linux-kernel - Re: [PATCH] net: 3com: 3c59x: remove set but not used variable 'mii

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Message-ID: <b4697457-51d2-c987-4138-b4b2b92e391d@gmail.com>
Date:   Fri, 3 Jan 2020 11:17:12 -0800
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Vladimir Oltean <olteanv@...il.com>, Andrew Lunn <andrew@...n.ch>
Cc:     Matthew Wilcox <willy@...radead.org>, yu kuai <yukuai3@...wei.com>,
        klassert@...nel.org, "David S. Miller" <davem@...emloft.net>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        hslester96@...il.com, mst@...hat.com, yang.wei9@....com.cn,
        netdev <netdev@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>, yi.zhang@...wei.com,
        zhengbin13@...wei.com
Subject: Re: [PATCH] net: 3com: 3c59x: remove set but not used variable
 'mii_reg1'

On 1/3/20 11:13 AM, Vladimir Oltean wrote:
> Hi Andrew,
> 
> On Fri, 3 Jan 2020 at 19:54, Andrew Lunn <andrew@...n.ch> wrote:
>>
>> I fully agree about the general case. However, reading the MII_BMSR
>> should not have any side affects. It would be an odd Ethernet PHY if
>> it did.
> 
> This is not really correct. As far as I know the clause 22 spec
> requires the link status bit in BMSR to be latching low, so that
> momentary losses of link can be caught post-facto.
> In fact, even genphy_update_link treats this case:
> 
>     /* The link state is latched low so that momentary link
>      * drops can be detected. Do not double-read the status
>      * in polling mode to detect such short link drops.
>      */
>     if (!phy_polling_mode(phydev)) {
>         status = phy_read(phydev, MII_BMSR);
>         if (status < 0)
>             return status;
>         else if (status & BMSR_LSTATUS)
>             goto done;
>     }
> 
> So no, reading BMSR generally is not without side effects, and that
> does not make the PHY odd.
> 
> Whether clearing the latching-low status bits is of any relevance to
> the 3com 3c59x driver bookkeeping, that I have not clue.

And since more reviewers are on the same boat, the fix should probably
look to eliminate the warning by doing something like:

(void)mdio_read(dev, vp->phys[0], MII_BMSR);
-- 
Florian