Re: [PATCH 1/2] ixgbe: fix VF statistic wraparound handling macro

["Roger B. Melton" <rmelton@cisco.com>]

Agreed, this handles the off by one error on wrap around and should be 
faster.

-Roger


On 10/12/15 11:41 AM, Alexander Duyck wrote:
> On 10/12/2015 06:33 AM, Harry van Haaren wrote:
>> Fix a misinterpretation of VF stats in ixgbe
>>
>> Signed-off-by: Harry van Haaren <harry.van.haaren@intel.com>
>> ---
>>   drivers/net/ixgbe/ixgbe_ethdev.c | 8 ++++++--
>>   1 file changed, 6 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/net/ixgbe/ixgbe_ethdev.c 
>> b/drivers/net/ixgbe/ixgbe_ethdev.c
>> index ec2918c..d226e8d 100644
>> --- a/drivers/net/ixgbe/ixgbe_ethdev.c
>> +++ b/drivers/net/ixgbe/ixgbe_ethdev.c
>> @@ -329,10 +329,14 @@ static int 
>> ixgbe_timesync_read_tx_timestamp(struct rte_eth_dev *dev,
>>   /*
>>    * Define VF Stats MACRO for Non "cleared on read" register
>>    */
>> -#define UPDATE_VF_STAT(reg, last, cur)                            \
>> +#define UPDATE_VF_STAT(reg, last, cur) \
>> { \
>>       uint32_t latest = IXGBE_READ_REG(hw, reg);              \
>> -    cur += latest - last;                                   \
>> +    if(likely(latest > last)) { \
>> +        cur += latest - last;                           \
>> +    } else {                                                \
>> +        cur += (UINT_MAX - last) + latest;              \
>> +    }                                                       \
>>       last = latest;                                          \
>>   }
>
> From what I can tell your math is adding an off by one error.  You 
> should probably be using UINT_MAX as a mask for the result, not as a 
> part of the calculation itself.
>
> So the correct way to compute this would be "cur += (latest - last) & 
> UINT_MAX".  Also the mask approach should be faster as it avoids any 
> conditional jumps.
>
> - Alex
> .
>

-- 
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