linux-kernel - Re: [PATCH net] amd-xgbe: read link status twice to avoid inconsistencies

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Message-ID: <acca227d-c2a9-4fc3-a6fc-3001472370a3@lunn.ch>
Date: Thu, 15 May 2025 02:07:13 +0200
From: Andrew Lunn <andrew@...n.ch>
To: Raju Rangoju <Raju.Rangoju@....com>, g@...n.ch
Cc: andrew+netdev@...n.ch, davem@...emloft.net, edumazet@...gle.com,
	kuba@...nel.org, pabeni@...hat.com, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, Shyam-sundar.S-k@....com
Subject: Re: [PATCH net] amd-xgbe: read link status twice to avoid
 inconsistencies

On Thu, May 15, 2025 at 01:11:45AM +0530, Raju Rangoju wrote:
> The link status is latched low, so read the register twice to get the
> current status and avoid link inconsistencies.
> 
> As per IEEE 802.3 "Table 22-8 - Status register bit definitions"
> 1.2  Link Status  1 = link is up    0 = link is down    RO/LL
> 
> Fixes: 4f3b20bfbb75 ("amd-xgbe: add support for rx-adaptation")
> Signed-off-by: Raju Rangoju <Raju.Rangoju@....com>
> ---
>  drivers/net/ethernet/amd/xgbe/xgbe-phy-v2.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/drivers/net/ethernet/amd/xgbe/xgbe-phy-v2.c b/drivers/net/ethernet/amd/xgbe/xgbe-phy-v2.c
> index 268399dfcf22..d233e3faa1a9 100644
> --- a/drivers/net/ethernet/amd/xgbe/xgbe-phy-v2.c
> +++ b/drivers/net/ethernet/amd/xgbe/xgbe-phy-v2.c
> @@ -2914,6 +2914,10 @@ static int xgbe_phy_link_status(struct xgbe_prv_data *pdata, int *an_restart)
>  		}
>  
>  		/* check again for the link and adaptation status */
> +		/* Link status is latched low, so read once to clear
> +		 * and then read again to get current state
> +		 */
> +		reg = XMDIO_READ(pdata, MDIO_MMD_PCS, MDIO_STAT1);


Since this is not a phylib user, i don't care too much, but:

https://elixir.bootlin.com/linux/v6.14.6/source/drivers/net/phy/phy_device.c#L2514

	/* The link state is latched low so that momentary link
	 * drops can be detected. Do not double-read the status
	 * in polling mode to detect such short link drops except
	 * the link was already down.
	 */

So you don't care about short link drops? You don't want to tell user
space the link went away and came back? It never happened.

	Andrew