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Diffstat (limited to 'drivers/net/ethernet/intel/e1000e/e1000.h')
-rw-r--r--drivers/net/ethernet/intel/e1000e/e1000.h20
1 files changed, 20 insertions, 0 deletions
diff --git a/drivers/net/ethernet/intel/e1000e/e1000.h b/drivers/net/ethernet/intel/e1000e/e1000.h
index a187582d2299..ba9c19e6994c 100644
--- a/drivers/net/ethernet/intel/e1000e/e1000.h
+++ b/drivers/net/ethernet/intel/e1000e/e1000.h
@@ -360,23 +360,43 @@ s32 e1000e_get_base_timinca(struct e1000_adapter *adapter, u32 *timinca);
* As a result, a shift of INCVALUE_SHIFT_n is used to fit a value of
* INCVALUE_n into the TIMINCA register allowing 32+8+(24-INCVALUE_SHIFT_n)
* bits to count nanoseconds leaving the rest for fractional nonseconds.
+ *
+ * Any given INCVALUE also has an associated maximum adjustment value. This
+ * maximum adjustment value is the largest increase (or decrease) which can be
+ * safely applied without overflowing the INCVALUE. Since INCVALUE has
+ * a maximum range of 24 bits, its largest value is 0xFFFFFF.
+ *
+ * To understand where the maximum value comes from, consider the following
+ * equation:
+ *
+ * new_incval = base_incval + (base_incval * adjustment) / 1billion
+ *
+ * To avoid overflow that means:
+ * max_incval = base_incval + (base_incval * max_adj) / billion
+ *
+ * Re-arranging:
+ * max_adj = floor(((max_incval - base_incval) * 1billion) / 1billion)
*/
#define INCVALUE_96MHZ 125
#define INCVALUE_SHIFT_96MHZ 17
#define INCPERIOD_SHIFT_96MHZ 2
#define INCPERIOD_96MHZ (12 >> INCPERIOD_SHIFT_96MHZ)
+#define MAX_PPB_96MHZ 23999900 /* 23,999,900 ppb */
#define INCVALUE_25MHZ 40
#define INCVALUE_SHIFT_25MHZ 18
#define INCPERIOD_25MHZ 1
+#define MAX_PPB_25MHZ 599999900 /* 599,999,900 ppb */
#define INCVALUE_24MHZ 125
#define INCVALUE_SHIFT_24MHZ 14
#define INCPERIOD_24MHZ 3
+#define MAX_PPB_24MHZ 999999999 /* 999,999,999 ppb */
#define INCVALUE_38400KHZ 26
#define INCVALUE_SHIFT_38400KHZ 19
#define INCPERIOD_38400KHZ 1
+#define MAX_PPB_38400KHZ 230769100 /* 230,769,100 ppb */
/* Another drawback of scaling the incvalue by a large factor is the
* 64-bit SYSTIM register overflows more quickly. This is dealt with