net: cdc_ncm: use ethtool to tune coalescing settings

Datagram coalescing is an integral part of the NCM and MBIM
protocols, intended to reduce the interrupt load primarily
on the device end of the USB link.  As with all coalescing
solutions, there is a trade-off between buffering and
interrupts.

The current defaults are based on the assumption that device
side buffers should be the limiting factor.  However, many
modern high speed LTE modems suffers from buffer-bloat,
making this assumption fail. This results in sub-optimal
performance due to excessive coalescing.  And in cases where
such modems are connected to cheap embedded hosts there is
often severe buffer allocation issues, giving very noticeable
performance degradation .

A start on improving this is going from build time hard
coded limits to per device user configurable limits.  The
ethtool coalescing API was selected as user interface
because, although the tuned values are buffer sizes, these
settings directly control datagram coalescing.

Signed-off-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 5 insertions, 1 deletions

diff --git a/include/linux/usb/cdc_ncm.h b/include/linux/usb/cdc_ncm.h
index 55b6feead93b..5c1066b4dc41 100644
--- a/include/linux/usb/cdc_ncm.h
+++ b/include/linux/usb/cdc_ncm.h
@@ -72,7 +72,9 @@
 /* Restart the timer, if amount of datagrams is less than given value */
 #define CDC_NCM_RESTART_TIMER_DATAGRAM_CNT      3
 #define CDC_NCM_TIMER_PENDING_CNT               2
-#define CDC_NCM_TIMER_INTERVAL                  (400UL * NSEC_PER_USEC)
+#define CDC_NCM_TIMER_INTERVAL_USEC             400UL
+#define CDC_NCM_TIMER_INTERVAL_MIN              5UL
+#define CDC_NCM_TIMER_INTERVAL_MAX              (15UL * USEC_PER_SEC)
 
 /* The following macro defines the minimum header space */
 #define CDC_NCM_MIN_HDR_SIZE \
@@ -107,6 +109,8 @@ struct cdc_ncm_ctx {
         spinlock_t mtx;
         atomic_t stop;
 
+        u64 timer_interval;
+
         u32 tx_timer_pending;
         u32 tx_curr_frame_num;
         u32 rx_max;