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

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Message-ID: <230147e8-e27b-48e1-9a62-7aa8abc3f492@lunn.ch>
Date: Fri, 5 Dec 2025 14:52:11 +0100
From: Andrew Lunn <andrew@...n.ch>
To: Jacky Chou <jacky_chou@...eedtech.com>
Cc: Heiner Kallweit <hkallweit1@...il.com>,
	Russell King <linux@...linux.org.uk>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Joel Stanley <joel@....id.au>,
	Andrew Jeffery <andrew@...econstruct.com.au>,
	netdev@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-aspeed@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next] net: mdio: aspeed: add dummy read to avoid
 read-after-write issue

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@...eedtech.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

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