[net-next,11/14] ixgbe: Prevent u8 wrapping of ITR value to something less than 10us

From: Alexander Duyck <alexander.h.duyck@linux.intel.com>

There were a couple cases where the ITR value generated via the adaptive
ITR scheme could exceed 126. This resulted in the value becoming either 0
or something less than 10. Switching back and forth between a value less
than 10 and a value greater than 10 can cause issues as certain hardware
features such as RSC to not function well when the ITR value has dropped
that low.

Reported-by: Gregg Leventhal <gleventhal@janestreet.com>
Signed-off-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
---
 drivers/net/ethernet/intel/ixgbe/ixgbe_main.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

On Tue, 2019-09-10 at 09:34 -0700, Jeff Kirsher wrote:
> From: Alexander Duyck <alexander.h.duyck@linux.intel.com>
> 
> There were a couple cases where the ITR value generated via the adaptive
> ITR scheme could exceed 126. This resulted in the value becoming either 0
> or something less than 10. Switching back and forth between a value less
> than 10 and a value greater than 10 can cause issues as certain hardware
> features such as RSC to not function well when the ITR value has dropped
> that low.

One quick thing we can add on here is:
Fixes: b4ded8327fea ("ixgbe: Update adaptive ITR algorithm")

This is likely something that we may want to backport to stable.

> Reported-by: Gregg Leventhal <gleventhal@janestreet.com>
> Signed-off-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>
> Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
> Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
> ---
>  drivers/net/ethernet/intel/ixgbe/ixgbe_main.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
> index dc034f4e8cf6..a5398b691aa8 100644
> --- a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
> +++ b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
> @@ -2623,7 +2623,7 @@ static void ixgbe_update_itr(struct ixgbe_q_vector *q_vector,
>  		/* 16K ints/sec to 9.2K ints/sec */
>  		avg_wire_size *= 15;
>  		avg_wire_size += 11452;
> -	} else if (avg_wire_size <= 1980) {
> +	} else if (avg_wire_size < 1968) {
>  		/* 9.2K ints/sec to 8K ints/sec */
>  		avg_wire_size *= 5;
>  		avg_wire_size += 22420;
> @@ -2656,6 +2656,8 @@ static void ixgbe_update_itr(struct ixgbe_q_vector *q_vector,
>  	case IXGBE_LINK_SPEED_2_5GB_FULL:
>  	case IXGBE_LINK_SPEED_1GB_FULL:
>  	case IXGBE_LINK_SPEED_10_FULL:
> +		if (avg_wire_size > 8064)
> +			avg_wire_size = 8064;
>  		itr += DIV_ROUND_UP(avg_wire_size,
>  				    IXGBE_ITR_ADAPTIVE_MIN_INC * 64) *
>  		       IXGBE_ITR_ADAPTIVE_MIN_INC;

From: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Date: Tue, 10 Sep 2019 09:34:31 -0700

> From: Alexander Duyck <alexander.h.duyck@linux.intel.com>
> 
> There were a couple cases where the ITR value generated via the adaptive
> ITR scheme could exceed 126. This resulted in the value becoming either 0
> or something less than 10. Switching back and forth between a value less
> than 10 and a value greater than 10 can cause issues as certain hardware
> features such as RSC to not function well when the ITR value has dropped
> that low.
> 
> Reported-by: Gregg Leventhal <gleventhal@janestreet.com>
> Signed-off-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>
> Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
> Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>

Please remove this from the series, add the Fixes: tag, and submit for 'net'
since Alexander says it is -stable material.

Thanks.