linux-kernel - Re: [PATCH net-next v2 3/3] phy: dp83td510: Utilize ALCD for cable length measurement when link is active

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Message-ID: <a02698f3-94b6-4e6c-b13a-7fbeba2ce42f@lunn.ch>
Date: Tue, 20 Aug 2024 18:49:00 +0200
From: Andrew Lunn <andrew@...n.ch>
To: Oleksij Rempel <o.rempel@...gutronix.de>
Cc: Heiner Kallweit <hkallweit1@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Jonathan Corbet <corbet@....net>, kernel@...gutronix.de,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	Russell King <linux@...linux.org.uk>
Subject: Re: [PATCH net-next v2 3/3] phy: dp83td510: Utilize ALCD for cable
 length measurement when link is active

On Tue, Aug 20, 2024 at 12:12:56PM +0200, Oleksij Rempel wrote:
> In industrial environments where 10BaseT1L PHYs are replacing existing
> field bus systems like CAN, it's often essential to retain the existing
> cable infrastructure. After installation, collecting metrics such as
> cable length is crucial for assessing the quality of the infrastructure.
> Traditionally, TDR (Time Domain Reflectometry) is used for this purpose.
> However, TDR requires interrupting the link, and if the link partner
> remains active, the TDR measurement will fail.
> 
> Unlike multi-pair systems, where TDR can be attempted during the MDI-X
> switching window, 10BaseT1L systems face greater challenges. The TDR
> sequence on 10BaseT1L is longer and coincides with uninterrupted
> autonegotiation pulses, making TDR impossible when the link partner is
> active.
> 
> The DP83TD510 PHY provides an alternative through ALCD (Active Link
> Cable Diagnostics), which allows for cable length measurement without
> disrupting an active link. Since a live link indicates no short or open
> cable states, ALCD can be used effectively to gather cable length
> information.
> 
> Enhance the dp83td510 driver by:
> - Leveraging ALCD to measure cable length when the link is active.
> - Bypassing TDR when a link is detected, as ALCD provides the required
>   information without disruption.
> 
> Signed-off-by: Oleksij Rempel <o.rempel@...gutronix.de>
> ---
>  drivers/net/phy/dp83td510.c | 83 ++++++++++++++++++++++++++++++++++---
>  1 file changed, 77 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/net/phy/dp83td510.c b/drivers/net/phy/dp83td510.c
> index 551e37786c2da..10a46354ad2b8 100644
> --- a/drivers/net/phy/dp83td510.c
> +++ b/drivers/net/phy/dp83td510.c
> @@ -169,6 +169,10 @@ static const u16 dp83td510_mse_sqi_map[] = {
>  #define DP83TD510E_UNKN_030E				0x30e
>  #define DP83TD510E_030E_VAL				0x2520
>  
> +#define DP83TD510E_ALCD_STAT				0xa9f
> +#define DP83TD510E_ALCD_COMPLETE			BIT(15)
> +#define DP83TD510E_ALCD_CABLE_LENGTH			GENMASK(10, 0)
> +
>  static int dp83td510_config_intr(struct phy_device *phydev)
>  {
>  	int ret;
> @@ -327,6 +331,16 @@ static int dp83td510_cable_test_start(struct phy_device *phydev)
>  {
>  	int ret;
>  
> +	/* If link partner is active, we won't be able to use TDR, since
> +	 * we can't force link partner to be silent. The autonegotiation
> +	 * pulses will be too frequent and the TDR sequence will be
> +	 * too long. So, TDR will always fail. Since the link is established
> +	 * we already know that the cable is working, so we can get some
> +	 * extra information line the cable length using ALCD.
> +	 */
> +	if (phydev->link)
> +		return 0;
> +

> +static int dp83td510_cable_test_get_status(struct phy_device *phydev,
> +					   bool *finished)
> +{
> +	*finished = false;
> +
> +	if (!phydev->link)
> +		return dp83td510_cable_test_get_tdr_status(phydev, finished);
> +
> +	return dp83td510_cable_test_get_alcd_status(phydev, finished);

Sorry, missed this earlier. It seems like there is a race here. It
could be the cable test was started without link, but when phylib
polls a few seconds later link could of established. Will valid ALCD
results be returned?

	Andrew