[Intel-wired-lan] [PATCH net-next] igb: Clarify idleslope config constraints

[Guedes, Andre]

Reviewed-by: Andre Guedes <andre.guedes at intel.com>

On Fri, 2017-11-10 at 14:21 -0800, Jesus Sanchez-Palencia wrote:
> By design, the idleslope increments are restricted to 16.384kbps steps.
> Add a comment to igb_main.c making that explicit and add one example
> that illustrates the impact of that.
> 
> Signed-off-by: Jesus Sanchez-Palencia <jesus.sanchez-palencia at intel.com>
> ---
>  drivers/net/ethernet/intel/igb/igb_main.c | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> diff --git a/drivers/net/ethernet/intel/igb/igb_main.c
> b/drivers/net/ethernet/intel/igb/igb_main.c
> index 43cf39527660..d56c998f3f14 100644
> --- a/drivers/net/ethernet/intel/igb/igb_main.c
> +++ b/drivers/net/ethernet/intel/igb/igb_main.c
> @@ -1744,6 +1744,20 @@ static void igb_configure_cbs(struct igb_adapter
> *adapter, int queue,
>  		 *     value = idleSlope * 61034
>  		 *             --------------
> ---                          (E6)
>  		 *                  1000000
> +		 *
> +		 * NOTE: For i210, given the above, we can see that
> idleslope
> +		 *       is represented in 16.38431 kbps units by the value
> at
> +		 *       the TQAVCC register (1Gbps / 61034), which reduces
> +		 *       the granularity for idleslope increments.
> +		 *       For instance, if you want to configure a 2576kbps
> +		 *       idleslope, the value to be written on the register
> +		 *       would have to be 157.23. If rounded down, you end
> +		 *       up with less bandwidth available than originally
> +		 *       required (~2572 kbps). If rounded up, you end up
> +		 *       with a higher bandwidth (~2589 kbps). Below the
> +		 *       approach we take is to always round up the
> +		 *       calculated value, so the resulting bandwidth might
> +		 *       be slightly higher for some configurations.
>  		 */
>  		value = DIV_ROUND_UP_ULL(idleslope * 61034ULL, 1000000);
>  
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