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

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Message-ID:
 <SEYPR06MB5134571388AC51EEAC45AF739DA2A@SEYPR06MB5134.apcprd06.prod.outlook.com>
Date: Mon, 8 Dec 2025 03:13:24 +0000
From: Jacky Chou <jacky_chou@...eedtech.com>
To: Andrew Lunn <andrew@...n.ch>
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"
	<netdev@...r.kernel.org>, "linux-arm-kernel@...ts.infradead.org"
	<linux-arm-kernel@...ts.infradead.org>, "linux-aspeed@...ts.ozlabs.org"
	<linux-aspeed@...ts.ozlabs.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>
Subject: [PATCH net-next] net: mdio: aspeed: add dummy read to avoid
 read-after-write issue

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@...eedtech.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