There are inconsistencies with the timing report where it incorrectly reports failing paths.
In the start of the timing report, the following incorrect timing errors appear:
================================================================================
Timing constraint: TIMEGRP "IN_GMII" OFFSET = IN 2 ns VALID 2 ns BEFORE COMP
"gmii_tx_clk";
10 paths analyzed, 10 endpoints analyzed, 10 failing endpoints
20 timing errors detected. (10 setup errors, 10 hold errors)
Minimum allowable offset is 2.069ns.
----------------------------------------------------------------------------------------------------------------------
Then, at the end of the report (in the details), there are no timing failures:
TIMEGRP "IN_GMII" OFFSET = IN 2 ns VALID 2 ns BEFORE COMP "gmii_tx_clk";
Worst Case Data Window 0.944; Ideal Clock Offset To Actual Clock -0.154;
------------------+------------+------------+---------+---------+-------------+
| | | Setup | Hold |Source Offset|
Source | Setup | Hold | Slack | Slack | To Center |
------------------+------------+------------+---------+---------+-------------+
gmii_tx_en | 1.271(R)| -0.386(R)| 0.729| 0.386| 0.171|
gmii_tx_er | 1.307(R)| -0.413(R)| 0.693| 0.413| 0.140|
gmii_txd<0> | 1.291(R)| -0.400(R)| 0.709| 0.400| 0.154|
gmii_txd<1> | 1.318(R)| -0.426(R)| 0.682| 0.426| 0.128|
gmii_txd<2> | 1.281(R)| -0.391(R)| 0.719| 0.391| 0.164|
gmii_txd<3> | 1.305(R)| -0.415(R)| 0.695| 0.415| 0.140|
gmii_txd<4> | 1.268(R)| -0.378(R)| 0.732| 0.378| 0.177|
gmii_txd<5> | 1.283(R)| -0.393(R)| 0.717| 0.393| 0.162|
gmii_txd<6> | 1.261(R)| -0.374(R)| 0.739| 0.374| 0.183|
gmii_txd<7> | 1.263(R)| -0.379(R)| 0.737| 0.379| 0.179|
------------------+------------+------------+---------+---------+-------------+
Worst Case Summary| 1.318| -0.374| 0.682| 0.374| |
------------------+------------+------------+---------+---------+-------------+
This is a known issue and is scheduled to be fixed in the ISE 11.2 software release.
Please be sure to use the detail of the report when analyzing the timing reports.