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

[Jakub Kicinski <kuba@kernel.org>]

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---
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.

[ ... ]
> +		/* 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.

>  		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.
-- 
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