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

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Date:   Wed, 5 Feb 2020 15:51:43 -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 2/5/20 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 just don't have a requirement from our customer to make it adjustable 
so I did not want to add something extra.

I can add in for v3.

>
> And I have to admit that I'm not too happy with the term "speed optimization".
> This sounds like the PHY has some magic to establish a 1.2Gbps link.
> Even though the vendor may call it this way in the datasheet, the standard
> term is "downshift". I'm fine with using "speed optimization" in constants
> to be in line with the datasheet. Just a comment in the code would be helpful
> that speed optimization is the vendor's term for downshift.

Ack.  The data sheet actually says "Speed optimization, also known as 
link downshift"

So I probably will just rename everything down shift.

>
>>   drivers/net/phy/dp83867.c | 55 +++++++++++++++++++++++++++++++++++++++
>>   1 file changed, 55 insertions(+)
>>
>> diff --git a/drivers/net/phy/dp83867.c b/drivers/net/phy/dp83867.c
>> index 967f57ed0b65..6f86ca1ebb51 100644
>> --- a/drivers/net/phy/dp83867.c
>> +++ b/drivers/net/phy/dp83867.c
>> @@ -21,6 +21,7 @@
>>   #define DP83867_DEVADDR		0x1f
>>   
>>   #define MII_DP83867_PHYCTRL	0x10
>> +#define MII_DP83867_PHYSTS	0x11
>>   #define MII_DP83867_MICR	0x12
>>   #define MII_DP83867_ISR		0x13
>>   #define DP83867_CFG2		0x14
>> @@ -118,6 +119,15 @@
>>   #define DP83867_IO_MUX_CFG_CLK_O_SEL_MASK	(0x1f << 8)
>>   #define DP83867_IO_MUX_CFG_CLK_O_SEL_SHIFT	8
>>   
>> +/* PHY STS bits */
>> +#define DP83867_PHYSTS_1000			BIT(15)
>> +#define DP83867_PHYSTS_100			BIT(14)
>> +#define DP83867_PHYSTS_DUPLEX			BIT(13)
>> +#define DP83867_PHYSTS_LINK			BIT(10)
>> +
>> +/* CFG2 bits */
>> +#define DP83867_SPEED_OPTIMIZED_EN		(BIT(8) | BIT(9))
>> +
>>   /* CFG3 bits */
>>   #define DP83867_CFG3_INT_OE			BIT(7)
>>   #define DP83867_CFG3_ROBUST_AUTO_MDIX		BIT(9)
>> @@ -287,6 +297,43 @@ static int dp83867_config_intr(struct phy_device *phydev)
>>   	return phy_write(phydev, MII_DP83867_MICR, micr_status);
>>   }
>>   
>> +static void dp83867_link_change_notify(struct phy_device *phydev)
>> +{
>> +	if (phydev->state != PHY_RUNNING)
>> +		return;
>> +
>> +	if (phydev->speed == SPEED_100 || phydev->speed == SPEED_10)
>> +		phydev_warn(phydev, "Downshift detected connection is %iMbps\n",
>> +			    phydev->speed);
> The link partner may simply not advertise 1Gbps. How do you know that
> a link speed of e.g. 100Mbps is caused by a downshift?
> Some PHY's I've seen with this feature have a flag somewhere indicating
> that downshift occurred. How about the PHY here?

I don't see a register that gives us that status

I will ask the hardware team if there is one.

This is a 1Gbps PHY by default so if a slower connection is established 
due to faulty cabling or LP advertisement then this would be a down 
shift IMO.

Dan