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@xxxxxxxxxxxxxx>
>> ---
>> 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,
--
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