netdev - Re: [PATCH net-next v10 3/5] net: phy: micrel: Add loopback support

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Message-ID: <9b61088921bafd7da5739c786274c083@engleder-embedded.com>
Date: Thu, 13 Mar 2025 10:35:47 +0100
From: Gerhard Engleder <gerhard@...leder-embedded.com>
To: Maxime Chevallier <maxime.chevallier@...tlin.com>
Cc: andrew@...n.ch, hkallweit1@...il.com, linux@...linux.org.uk,
 davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
 pabeni@...hat.com, netdev@...r.kernel.org
Subject: Re: [PATCH net-next v10 3/5] net: phy: micrel: Add loopback support

Am 13.03.2025 09:57, schrieb Maxime Chevallier:
> Hello Gerhard,
> 
> On Wed, 12 Mar 2025 21:30:08 +0100
> Gerhard Engleder <gerhard@...leder-embedded.com> wrote:
> 
>> The KSZ9031 PHYs requires full duplex for loopback mode. Add PHY
>> specific set_loopback() to ensure this.
>> 
>> Signed-off-by: Gerhard Engleder <gerhard@...leder-embedded.com>
>> ---
>>  drivers/net/phy/micrel.c | 24 ++++++++++++++++++++++++
>>  1 file changed, 24 insertions(+)
>> 
>> diff --git a/drivers/net/phy/micrel.c b/drivers/net/phy/micrel.c
>> index 289e1d56aa65..24882d30f685 100644
>> --- a/drivers/net/phy/micrel.c
>> +++ b/drivers/net/phy/micrel.c
>> @@ -1032,6 +1032,29 @@ static int ksz9021_config_init(struct 
>> phy_device *phydev)
>>  #define MII_KSZ9031RN_EDPD		0x23
>>  #define MII_KSZ9031RN_EDPD_ENABLE	BIT(0)
>> 
>> +static int ksz9031_set_loopback(struct phy_device *phydev, bool 
>> enable,
>> +				int speed)
>> +{
>> +	u16 ctl = BMCR_LOOPBACK;
>> +	int val;
>> +
>> +	if (!enable)
>> +		return genphy_loopback(phydev, enable, 0);
>> +
>> +	if (speed == SPEED_10 || speed == SPEED_100 || speed == SPEED_1000)
>> +		phydev->speed = speed;
>> +	else if (speed)
>> +		return -EINVAL;
>> +	phydev->duplex = DUPLEX_FULL;
>> +
>> +	ctl |= mii_bmcr_encode_fixed(phydev->speed, phydev->duplex);
>> +
>> +	phy_write(phydev, MII_BMCR, ctl);
>> +
>> +	return phy_read_poll_timeout(phydev, MII_BMSR, val, val & 
>> BMSR_LSTATUS,
>> +				     5000, 500000, true);
> 
> Maybe I don't fully get it, but it looks to me that you poll, waiting
> for the link to become up. As you are in local loopback mode, how does
> that work ? Do you need connection to an active LP for loopback to
> work, or does the BMSR_LSTATUS bit behave differently under local
> loopback ?

Yes I'm polling for link to come up. This is identical to 
genphy_loopback().
There is no need for a link partner to get a link up in loopback mode.
BMSR_LSTATUS reflects the loopback as link in this case.

This series allows to configure a loopback with defined speed without 
any
link partner. Currently for PHY loopback a link partner is needed in 
some
cases (starting with 1 Gbps loopback).

Gerhard