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: <bb7d6644-38b9-c807-6ef2-45a9d5acefe9@ti.com>
Date:   Tue, 9 May 2023 15:57:33 +0530
From:   Siddharth Vadapalli <s-vadapalli@...com>
To:     Heiner Kallweit <hkallweit1@...il.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>,
        <s-vadapalli@...com>
Subject: Re: [PATCH net v2] net: phy: dp83867: add w/a for packet errors seen
 with short cables



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."

> 
>> +	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)
> 

-- 
Regards,
Siddharth.