	}
>> +
> 
> [Severity: Medium]
> The new comment says the MAC silently drops CHECKSUM_PARTIAL frames 
> longer
> than tx_thld_len. Can that already happen before this patch?
> 
> An earlier patch in this series, "net: bcmgenet: pad transmit frames 
> out
> of the packet ready window", added bcmgenet_tx_pkt_rdy_thld():
> 
> 	while (thld > ENET_THLD_DEFAULT &&
> 	       ENET_MAX_FRAME_LEN(mtu) - ETH_FCS_LEN > thld * ENET_THLD_UNIT 
> &&
> 	       thld * ENET_THLD_UNIT + ENET_TX_SAFE_MARGIN > mtu + ETH_HLEN)
> 		thld -= ENET_THLD_BURST / ENET_THLD_UNIT;
> 
> On 16K and 64K page kernels ENET_THLD_MAX_LEN is 3840, which makes the 
> old
> ENET_MAX_MTU 3820. For MTUs from 3809 to 3820:
> 
>     mtu + 32 > 3840
>     3840 + 64 > mtu + 14
> 
> So the loop lowers the threshold to 0xe0 and tx_thld_len becomes 3584.
> Untagged frames can still be up to mtu + 14 bytes long.
> 
> In that earlier commit, CHECKSUM_PARTIAL frames longer than 3584 bytes 
> go
> out with DMA_TX_DO_CSUM and have no software fallback. Going by the 
> comment
> above, the MAC loses them.
> 
> Could this fallback move into, or ahead of, the patch that first lets
> tx_thld_len drop below the largest frame? Bisects and backports that 
> stop
> at that patch would then not see silent TX loss on large-page kernels.
> 
> The commit message only mentions the fallback in passing:
> 
>     The MAC only checksums a frame it holds in full, so anything longer 
> than
>     the threshold falls back to software.
> 
> Could it also say that this fixes TX for the earlier patch?
> 
>>  	/* Keep the frame out of the window just past the threshold */
>>  	if (unlikely(skb->len > priv->tx_thld_len &&
>>  		     skb->len < priv->tx_thld_len + ENET_TX_SAFE_MARGIN)) {
> 
> [ ... ]
> 
>> @@ -4272,7 +4358,7 @@ static int bcmgenet_probe(struct platform_device 
>> *pdev)
>>  	/* v1 cannot program the thresholds, so it stays at the default MTU 
>> */
>>  	priv->rx_buf_len = bcmgenet_rx_buf_len(dev->mtu);
>>  	if (!GENET_IS_V1(priv))
>> -		dev->max_mtu = ENET_MAX_MTU;
>> +		dev->max_mtu = ENET_MAX_JUMBO_MTU;
> 
> [Severity: Medium]
> Now that max_mtu is 16347, can the TX side describe frames this large?
> This patch leaves bcmgenet_xmit() unchanged. It still puts each skb 
> head or
> frag in one descriptor and shifts the size in without a mask:
> 
> 	len_stat = (size << DMA_BUFLENGTH_SHIFT) |
> 		   (priv->hw_params->qtag_mask << DMA_TX_QTAG_SHIFT);
> 
> The header defines that field as 12 bits:
> 
> /* Tx/Rx Dma Descriptor common bits*/
> #define DMA_BUFLENGTH_MASK		0x0fff
> #define DMA_BUFLENGTH_SHIFT		16
> 
> Buffers over 4095 bytes are common at jumbo MTUs. Near MTU 9000, a UDP 
> or
> ICMP datagram gets a single linear head from __ip_append_data(). A frag
> from an order-3 page_frag page in TCP or soft-GSO can be almost as 
> large as
> the MSS.
> 
> For a 9014 byte head the field would hold 822, and the upper bits would
> spill above the field.
> 
> If the hardware field really is 12 bits, would these frames go out
> truncated or corrupt while BQL and bytes_sent count the full length?

It is not 12 bits. On a CM4 at MTU 9000 I sent raw single descriptor 
frames
of 1498 to 8998 bytes (20 each). All frames arrived at full length with 
every
payload byte intact. With a 12 bit field 4098 would have gone out as 2 
bytes.

DMA_BUFLENGTH_MASK masks the length out of the RX status word, it is not 
the
hardware field width, so max_mtu needs no cap.

> [...]