Re: [PATCH net] net: airoha: Add missing bits in airoha_qdma_cleanup_tx_queue()

[Lorenzo Bianconi <lorenzo@kernel.org>]

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> This is an AI-generated review of your patch. The human sending this
> email has considered the AI review valid, or at least plausible.
> ---
> net: airoha: Add missing bits in airoha_qdma_cleanup_tx_queue()
> 
> This commit resets DMA TX descriptors in airoha_qdma_cleanup_tx_queue
> and synchronizes TX_DMA_IDX to TX_CPU_IDX to notify the hardware that
> the ring is empty.
> 
> > diff --git a/drivers/net/ethernet/airoha/airoha_eth.c b/drivers/net/ethernet/airoha/airoha_eth.c
> > index 9285a68f435fe..963ab7b8d166b 100644
> > --- a/drivers/net/ethernet/airoha/airoha_eth.c
> > +++ b/drivers/net/ethernet/airoha/airoha_eth.c
> > @@ -1044,13 +1044,17 @@ static int airoha_qdma_init_tx(struct airoha_qdma *qdma)
> [ ... ]
> >  	spin_lock_bh(&q->lock);
> >  	for (i = 0; i < q->ndesc; i++) {
> > -		struct airoha_queue_entry *e = &q->entry[i];
> > +		struct airoha_qdma_desc *desc = &q->desc[i];
> >  
> > +		e = &q->entry[i];
> 
> Can this result in a NULL pointer dereference?
> 
> If airoha_qdma_init_tx_queue() sets q->ndesc before allocating q->entry and
> q->desc, and devm_kzalloc() fails, the error path jumps to
> airoha_hw_cleanup().
> 
> Because q->ndesc > 0, this cleanup loop would execute and dereference
> q->entry[i] and q->desc[i], which may not have been allocated.

ack, I will fix it in v2.

> 
> [ ... ]
> > +		/* Reset DMA descriptor */
> > +		WRITE_ONCE(desc->ctrl, 0);
> > +		WRITE_ONCE(desc->addr, 0);
> > +		WRITE_ONCE(desc->data, 0);
> > +		WRITE_ONCE(desc->msg0, 0);
> > +		WRITE_ONCE(desc->msg1, 0);
> > +		WRITE_ONCE(desc->msg2, 0);
> 
> Is it safe to modify these DMA descriptors and reset the ring indices below
> without waiting for the hardware DMA engine to become idle?
> 
> In airoha_dev_stop(), the driver clears GLOBAL_CFG_TX_DMA_EN_MASK to request
> the hardware DMA engine to stop, but it does not appear to poll
> GLOBAL_CFG_TX_DMA_BUSY_MASK to ensure it has actually stopped.
> 
> Modifying the DMA descriptors and internal ring indices while the hardware
> is still actively processing in-flight descriptors could potentially lead
> to corrupted DMA fetches or IOMMU faults.

This is already fixed here:
https://lore.kernel.org/netdev/20260411-airoha-fix-ndo_stop-v1-1-caddaa181739@kernel.org/

> 
> >  		q->queued--;
> >  	}
> > +
> > +	e = list_first_entry(&q->tx_list, struct airoha_queue_entry,
> > +			     list);
> > +	index = e - q->entry;
> > +	/* Set TX_DMA_IDX to TX_CPU_IDX to notify the hw the QDMA TX ring is
> > +	 * empty.
> > +	 */
> > +	airoha_qdma_rmw(qdma, REG_TX_CPU_IDX(qid), TX_RING_CPU_IDX_MASK,
> > +			FIELD_PREP(TX_RING_CPU_IDX_MASK, index));
> > +	airoha_qdma_rmw(qdma, REG_TX_DMA_IDX(qid), TX_RING_DMA_IDX_MASK,
> > +			FIELD_PREP(TX_RING_DMA_IDX_MASK, index));
> 
> Could list_first_entry() be called on an empty list here?
> 
> If q->tx_list is empty (for example, if allocation failed in
> airoha_qdma_init_tx_queue or if no entries had a valid dma_addr), calling
> list_first_entry() returns an invalid pointer.
> 
> The subsequent pointer arithmetic would produce a garbage index value,
> which is then written to the REG_TX_CPU_IDX and REG_TX_DMA_IDX registers.

ack, I will fix it in v2.

Regards,
Lorenzo

> -- 
> pw-bot: cr

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