[PATCH net-next] net: mdio: aspeed: add dummy read to avoid read-after-write issue

[PATCH net-next] net: mdio: aspeed: add dummy read to avoid read-after-write issue

[Jacky Chou]

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.

Signed-off-by: Jacky Chou <jacky_chou@aspeedtech.com>
---
 drivers/net/mdio/mdio-aspeed.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/net/mdio/mdio-aspeed.c b/drivers/net/mdio/mdio-aspeed.c
index e55be6dc9ae7..00e61b922876 100644
--- a/drivers/net/mdio/mdio-aspeed.c
+++ b/drivers/net/mdio/mdio-aspeed.c
@@ -62,6 +62,12 @@ static int aspeed_mdio_op(struct mii_bus *bus, u8 st, u8 op, u8 phyad, u8 regad,
 		| FIELD_PREP(ASPEED_MDIO_DATA_MIIRDATA, data);
 
 	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.
+	 */
+	(void)ioread32(ctx->base + ASPEED_MDIO_CTRL);
 
 	return readl_poll_timeout(ctx->base + ASPEED_MDIO_CTRL, ctrl,
 				!(ctrl & ASPEED_MDIO_CTRL_FIRE),

---
base-commit: 8f7aa3d3c7323f4ca2768a9e74ebbe359c4f8f88
change-id: 20251205-aspeed_mdio_add_dummy_read-d58d1e49412d

Best regards,
-- 
Jacky Chou <jacky_chou@aspeedtech.com>

Re: [PATCH net-next] net: mdio: aspeed: add dummy read to avoid read-after-write issue

[Andrew Lunn]

On Fri, Dec 05, 2025 at 09:37:22AM +0800, Jacky Chou wrote:
> 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.
> 
> Signed-off-by: Jacky Chou <jacky_chou@aspeedtech.com>

This seems like a bug fix. Please add a Fixes: tag, for base it on
net, not net-next.

> ---
>  drivers/net/mdio/mdio-aspeed.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/drivers/net/mdio/mdio-aspeed.c b/drivers/net/mdio/mdio-aspeed.c
> index e55be6dc9ae7..00e61b922876 100644
> --- a/drivers/net/mdio/mdio-aspeed.c
> +++ b/drivers/net/mdio/mdio-aspeed.c
> @@ -62,6 +62,12 @@ static int aspeed_mdio_op(struct mii_bus *bus, u8 st, u8 op, u8 phyad, u8 regad,
>  		| FIELD_PREP(ASPEED_MDIO_DATA_MIIRDATA, data);
>  
>  	iowrite32(ctrl, ctx->base + ASPEED_MDIO_CTRL);
> +	/* Workaround for read-after-write issue.

Blank line before the comment please.

    Andrew

---
pw-bot: cr

[PATCH net-next] net: mdio: aspeed: add dummy read to avoid read-after-write issue

[Jacky Chou]

Hi Andrew,

Thank you for your reply

> > 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.
> >
> > Signed-off-by: Jacky Chou <jacky_chou@aspeedtech.com>
> 
> This seems like a bug fix. Please add a Fixes: tag, for base it on net, not
> net-next.
> 

Yes, it is a bug fix on HW design.
I will send it to net again and add Fixes tag.

> > ---
> >  drivers/net/mdio/mdio-aspeed.c | 6 ++++++
> >  1 file changed, 6 insertions(+)
> >
> > diff --git a/drivers/net/mdio/mdio-aspeed.c
> > b/drivers/net/mdio/mdio-aspeed.c index e55be6dc9ae7..00e61b922876
> > 100644
> > --- a/drivers/net/mdio/mdio-aspeed.c
> > +++ b/drivers/net/mdio/mdio-aspeed.c
> > @@ -62,6 +62,12 @@ static int aspeed_mdio_op(struct mii_bus *bus, u8 st,
> u8 op, u8 phyad, u8 regad,
> >  		| FIELD_PREP(ASPEED_MDIO_DATA_MIIRDATA, data);
> >
> >  	iowrite32(ctrl, ctx->base + ASPEED_MDIO_CTRL);
> > +	/* Workaround for read-after-write issue.
> 
> Blank line before the comment please.
> 

Agreed.

Thanks,
Jacky