[PATCH v2 15/19] hw/net/bcm2838_genet: fix PHY autonegotiation-restart deadlock

[Marcelo Manzo <marcelomanzo@gmail.com>]

The guest PHY driver requests an autonegotiation restart by setting
BMCR.ANRESTART, then polls BMCR waiting for the hardware to clear
that bit as an acknowledgment. bcm2838_genet_mdio_cmd() called
FIELD_DP16() to clear ANRESTART in phy_reg_data but discarded its
return value -- FIELD_DP16() does not modify its argument in place,
it returns a new value -- so the bit was never actually cleared in
the register the guest reads back. The guest driver then spun
forever waiting for an acknowledgment that would never come,
appearing to hang the whole boot.

Assign the FIELD_DP16() result back to phy_reg_data, and also call
bcm2838_genet_phy_update_link() once the restart is handled so link
status actually gets refreshed, matching the surrounding code's own
comment ("Initiate auto-negotiation once it has been restarted").

Found by booting a real guest against this series and tracing the
MDIO command register with targeted debug prints: the guest was
observed polling BMCR indefinitely, always reading back the
ANRESTART bit still set. Confirmed fixed across repeated boots: link
comes up and autonegotiation completes every time.

Signed-off-by: Marcelo Manzo <marcelomanzo@gmail.com>
---
 hw/net/bcm2838_genet.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/hw/net/bcm2838_genet.c b/hw/net/bcm2838_genet.c
index 746e416e53..dfdc5ac728 100644
--- a/hw/net/bcm2838_genet.c
+++ b/hw/net/bcm2838_genet.c
@@ -20,6 +20,8 @@
 #include "hw/net/bcm2838_genet.h"
 #include "trace.h"
 
+static void bcm2838_genet_phy_update_link(BCM2838GenetState *s);
+
 /* GENET layouts */
 REG32(GENET_SYS_REV_CTRL,              0)
 FIELD(GENET_SYS_REV_CTRL, GPHY_REV,    0, 16)
@@ -392,10 +394,15 @@ static uint64_t bcm2838_genet_mdio_cmd(BCM2838GenetState *s, uint64_t cmd)
                 if (phy_reg_id == BCM2838_GENET_PHY_BMCR) {
                     /* Initiate auto-negotiation once it has been restarted */
                     if (anrestart == 1) {
-                        FIELD_DP16(phy_reg_data, GENET_PHY_BMCR, ANRESTART, 0);
+                        phy_reg_data = FIELD_DP16(phy_reg_data,
+                                                  GENET_PHY_BMCR,
+                                                  ANRESTART, 0);
                     }
                 }
                 *phy_reg = phy_reg_data;
+                if (phy_reg_id == BCM2838_GENET_PHY_BMCR && anrestart == 1) {
+                    bcm2838_genet_phy_update_link(s);
+                }
             }
         }
     }
-- 
2.47.1