netdev - Re: [PATCH net v2] phy: avoid unnecessary link-up delay in polling mode

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Date:   Wed, 29 Jan 2020 22:01:28 +0100
From:   Heiner Kallweit <hkallweit1@...il.com>
To:     Petr Oros <poros@...hat.com>
Cc:     netdev@...r.kernel.org, andrew@...n.ch, f.fainelli@...il.com,
        ivecera@...hat.com
Subject: Re: [PATCH net v2] phy: avoid unnecessary link-up delay in polling
 mode

On 29.01.2020 13:19, Petr Oros wrote:
> commit 93c0970493c71f ("net: phy: consider latched link-down status in
> polling mode") removed double-read of latched link-state register for
> polling mode from genphy_update_link(). This added extra ~1s delay into
> sequence link down->up.
> Following scenario:
>  - After boot link goes up
>  - phy_start() is called triggering an aneg restart, hence link goes
>    down and link-down info is latched.
>  - After aneg has finished link goes up. In phy_state_machine is checked
>    link state but it is latched "link is down". The state machine is
>    scheduled after one second and there is detected "link is up". This
>    extra delay can be avoided when we keep link-state register double read
>    in case when link was down previously.
> 
> With this solution we don't miss a link-down event in polling mode and
> link-up is faster.
> 

I have a little problem to understand why it should be faster this way.
Let's take an example: aneg takes 3.5s
Current behavior:

T0: aneg is started, link goes down, link-down status is latched
    (phydev->link is still 1)
T0+1s: state machine runs, latched link-down is read,
       phydev->link goes down, state change PHY_UP to PHY_NOLINK
T0+2s: state machine runs, up-to-date link-down is read
T0+3s: state machine runs, up-to-date link-down is read
T0+4s: state machine runs, aneg is finished, up-to-date link-up is read,
       phydev->link goes up, state change PHY_NOLINK to PHY_RUNNING

Your patch changes the behavior of T0+1s only. So it should make a
difference only if aneg takes less than 1s.
Can you explain, based on the given example, how your change is
supposed to improve this?

And on a side note: I wouldn't consider this change a fix, therefore
it would be material for net-next that is closed at the moment.

Heiner

> Changes in v2:
> - Fixed typos in phy_polling_mode() argument
> 
> Fixes: 93c0970493c71f ("net: phy: consider latched link-down status in polling mode")
> Signed-off-by: Petr Oros <poros@...hat.com>
> ---
>  drivers/net/phy/phy-c45.c    | 5 +++--
>  drivers/net/phy/phy_device.c | 5 +++--
>  2 files changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/net/phy/phy-c45.c b/drivers/net/phy/phy-c45.c
> index a1caeee1223617..bceb0dcdecbd61 100644
> --- a/drivers/net/phy/phy-c45.c
> +++ b/drivers/net/phy/phy-c45.c
> @@ -239,9 +239,10 @@ int genphy_c45_read_link(struct phy_device *phydev)
>  
>  		/* 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.
> +		 * in polling mode to detect such short link drops except
> +		 * the link was already down.
>  		 */
> -		if (!phy_polling_mode(phydev)) {
> +		if (!phy_polling_mode(phydev) || !phydev->link) {
>  			val = phy_read_mmd(phydev, devad, MDIO_STAT1);
>  			if (val < 0)
>  				return val;
> diff --git a/drivers/net/phy/phy_device.c b/drivers/net/phy/phy_device.c
> index 6a5056e0ae7757..05417419c484fa 100644
> --- a/drivers/net/phy/phy_device.c
> +++ b/drivers/net/phy/phy_device.c
> @@ -1930,9 +1930,10 @@ int genphy_update_link(struct phy_device *phydev)
>  
>  	/* 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.
> +	 * in polling mode to detect such short link drops except
> +	 * the link was already down.
>  	 */
> -	if (!phy_polling_mode(phydev)) {
> +	if (!phy_polling_mode(phydev) || !phydev->link) {
>  		status = phy_read(phydev, MII_BMSR);
>  		if (status < 0)
>  			return status;
>