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authorJesus Sanchez-Palencia <jesus.sanchez-palencia@intel.com>2017-11-10 14:21:50 -0800
committerJeff Kirsher <jeffrey.t.kirsher@intel.com>2018-01-24 12:27:48 -0800
commit0da6090ff70935d71f1c023def59e741f3ae322b (patch)
tree6d7ca4f60b1ab69bf3438d2239c6eecc40cb17d3 /drivers/net/ethernet/intel/igb
parente1000e: Set HTHRESH when PTHRESH is used (diff)
downloadlinux-dev-0da6090ff70935d71f1c023def59e741f3ae322b.tar.xz
linux-dev-0da6090ff70935d71f1c023def59e741f3ae322b.zip
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 <jesus.sanchez-palencia@intel.com> Tested-by: Aaron Brown <aaron.f.brown@intel.com> Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Diffstat (limited to 'drivers/net/ethernet/intel/igb')
-rw-r--r--drivers/net/ethernet/intel/igb/igb_main.c14
1 files changed, 14 insertions, 0 deletions
diff --git a/drivers/net/ethernet/intel/igb/igb_main.c b/drivers/net/ethernet/intel/igb/igb_main.c
index 21a34e9e2645..78fe10819c3a 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);