Subject: Re: [PATCH net v2] net: xilinx: axienet: Fix packet counting

From: Sean Anderson <[email-masked]>
Date: Tue, 10 Sep 2024 10:24:36 -0400

On 9/9/24 21:00, Jakub Kicinski wrote:
> On Fri,  6 Sep 2024 12:42:27 -0400 Sean Anderson wrote:
>> axienet_free_tx_chain returns the number of DMA descriptors it's
>> handled. However, axienet_tx_poll treats the return as the number of
>> packets. When scatter-gather SKBs are enabled, a single packet may use
>> multiple DMA descriptors, which causes incorrect packet counts. Fix this
>> by explicitly keepting track of the number of packets processed as
>> separate from the DMA descriptors.
>> 
>> Fixes: 8a3b7a252dca ("drivers/net/ethernet/xilinx: added Xilinx AXI Ethernet driver")
>> Signed-off-by: Sean Anderson <[email-masked]>
> 
>> diff --git a/drivers/net/ethernet/xilinx/xilinx_axienet_main.c b/drivers/net/ethernet/xilinx/xilinx_axienet_main.c
>> index 9aeb7b9f3ae4..556033849d55 100644
>> --- a/drivers/net/ethernet/xilinx/xilinx_axienet_main.c
>> +++ b/drivers/net/ethernet/xilinx/xilinx_axienet_main.c
>> @@ -670,21 +670,21 @@ static int axienet_device_reset(struct net_device *ndev)
>>   * @force:	Whether to clean descriptors even if not complete
>>   * @sizep:	Pointer to a u32 filled with the total sum of all bytes
>>   *		in all cleaned-up descriptors. Ignored if NULL.
>> - * @budget:	NAPI budget (use 0 when not called from NAPI poll)
>> + * @budget:	NAPI budget (use INT_MAX when not called from NAPI poll)
> 
> use INT_MAX and force=true when ... ?
> To make sure the dependency is clear.
> But actually...
> 
>>   *
>>   * Would either be called after a successful transmit operation, or after
>>   * there was an error when setting up the chain.
>> - * Returns the number of descriptors handled.
>> + * Returns the number of packets handled.
>>   */
>>  static int axienet_free_tx_chain(struct axienet_local *lp, u32 first_bd,
>>  				 int nr_bds, bool force, u32 *sizep, int budget)
>>  {
>>  	struct axidma_bd *cur_p;
>>  	unsigned int status;
>> +	int i, packets = 0;
>>  	dma_addr_t phys;
>> -	int i;
>>  
>> -	for (i = 0; i < nr_bds; i++) {
>> +	for (i = 0; i < nr_bds && packets < budget; i++) {
> 
> why are you doing this? To make sure drivers doesn't complete more 
> than "budget" Tx skbs? The budget is really for Rx, for Tx you can
> use a reasonable fixed value, independent of what budget core
> passes in, e.g. 128. See:
> https://www.kernel.org/doc/html/next/networking/napi.html#datapath-api

I read this but it was unclear to me because it seems oriented towards
"combined" NAPI instances, while we have separate instances for RX and
TX. So even for TX-only instances, we can ignore budget?

>>  		cur_p = &lp->tx_bd_v[(first_bd + i) % lp->tx_bd_num];
>>  		status = cur_p->status;
>>  
>> @@ -701,8 +701,10 @@ static int axienet_free_tx_chain(struct axienet_local *lp, u32 first_bd,
>>  				 (cur_p->cntrl & XAXIDMA_BD_CTRL_LENGTH_MASK),
>>  				 DMA_TO_DEVICE);
>>  
>> -		if (cur_p->skb && (status & XAXIDMA_BD_STS_COMPLETE_MASK))
>> -			napi_consume_skb(cur_p->skb, budget);
>> +		if (cur_p->skb && (status & XAXIDMA_BD_STS_COMPLETE_MASK)) {
>> +			napi_consume_skb(cur_p->skb, force ? 0 : budget);
>> +			packets++;
>> +		}
>>  
>>  		cur_p->app0 = 0;
>>  		cur_p->app1 = 0;
>> @@ -718,7 +720,13 @@ static int axienet_free_tx_chain(struct axienet_local *lp, u32 first_bd,
>>  			*sizep += status & XAXIDMA_BD_STS_ACTUAL_LEN_MASK;
>>  	}
>>  
>> -	return i;
>> +	if (!force) {
>> +		lp->tx_bd_ci += i;
>> +		if (lp->tx_bd_ci >= lp->tx_bd_num)
>> +			lp->tx_bd_ci %= lp->tx_bd_num;
>> +	}
> 
> Moving this chunk into axienet_free_tx_chain() is a noop, right?
> Please avoid code cleanups in fixes.

The relevant variable (number of descriptors handled) is no longer
returned to axienet_tx_poll, so it can't update the current descriptor
properly.

--Sean

>> +	return packets;
>>  }
>>  /**