Subject: Re: [PATCH net] net: xilinx: axienet: Fix IRQ coalescing packet count overflow

From: Sean Anderson <[email-masked]>
Date: Thu, 5 Sep 2024 10:34:15 -0400

On 9/4/24 12:00, Simon Horman wrote:
> On Tue, Sep 03, 2024 at 02:00:59PM -0400, Sean Anderson wrote:
>> If coalesce_count is greater than 255 it will not fit in the register and
>> will overflow. Clamp it to 255 for more-predictable results.
> 
> Hi Sean,
> 
> Can this occur in practice?

Yes. Simply do `ethtool -C ethX rx-frames 300` or something similar and
you will end up with a limit of 44 instead. I ran into this with DIM and
was wondering why the highest-throughput setting (256) was behaving so
poorly...

>> 
>> Signed-off-by: Sean Anderson <[email-masked]>
>> Fixes: 8a3b7a252dca ("drivers/net/ethernet/xilinx: added Xilinx AXI Ethernet driver")
> 
> nit: I think it is usual for the order of these tags to be reversed.

OK

>> ---
>> 
>>  drivers/net/ethernet/xilinx/xilinx_axienet_main.c | 6 ++++--
>>  1 file changed, 4 insertions(+), 2 deletions(-)
>> 
>> diff --git a/drivers/net/ethernet/xilinx/xilinx_axienet_main.c b/drivers/net/ethernet/xilinx/xilinx_axienet_main.c
>> index 9aeb7b9f3ae4..5f27fc1c4375 100644
>> --- a/drivers/net/ethernet/xilinx/xilinx_axienet_main.c
>> +++ b/drivers/net/ethernet/xilinx/xilinx_axienet_main.c
>> @@ -252,7 +252,8 @@ static u32 axienet_usec_to_timer(struct axienet_local *lp, u32 coalesce_usec)
>>  static void axienet_dma_start(struct axienet_local *lp)
>>  {
>>  	/* Start updating the Rx channel control register */
>> -	lp->rx_dma_cr = (lp->coalesce_count_rx << XAXIDMA_COALESCE_SHIFT) |
>> +	lp->rx_dma_cr = (min(lp->coalesce_count_rx, 255) <<
>> +			 XAXIDMA_COALESCE_SHIFT) |
>>  			XAXIDMA_IRQ_IOC_MASK | XAXIDMA_IRQ_ERROR_MASK;
> 
> nit: it would be nice to avoid using a naked 255 here.
>      Perhaps: #define XAXIDMA_COALESCE_MAX 0xff

OK, but this is the same as the limit used in axienet_usec_to_timer.

--Sean

>>  	/* Only set interrupt delay timer if not generating an interrupt on
>>  	 * the first RX packet. Otherwise leave at 0 to disable delay interrupt.
>> @@ -264,7 +265,8 @@ static void axienet_dma_start(struct axienet_local *lp)
>>  	axienet_dma_out32(lp, XAXIDMA_RX_CR_OFFSET, lp->rx_dma_cr);
>>  
>>  	/* Start updating the Tx channel control register */
>> -	lp->tx_dma_cr = (lp->coalesce_count_tx << XAXIDMA_COALESCE_SHIFT) |
>> +	lp->tx_dma_cr = (min(lp->coalesce_count_tx, 255) <<
>> +			 XAXIDMA_COALESCE_SHIFT) |
>>  			XAXIDMA_IRQ_IOC_MASK | XAXIDMA_IRQ_ERROR_MASK;
>>  	/* Only set interrupt delay timer if not generating an interrupt on
>>  	 * the first TX packet. Otherwise leave at 0 to disable delay interrupt.
>> -- 
>> 2.35.1.1320.gc452695387.dirty
>> 
>>