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

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Message-ID: <CA+h21hqcz=QF8bq285JjdOn+gsOGvGSnDiWzDOS5-XGAGGGr9w@mail.gmail.com>
Date:   Fri, 3 Jan 2020 21:13:07 +0200
From:   Vladimir Oltean <olteanv@...il.com>
To:     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'

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.

>
>    Andrew

Regards,
-Vladimir