e1000e: Ignore TSYNCRXCTL when getting I219 clock attributes

There have been multiple reports of crashes that look like
kernel: RIP: 0010:[<ffffffff8110303f>] timecounter_read+0xf/0x50
[...]
kernel: Call Trace:
kernel:  [<ffffffffa0806b0f>] e1000e_phc_gettime+0x2f/0x60 [e1000e]
kernel:  [<ffffffffa0806c5d>] e1000e_systim_overflow_work+0x1d/0x80 [e1000e]
kernel:  [<ffffffff810992c5>] process_one_work+0x155/0x440
kernel:  [<ffffffff81099e16>] worker_thread+0x116/0x4b0
kernel:  [<ffffffff8109f422>] kthread+0xd2/0xf0
kernel:  [<ffffffff8163184f>] ret_from_fork+0x3f/0x70

These can be traced back to the fact that e1000e_systim_reset() skips the
timecounter_init() call if e1000e_get_base_timinca() returns -EINVAL, which
leads to a null deref in timecounter_read().

Commit 83129b37 ("e1000e: fix systim issues", v4.2-rc1) reworked
e1000e_get_base_timinca() in such a way that it can return -EINVAL for
e1000_pch_spt if the SYSCFI bit is not set in TSYNCRXCTL.

Some experimentation has shown that on I219 (e1000_pch_spt, "MAC: 12")
adapters, the E1000_TSYNCRXCTL_SYSCFI flag is unstable; TSYNCRXCTL reads
sometimes don't have the SYSCFI bit set. Retrying the read shortly after
finds the bit to be set. This was observed at boot (probe) but also link up
and link down.

Moreover, the phc (PTP Hardware Clock) seems to operate normally even after
reads where SYSCFI=0. Therefore, remove this register read and
unconditionally set the clock parameters.
Reported-by: NAchim Mildenberger <admin@fph.physik.uni-karlsruhe.de>
Message-Id: <20180425065243.g5mqewg5irkwgwgv@f2>
Bugzilla: https://bugzilla.suse.com/show_bug.cgi?id=1075876
Fixes: 83129b37 ("e1000e: fix systim issues")
Signed-off-by: NBenjamin Poirier <bpoirier@suse.com>
Tested-by: NAaron Brown <aaron.f.brown@intel.com>
Signed-off-by: NJeff Kirsher <jeffrey.t.kirsher@intel.com>

drivers/net/ethernet/intel/e1000e/netdev.c

@@ -3527,15 +3527,12 @@ s32 e1000e_get_base_timinca(struct e1000_adapter *adapter, u32 *timinca)
 		}
 		break;
 	case e1000_pch_spt:
-		if (er32(TSYNCRXCTL) & E1000_TSYNCRXCTL_SYSCFI) {
-			/* Stable 24MHz frequency */
-			incperiod = INCPERIOD_24MHZ;
-			incvalue = INCVALUE_24MHZ;
-			shift = INCVALUE_SHIFT_24MHZ;
-			adapter->cc.shift = shift;
-			break;
-		}
-		return -EINVAL;
+		/* Stable 24MHz frequency */
+		incperiod = INCPERIOD_24MHZ;
+		incvalue = INCVALUE_24MHZ;
+		shift = INCVALUE_SHIFT_24MHZ;
+		adapter->cc.shift = shift;
+		break;
 	case e1000_pch_cnp:
 		if (er32(TSYNCRXCTL) & E1000_TSYNCRXCTL_SYSCFI) {
 			/* Stable 24MHz frequency */