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

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Message-ID: <8fa98423-9c3c-62c9-1e5a-29b2eef555e3@ti.com>
Date:   Thu, 6 Feb 2020 16:36:23 -0600
From:   Dan Murphy <dmurphy@...com>
To:     Heiner Kallweit <hkallweit1@...il.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

Heiner

On 2/6/20 4:28 PM, Heiner Kallweit wrote:
> 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.
True.  This register is only going to return 1,2,4 and 8.  And it is 
defaulted to 4 attempts.
>
> 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.

Thats a good idea but.. The register is defaulted to always report 4 
attempts were made. It never reports 0 attempts so we would never know 
the truth behind the reporting.  Kinda like matching the speeds.

Dan