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

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Message-ID: <ba3d6047-0edd-4830-a5c2-bd7af463b688@amd.com>
Date: Wed, 25 Jun 2025 15:12:33 +0530
From: "Rangoju, Raju" <raju.rangoju@....com>
To: Andrew Lunn <andrew@...n.ch>, 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 5/15/2025 5:37 AM, Andrew Lunn wrote:
> 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.

Thanks Andrew for point this out. I'll re-spin v2 addressing these.

> 
> 	Andrew