linux-kernel - Re: [PATCH net-next v2] net: phy: dp83867: Add speed optimization feature

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Date:   Thu, 6 Feb 2020 23:28:13 +0100
From:   Heiner Kallweit <hkallweit1@...il.com>
To:     Dan Murphy <dmurphy@...com>, andrew@...n.ch, f.fainelli@...il.com
Cc:     linux@...linux.org.uk, davem@...emloft.net, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next v2] net: phy: dp83867: Add speed optimization
 feature

On 06.02.2020 23:13, Dan Murphy wrote:
> Heiner
> 
> On 2/5/20 3:16 PM, Heiner Kallweit wrote:
>> On 04.02.2020 19:13, Dan Murphy wrote:
>>> Set the speed optimization bit on the DP83867 PHY.
>>> This feature can also be strapped on the 64 pin PHY devices
>>> but the 48 pin devices do not have the strap pin available to enable
>>> this feature in the hardware.  PHY team suggests to have this bit set.
>>>
>>> With this bit set the PHY will auto negotiate and report the link
>>> parameters in the PHYSTS register.  This register provides a single
>>> location within the register set for quick access to commonly accessed
>>> information.
>>>
>>> In this case when auto negotiation is on the PHY core reads the bits
>>> that have been configured or if auto negotiation is off the PHY core
>>> reads the BMCR register and sets the phydev parameters accordingly.
>>>
>>> This Giga bit PHY can throttle the speed to 100Mbps or 10Mbps to accomodate a
>>> 4-wire cable.  If this should occur the PHYSTS register contains the
>>> current negotiated speed and duplex mode.
>>>
>>> In overriding the genphy_read_status the dp83867_read_status will do a
>>> genphy_read_status to setup the LP and pause bits.  And then the PHYSTS
>>> register is read and the phydev speed and duplex mode settings are
>>> updated.
>>>
>>> Signed-off-by: Dan Murphy <dmurphy@...com>
>>> ---
>>> v2 - Updated read status to call genphy_read_status first, added link_change
>>> callback to notify of speed change and use phy_set_bits - https://lore.kernel.org/patchwork/patch/1188348/
>>>
>> As stated in the first review, it would be appreciated if you implement
>> also the downshift tunable. This could be a separate patch in this series.
>> Most of the implementation would be boilerplate code.
> 
> 
> I looked at this today and there are no registers that allow tuning the downshift attempts.  There is only a RO register that tells you how many attempts it took to achieve a link.  So at the very least we could put in the get_tunable but there will be no set.
> 
The get operation for the downshift tunable should return after how many failed
attempts the PHY starts a downshift. This doesn't match with your description of
this register, so yes: Implementing the tunable for this PHY doesn't make sense.

However this register may be useful in the link_change_notify() callback to
figure out whether a downshift happened, to trigger the info message you had in
your first version.

> So we should probably skip this for this PHY.
> 
> Dan
> 
Heiner