Re: [PATCH net 2/4] net: ravb: Allow RX loop to move past DMA mapping errors

[Paul Barker <paul.barker.ct@bp.renesas.com>]

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On 14/04/2024 13:17, Niklas Söderlund wrote:
> Hi Paul,
> 
> Thanks for your patch.
> 
> On 2024-04-11 12:44:31 +0100, Paul Barker wrote:
>> The RX loops in ravb_rx_gbeth() and ravb_rx_rcar() skip to the next loop
>> interation if a zero-length descriptor is seen (indicating a DMA mapping
>> error). However, the current rx descriptor index `priv->cur_rx[q]` was
>> incremented at the end of the loop and so would not be incremented when
>> we skip to the next loop iteration. This would cause the loop to keep
>> seeing the same zero-length descriptor instead of moving on to the next
>> descriptor.
>>
>> As the loop counter `i` still increments, the loop would eventually
>> terminate so there is no risk of being stuck here forever - but we
>> should still fix this to avoid wasting cycles.
>>
>> To fix this, the rx descriptor index is incremented at the top of the
>> loop, in the for statement itself. The assignments of `entry` and `desc`
>> are brought into the loop to avoid the need for duplication.
>>
>> Fixes: d8b48911fd24 ("ravb: fix ring memory allocation")
>> Signed-off-by: Paul Barker <paul.barker.ct@bp.renesas.com>
>> ---
>>  drivers/net/ethernet/renesas/ravb_main.c | 25 ++++++++++++------------
>>  1 file changed, 13 insertions(+), 12 deletions(-)
>>
>> diff --git a/drivers/net/ethernet/renesas/ravb_main.c b/drivers/net/ethernet/renesas/ravb_main.c
>> index 70f2900648d4..028ab5c6aaf7 100644
>> --- a/drivers/net/ethernet/renesas/ravb_main.c
>> +++ b/drivers/net/ethernet/renesas/ravb_main.c
>> @@ -775,12 +775,15 @@ static bool ravb_rx_gbeth(struct net_device *ndev, int *quota, int q)
>>  	int limit;
>>  	int i;
>>  
>> -	entry = priv->cur_rx[q] % priv->num_rx_ring[q];
>>  	limit = priv->dirty_rx[q] + priv->num_rx_ring[q] - priv->cur_rx[q];
>>  	stats = &priv->stats[q];
>>  
>> -	desc = &priv->rx_ring[q].desc[entry];
>> -	for (i = 0; i < limit && rx_packets < *quota && desc->die_dt != DT_FEMPTY; i++) {
>> +	for (i = 0; i < limit && rx_packets < *quota; i++, priv->cur_rx[q]++) {
>> +		entry = priv->cur_rx[q] % priv->num_rx_ring[q];
>> +		desc = &priv->rx_ring[q].desc[entry];
>> +		if (desc->die_dt == DT_FEMPTY)
>> +			break;
>> +
>>  		/* Descriptor type must be checked before all other reads */
>>  		dma_rmb();
>>  		desc_status = desc->msc;
>> @@ -848,9 +851,6 @@ static bool ravb_rx_gbeth(struct net_device *ndev, int *quota, int q)
>>  				break;
>>  			}
>>  		}
>> -
>> -		entry = (++priv->cur_rx[q]) % priv->num_rx_ring[q];
>> -		desc = &priv->rx_ring[q].desc[entry];
>>  	}
>>  
>>  	/* Refill the RX ring buffers. */
>> @@ -891,7 +891,6 @@ static bool ravb_rx_rcar(struct net_device *ndev, int *quota, int q)
>>  {
>>  	struct ravb_private *priv = netdev_priv(ndev);
>>  	const struct ravb_hw_info *info = priv->info;
>> -	int entry = priv->cur_rx[q] % priv->num_rx_ring[q];
>>  	struct net_device_stats *stats = &priv->stats[q];
>>  	struct ravb_ex_rx_desc *desc;
>>  	struct sk_buff *skb;
>> @@ -900,12 +899,17 @@ static bool ravb_rx_rcar(struct net_device *ndev, int *quota, int q)
>>  	int rx_packets = 0;
>>  	u8  desc_status;
>>  	u16 pkt_len;
>> +	int entry;
>>  	int limit;
>>  	int i;
>>  
>>  	limit = priv->dirty_rx[q] + priv->num_rx_ring[q] - priv->cur_rx[q];
>> -	desc = &priv->rx_ring[q].ex_desc[entry];
>> -	for (i = 0; i < limit && rx_packets < *quota && desc->die_dt != DT_FEMPTY; i++) {
>> +	for (i = 0; i < limit && rx_packets < *quota; i++, priv->cur_rx[q]++) {
>> +		entry = priv->cur_rx[q] % priv->num_rx_ring[q];
>> +		desc = &priv->rx_ring[q].ex_desc[entry];
>> +		if (desc->die_dt == DT_FEMPTY)
>> +			break;
> 
> I really like moving the assignment of entry and desc to the top of the 
> loop. But I don't like the loop limits as it's hard, at least for me, to 
> immediately see what's going on. How about,
> 
>     limit = min(priv->dirty_rx[q] + priv->num_rx_ring[q] - priv->cur_rx[q], *quota);
> 
>     for (i = 0; i < limit; i++) {
>         entry = priv->cur_rx[q] % priv->num_rx_ring[q];
>         desc = &priv->rx_ring[q].ex_desc[entry];
> 
>         /* There are no more valid descriptors after an empty one. */
>         if (desc->die_dt == DT_FEMPTY)
>                 break;
> 
>         ...
>     }

We need to count received packets separately from the number of
descriptors processed, as done in the previous commit in this series,
so we can't just have a single check against limit.

We also need to increment priv->cur_rx[q]. If we put `priv->cur_rx[q]++`
at the end of the loop then we're back to having to worry about it when
we have a continue statement.

We could move the `rx_packets < *quota` check inside the loop itself,
but I don't see that as any clearer myself.

Thanks,

-- 
Paul Barker

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