Re: [PATCH v2] net: stmmac: xgmac2: disable RBUE in default RX interrupt mask

[Nazle Asmade, Muhammad Nazim Amirul]

On 9/6/2026 5:12 pm, Paolo Abeni wrote:
> On 6/8/26 5:27 PM, Simon Horman wrote:
>> From: 'Simon Horman' <horms@xxxxxxxxxx>
>>
>> This is an AI-generated review of your patch. The human sending this
>> email has considered the AI review valid, or at least plausible.
>> Full review at: https://sashiko.dev/
>> ---
>> net: stmmac: xgmac2: disable RBUE in default RX interrupt mask
>>
>> This commit removes the RX Buffer Unavailable (RBUE) interrupt from the
>> default enable masks for XGMAC2 devices. This is intended to prevent MAC
>> interrupt storms that can occur under heavy RX pressure when the DMA
>> runs out of RX descriptors.
>>
>>> diff --git a/drivers/net/ethernet/stmicro/stmmac/dwxgmac2.h b/drivers/net/ethernet/stmicro/stmmac/dwxgmac2.h
>>> index 51943705a2b03..b5fdc253a2f24 100644
>>> --- a/drivers/net/ethernet/stmicro/stmmac/dwxgmac2.h
>>> +++ b/drivers/net/ethernet/stmicro/stmmac/dwxgmac2.h
>>> @@ -379,9 +379,9 @@
>>>   #define XGMAC_RIE                 BIT(6)
>>>   #define XGMAC_TBUE                        BIT(2)
>>>   #define XGMAC_TIE                 BIT(0)
>>> -#define XGMAC_DMA_INT_DEFAULT_EN   (XGMAC_NIE | XGMAC_AIE | XGMAC_RBUE | \
>>> +#define XGMAC_DMA_INT_DEFAULT_EN   (XGMAC_NIE | XGMAC_AIE | \
>>>                                     XGMAC_RIE | XGMAC_TIE)
>>> -#define XGMAC_DMA_INT_DEFAULT_RX   (XGMAC_RBUE | XGMAC_RIE)
>>> +#define XGMAC_DMA_INT_DEFAULT_RX   (XGMAC_RIE)
>>
>> [Severity: High]
>> Does removing the RBUE interrupt break the driver's ability to recover from
>> RX out-of-memory conditions, leading to a permanent RX queue stall?
>>
>> When stmmac_rx() processes packets, it calls stmmac_rx_refill() to replenish
>> the ring descriptors. If page_pool_alloc_pages() fails due to memory
>> pressure, the descriptors remain empty:
>>
>> drivers/net/ethernet/stmicro/stmmac/stmmac_main.c:stmmac_rx_refill() {
>>      ...
>>      buf->page = page_pool_alloc_pages(rx_q->page_pool, gfp);
>>      if (!buf->page)
>>          break;
>>      ...
>> }
>>
>> stmmac_rx() then returns the number of processed packets, which will be
>> less than the NAPI budget if the ring is empty. This causes
>> stmmac_napi_poll_rx() to complete NAPI and re-enable interrupts:
>>
>> drivers/net/ethernet/stmicro/stmmac/stmmac_main.c:stmmac_napi_poll_rx() {
>>      ...
>>      work_done = stmmac_rx(priv, budget, chan);
>>      if (work_done < budget && napi_complete_done(napi, work_done)) {
>>          ...
>>          stmmac_enable_dma_irq(priv, priv->ioaddr, chan, 1, 0);
>>      }
>>      ...
>> }
>>
>> When the hardware subsequently tries to receive a packet, it finds no
>> descriptors and enters the Receive Buffer Unavailable (RBU) state. By
>> removing XGMAC_RBUE from the RX interrupt enable mask, the MAC will no
>> longer trigger an interrupt for the RBU state.
>>
>> Because no packets can be successfully received without descriptors, the
>> normal Receive Interrupt (RIE) will also never fire. With NAPI stopped and
>> no interrupts firing, the driver has no polling timer to retry allocation,
>> which could result in a permanent stall of the RX queue.
>
> Nazim, you need at least to clarify in the commit message why this patch
> is safe.
>
> /P
>
Hi Simon, Paolo

Thanks for the review! Will send out v3 with commit message updated.

BR,
Nazim