netdev - Re: [PATCH net-next] net: bcmgenet: collect Rx discarded packet count

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Date:	Tue, 10 Mar 2015 13:52:32 -0700
From:	Petri Gynther <pgynther@...gle.com>
To:	Florian Fainelli <f.fainelli@...il.com>
Cc:	netdev@...r.kernel.org, David Miller <davem@...emloft.net>,
	Jaedon Shin <jaedon.shin@...il.com>
Subject: Re: [PATCH net-next] net: bcmgenet: collect Rx discarded packet count

Hi Florian,

On Tue, Mar 10, 2015 at 12:43 PM, Florian Fainelli <f.fainelli@...il.com> wrote:
> On 10/03/15 12:18, Petri Gynther wrote:
>> Bits 31:16 of RDMA_PROD_INDEX contain Rx discarded packet count, which
>> are the Rx packets that had to be dropped by MAC hardware since there
>> was no room on the Rx queue. Add code to collect this information into
>> the netdev stats.
>>
>> Signed-off-by: Petri Gynther <pgynther@...gle.com>
>> ---
>>  drivers/net/ethernet/broadcom/genet/bcmgenet.c | 10 ++++++++++
>>  1 file changed, 10 insertions(+)
>>
>> diff --git a/drivers/net/ethernet/broadcom/genet/bcmgenet.c b/drivers/net/ethernet/broadcom/genet/bcmgenet.c
>> index 275be56..7aa1834 100644
>> --- a/drivers/net/ethernet/broadcom/genet/bcmgenet.c
>> +++ b/drivers/net/ethernet/broadcom/genet/bcmgenet.c
>> @@ -1384,9 +1384,19 @@ static unsigned int bcmgenet_desc_rx(struct bcmgenet_priv *priv,
>>       int len, err;
>>       unsigned int rxpktprocessed = 0, rxpkttoprocess;
>>       unsigned int p_index;
>> +     unsigned int discards;
>>       unsigned int chksum_ok = 0;
>>
>>       p_index = bcmgenet_rdma_ring_readl(priv, index, RDMA_PROD_INDEX);
>> +
>> +     discards = (p_index >> DMA_P_INDEX_DISCARD_CNT_SHIFT) &
>> +                DMA_P_INDEX_DISCARD_CNT_MASK;
>> +     if (discards > 0) {
>> +             bcmgenet_rdma_ring_writel(priv, index, 0, RDMA_PROD_INDEX);
>
> This adds an expensive register write (~300ns on MIPS, ~200ns on ARM) in
> the hot-path, and the counter saturation happens at 0xffff, which I
> would prefer we deal with explicitly, even though that means missing a
> bunch of discard events once we have already saturated, rather than
> resetting this counter *and* the producer index for every NAPI round we
> get called.
>
> You could also deal with this counter in the ethtool gstats functions
> and perform the saturation handle there, since this is a slow path already.
>

It is a rare event that discards > 0. Thus, the extra register write
happens very infrequently.

And, writing 0 to RDMA_PROD_INDEX does not reset the producer index
bits 15:0 that are used in Rx processing.

>> +             dev->stats.rx_missed_errors += discards;
>> +             dev->stats.rx_errors += discards;
>> +     }
>> +
>>       p_index &= DMA_P_INDEX_MASK;
>>
>>       if (likely(p_index >= ring->c_index))
>>
>
>
> --
> Florian
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