RE: [PATCH net v4 1/6] net: stmmac: Fix incorrect location to set real_num_rx|tx_queues

[Ong, Boon Leong]

From: David Miller <davem@xxxxxxxxxxxxx>
Date: Wednesday, February 5, 2020 9:39 PM

>From: Ong Boon Leong <boon.leong.ong@xxxxxxxxx>
>Date: Wed,  5 Feb 2020 16:55:05 +0800
>
>> diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
>b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
>> index 5836b21edd7e..4d9afa13eeb9 100644
>> --- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
>> +++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
>> @@ -2657,6 +2657,10 @@ static int stmmac_hw_setup(struct net_device
>*dev, bool init_ptp)
>>  >--->-------stmmac_enable_tbs(priv, priv->ioaddr, enable, chan);
>>  >---}
>>
>> +>---/* Configure real RX and TX queues */
>> +>---netif_set_real_num_rx_queues(dev, priv->plat->rx_queues_to_use);
>> +>---netif_set_real_num_tx_queues(dev, priv->plat->tx_queues_to_use);
>> +
>>  >---/* Start the ball rolling... */
>>  >---stmmac_start_all_dma(priv);
>>
>
>It is only safe to ignore the return values from
>netif_set_real_num_{rx,tx}_queues() if you call them before the
>network device is registered.  Because only in that case are these
>functions guaranteed to succeed.
>
>But now that you have moved these calls here, they can fail.
>
>Therefore you must check the return value and unwind the state
>completely upon failures.
>
>Honestly, I think this change will have several undesirable side effects:
>
>1) Lots of added new code complexity
>
>2) A new failure mode when resuming the device, users will find this
>   very hard to diagnose and recover from
>
>What real value do you get from doing these calls after probe?
>
>If you can't come up with a suitable answer to that question, you
>should reconsider this change.
>
>Thanks.

We have patch that implements get|set_channels() that depends on this fix.
Anyway, we understand your insight and perspective now. So, we will drop
this patch in v5 series.

Thanks