netdev - Re: [RFC PATCH 1/2] net: phy: dp83867: add w/a for packet errors seen with short cables

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Date:   Wed, 26 Apr 2023 10:39:02 +0530
From:   Siddharth Vadapalli <s-vadapalli@...com>
To:     Andrew Lunn <andrew@...n.ch>
CC:     <s-vadapalli@...com>, <hkallweit1@...il.com>,
        <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: [RFC PATCH 1/2] net: phy: dp83867: add w/a for packet errors seen
 with short cables



On 25/04/23 17:35, Andrew Lunn wrote:
> On Tue, Apr 25, 2023 at 11:14:28AM +0530, 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.
>>
>> Signed-off-by: Grygorii Strashko <grygorii.strashko@...com>
>> Signed-off-by: Siddharth Vadapalli <s-vadapalli@...com>
> 
> https://www.kernel.org/doc/html/latest/process/maintainer-netdev.html
> 
> Please set the tree in the Subject line to be net.
> Please also add a Fixes: tag, probably for the patch which added this driver.

I will update the subject to "RFC PATCH net" and also add the "Fixes" tag in the
v2 series.

> 
> 
> 
> 
>> ---
>>  drivers/net/phy/dp83867.c | 15 ++++++++++++++-
>>  1 file changed, 14 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/net/phy/dp83867.c b/drivers/net/phy/dp83867.c
>> index 5821f04c69dc..ba60cf35872e 100644
>> --- a/drivers/net/phy/dp83867.c
>> +++ b/drivers/net/phy/dp83867.c
>> @@ -42,6 +42,7 @@
>>  #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
>> @@ -934,8 +935,20 @@ 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;
>> +
>> +	phy_write_mmd(phydev, DP83867_DEVADDR, DP83867_DSP_FFE_CFG, 0X0E81);
> 
> Maybe check the return code for errors?

The return value of phy_write_mmd() doesn't have to be checked since it will be
zero for the following reasons:
The dp83867 driver does not have a custom .write_mmd method. Also, the dp83867
phy does not support clause 45. Due to this, within __phy_write_mmd(), the ELSE
statement will be executed, which results in the return value being zero.

> 
>       Andrew

-- 
Regards,
Siddharth.