The current implementation of the igc driver doesn't power up the PHY
before the link test in igc_ethtool_diag_test(), causing the link test
to always report FAIL when admin state is down and the PHY is
consequently powered down.

To test the link state regardless of admin state, power up the PHY
before the link test in the offline test path. After the link test, the
original PHY state is restored by igc_reset(), so additional code which
explicitly restores the original state is not necessary.

Note that this change is applied only for the offline test path. This is
because in the online path we shouldn't interrupt normal networking
operation and powering up the PHY and restoring the original state would
interrupt that.

This implementation also uses igc_power_up_phy_copper() without checking
the media type, since igc devices are currently only copper devices and
the function is called in other places without checking the media type.

Furthermore, the powering up is on a best-effort basis, that is, we
don't handle failures of powering up (e.g. bus error) and just let the
test report FAIL.

Tested on Intel Corporation Ethernet Controller I226-V (rev 04) with
cable connected and link available.

Set device down and do ethtool test.
  # ip link set dev enp0s5 down

Without patch:
  # ethtool --test enp0s5
  The test result is FAIL
  The test extra info:
  Register test  (offline)         0
  Eeprom test    (offline)         0
  Interrupt test (offline)         0
  Loopback test  (offline)         0
  Link test   (on/offline)         1

With patch:
  # ethtool --test enp0s5
  The test result is PASS
  The test extra info:
  Register test  (offline)         0
  Eeprom test    (offline)         0
  Interrupt test (offline)         0
  Loopback test  (offline)         0
  Link test   (on/offline)         0

Fixes: f026d8ca2904 ("igc: add support to eeprom, registers and link self-tests")
Signed-off-by: Kohei Enju <enjuk@amazon.com>
---
Changes:
v1->v2:
  - rephrase commit message to clarify:
    - applied only for offline test path
    - original power state is restored by igc_reset()
    - powering up the PHY is on a best-effort basis
v1: https://lore.kernel.org/intel-wired-lan/20250830170656.61496-1-enjuk@amazon.com/
---
 drivers/net/ethernet/intel/igc/igc_ethtool.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/net/ethernet/intel/igc/igc_ethtool.c b/drivers/net/ethernet/intel/igc/igc_ethtool.c
index f3e7218ba6f3..ca93629b1d3a 100644
--- a/drivers/net/ethernet/intel/igc/igc_ethtool.c
+++ b/drivers/net/ethernet/intel/igc/igc_ethtool.c
@@ -2094,6 +2094,9 @@ static void igc_ethtool_diag_test(struct net_device *netdev,
 		netdev_info(adapter->netdev, "Offline testing starting");
 		set_bit(__IGC_TESTING, &adapter->state);
 
+		/* power up PHY for link test */
+		igc_power_up_phy_copper(&adapter->hw);
+
 		/* Link test performed before hardware reset so autoneg doesn't
 		 * interfere with test result
 		 */
--