e1000e: workaround hw errata

There is a hardware errata in some revisions of the 82574 that needs
to be worked around in the driver by setting a register bit at init.

If this bit is not set A0 versions of the 82574 can generate
tx hangs.
Signed-off-by: NJesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: NBruce Allan <bruce.w.allan@intel.com>
Signed-off-by: NJeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/e1000e/82571.c

@@ -981,11 +981,15 @@ static void e1000_initialize_hw_bits_82571(struct e1000_hw *hw)
 		ew32(PBA_ECC, reg);
 	}
 
-	/* PCI-Ex Control Register */
+	/* PCI-Ex Control Registers */
 	if (hw->mac.type == e1000_82574) {
 		reg = er32(GCR);
 		reg |= (1 << 22);
 		ew32(GCR, reg);
+
+		reg = er32(GCR2);
+		reg |= 1;
+		ew32(GCR2, reg);
 	}
 
 	return;

drivers/net/e1000e/hw.h

@@ -206,6 +206,7 @@ enum e1e_registers {
 	E1000_MANC2H    = 0x05860, /* Management Control To Host - RW */
 	E1000_SW_FW_SYNC = 0x05B5C, /* Software-Firmware Synchronization - RW */
 	E1000_GCR	= 0x05B00, /* PCI-Ex Control */
+	E1000_GCR2      = 0x05B64, /* PCI-Ex Control #2 */
 	E1000_FACTPS    = 0x05B30, /* Function Active and Power State to MNG */
 	E1000_SWSM      = 0x05B50, /* SW Semaphore */
 	E1000_FWSM      = 0x05B54, /* FW Semaphore */