net: mdio: aspeed: add dummy read to avoid read-after-write issue

[ Upstream commit d1a1a4bade4b20c0858d0b2f81d2611de055f675 ]

The Aspeed MDIO controller may return incorrect data when a read operation
follows immediately after a write. Due to a controller bug, the subsequent
read can latch stale data, causing the polling logic to terminate earlier
than expected.

To work around this hardware issue, insert a dummy read after each write
operation. This ensures that the next actual read returns the correct
data and prevents premature polling exit.

This workaround has been verified to stabilize MDIO transactions on
affected Aspeed platforms.

Fixes: f160e99462c6 ("net: phy: Add mdio-aspeed")
Signed-off-by: Jacky Chou <jacky_chou@aspeedtech.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20251211-aspeed_mdio_add_dummy_read-v3-1-382868869004@aspeedtech.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/drivers/net/mdio/mdio-aspeed.c b/drivers/net/mdio/mdio-aspeed.c
index f22be2f069e9880562651dbbf6534f81f9b73a00..a929399a10d1e19c003e4b75622f542a794bd43d 100644 (file)
--- a/drivers/net/mdio/mdio-aspeed.c
+++ b/drivers/net/mdio/mdio-aspeed.c
@@ -57,6 +57,13 @@ static int aspeed_mdio_op(struct mii_bus *bus, u8 st, u8 op, u8 phyad, u8 regad,
 
        iowrite32(ctrl, ctx->base + ASPEED_MDIO_CTRL);
 
+       /* Workaround for read-after-write issue.
+        * The controller may return stale data if a read follows immediately
+        * after a write. A dummy read forces the hardware to update its
+        * internal state, ensuring that the next real read returns correct data.
+        */
+       ioread32(ctx->base + ASPEED_MDIO_CTRL);
+
        return readl_poll_timeout(ctx->base + ASPEED_MDIO_CTRL, ctrl,
                                !(ctrl & ASPEED_MDIO_CTRL_FIRE),
                                ASPEED_MDIO_INTERVAL_US,