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

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>

diff --git a/drivers/net/mdio/mdio-aspeed.c b/drivers/net/mdio/mdio-aspeed.c
index e55be6dc9ae70f9e76c4a76df427193e74a5e039..d6b9004c61dc161d44909d1eb7dfa7672de35254 100644 (file)
--- a/drivers/net/mdio/mdio-aspeed.c
+++ b/drivers/net/mdio/mdio-aspeed.c
@@ -63,6 +63,13 @@ 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,