netdev - Re: [PATCH net 2/3] net: phy: broadcom: Only set BMCR.PDOWN to suspend

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Date:   Wed, 10 Mar 2021 13:15:30 -0800
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Heiner Kallweit <hkallweit1@...il.com>, netdev@...r.kernel.org
Cc:     andrew@...n.ch, kuba@...nel.org, davem@...emloft.net
Subject: Re: [PATCH net 2/3] net: phy: broadcom: Only set BMCR.PDOWN to
 suspend

On 3/10/21 1:07 PM, Heiner Kallweit wrote:
> On 10.03.2021 21:41, Florian Fainelli wrote:
>> B50212E PHYs have been observed to get into an incorrect state with the
>> visible effect of having both activity and link LEDs flashing
>> alternatively instead of being turned off as intended when
>> genphy_suspend() was issued. The BCM54810 is a similar design and
>> equally suffers from that issue.
>>
>> The datasheet is not particularly clear whether a read/modify/write
>> sequence is acceptable and only indicates that BMCR.PDOWN=1 should be
>> utilized to enter the power down mode. When this was done the PHYs were
>> always measured to have power levels that match the expectations and
>> LEDs powered off.
>>
>> Fixes: fe26821fa614 ("net: phy: broadcom: Wire suspend/resume for BCM54810")
>> Signed-off-by: Florian Fainelli <f.fainelli@...il.com>
>> ---
>>  drivers/net/phy/broadcom.c | 17 ++++++++++++++++-
>>  1 file changed, 16 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/net/phy/broadcom.c b/drivers/net/phy/broadcom.c
>> index b8eb736fb456..b33ffd44f799 100644
>> --- a/drivers/net/phy/broadcom.c
>> +++ b/drivers/net/phy/broadcom.c
>> @@ -388,6 +388,21 @@ static int bcm54xx_config_init(struct phy_device *phydev)
>>  	return 0;
>>  }
>>  
>> +static int bcm54xx_suspend(struct phy_device *phydev)
>> +{
>> +	/* We cannot perform a read/modify/write like what genphy_suspend()
>> +	 * does because depending on the time we can observe the PHY having
>> +	 * both of its LEDs flashing indicating that it is in an incorrect
>> +	 * state and not powered down as expected.
>> +	 *
>> +	 * There is not a clear indication in the datasheet whether a
>> +	 * read/modify/write would be acceptable, but a blind write to the
>> +	 * register has been proven to be functional unlike the
>> +	 * Read/Modify/Write.
>> +	 */
>> +	return phy_write(phydev, MII_BMCR, BMCR_PDOWN);
> 
> This clears all other bits in MII_BMCR, incl. ANENABLE and the ones used in
> forced mode. So you have to rely on somebody calling genphy_config_aneg()
> to sync the register bits with the values cached in struct phy_device
> on resume. Typically the phylib state machine takes care, but do we have
> to consider use cases where this is not the case?

Good point, how about if we had forced the link before suspending, does
PHYLIB take care of re-applying the same parameters? It arguably should
do that in all cases given that power to the PHY can be cut depending on
the suspend mode.
-- 
Florian