linux-kernel - Re: [PATCH net v2] net: phy: dp83867: add w/a for packet errors seen with short cables

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Message-ID: <973341c8-a8b1-840d-6e6b-d8a73aa7a946@gmail.com>
Date:   Tue, 9 May 2023 12:40:37 +0200
From:   Heiner Kallweit <hkallweit1@...il.com>
To:     Siddharth Vadapalli <s-vadapalli@...com>
Cc:     andrew@...n.ch, linux@...linux.org.uk, davem@...emloft.net,
        edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, srk@...com
Subject: Re: [PATCH net v2] net: phy: dp83867: add w/a for packet errors seen
 with short cables

On 09.05.2023 12:27, Siddharth Vadapalli wrote:
> 
> 
> On 09/05/23 14:29, Heiner Kallweit wrote:
>> On 09.05.2023 07:21, Siddharth Vadapalli wrote:
>>> From: Grygorii Strashko <grygorii.strashko@...com>
>>>
>>> Introduce the W/A for packet errors seen with short cables (<1m) between
>>> two DP83867 PHYs.
>>>
>>> The W/A recommended by DM requires FFE Equalizer Configuration tuning by
>>> writing value 0x0E81 to DSP_FFE_CFG register (0x012C), surrounded by hard
>>> and soft resets as follows:
>>>
>>> write_reg(0x001F, 0x8000); //hard reset
>>> write_reg(DSP_FFE_CFG, 0x0E81);
>>> write_reg(0x001F, 0x4000); //soft reset
>>>
>>> Since  DP83867 PHY DM says "Changing this register to 0x0E81, will not
>>> affect Long Cable performance.", enable the W/A by default.
>>>
>>> Fixes: 2a10154abcb7 ("net: phy: dp83867: Add TI dp83867 phy")
>>> Signed-off-by: Grygorii Strashko <grygorii.strashko@...com>
>>> Signed-off-by: Siddharth Vadapalli <s-vadapalli@...com>
>>> ---
>>>
>>> V1 patch at:
>>> https://lore.kernel.org/r/20230508070019.356548-1-s-vadapalli@ti.com
>>>
>>> Changes since v1 patch:
>>> - Wrap the line invoking phy_write_mmd(), limiting it to 80 characters.
>>> - Replace 0X0E81 with 0x0e81 in the call to phy_write_mmd().
>>> - Replace 0X012C with 0x012c in the new define for DP83867_DSP_FFE_CFG.
>>>
>>> RFC patch at:
>>> https://lore.kernel.org/r/20230425054429.3956535-2-s-vadapalli@ti.com/
>>>
>>> Changes since RFC patch:
>>> - Change patch subject to PATCH net.
>>> - Add Fixes tag.
>>> - Check return value of phy_write_mmd().
>>>
>>>  drivers/net/phy/dp83867.c | 18 +++++++++++++++++-
>>>  1 file changed, 17 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/net/phy/dp83867.c b/drivers/net/phy/dp83867.c
>>> index d75f526a20a4..bbdcc595715d 100644
>>> --- a/drivers/net/phy/dp83867.c
>>> +++ b/drivers/net/phy/dp83867.c
>>> @@ -44,6 +44,7 @@
>>>  #define DP83867_STRAP_STS1	0x006E
>>>  #define DP83867_STRAP_STS2	0x006f
>>>  #define DP83867_RGMIIDCTL	0x0086
>>> +#define DP83867_DSP_FFE_CFG	0x012c
>>>  #define DP83867_RXFCFG		0x0134
>>>  #define DP83867_RXFPMD1	0x0136
>>>  #define DP83867_RXFPMD2	0x0137
>>> @@ -941,8 +942,23 @@ static int dp83867_phy_reset(struct phy_device *phydev)
>>>  
>>>  	usleep_range(10, 20);
>>>  
>>> -	return phy_modify(phydev, MII_DP83867_PHYCTRL,
>>> +	err = phy_modify(phydev, MII_DP83867_PHYCTRL,
>>>  			 DP83867_PHYCR_FORCE_LINK_GOOD, 0);
>>> +	if (err < 0)
>>> +		return err;
>>> +
>>
>> Would be good to add a comment here explaining what this magic write does.
> 
> Sure. Is the following comment acceptable?
> 
> "Configure the DSP Feedforward Equalizer Configuration register to improve short
> cable (< 1 meter) performance. This will not affect long cable performance."
> 
Sounds good. Important is just that the magic value write is explained, so that
readers of the source code don't have to scroll through the commit history.

>>
>>> +	err = phy_write_mmd(phydev, DP83867_DEVADDR, DP83867_DSP_FFE_CFG,
>>> +			    0x0e81);
>>> +	if (err < 0)
>>> +		return err;
>>> +
>>> +	err = phy_write(phydev, DP83867_CTRL, DP83867_SW_RESTART);
>>> +	if (err < 0)
>>> +		return err;
>>> +
>>> +	usleep_range(10, 20);
>>> +
>>> +	return 0;
>>>  }
>>>  
>>>  static void dp83867_link_change_notify(struct phy_device *phydev)
>>
>