From 0da6090ff70935d71f1c023def59e741f3ae322b Mon Sep 17 00:00:00 2001 From: Jesus Sanchez-Palencia Date: Fri, 10 Nov 2017 14:21:50 -0800 Subject: [PATCH] igb: Clarify idleslope config constraints 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 Tested-by: Aaron Brown Signed-off-by: Jeff Kirsher --- 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 21a34e9..78fe108 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); -- 2.7.4