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

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Message-ID: <20200103144623.GI6788@bombadil.infradead.org>
Date:   Fri, 3 Jan 2020 06:46:23 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     yu kuai <yukuai3@...wei.com>
Cc:     klassert@...nel.org, davem@...emloft.net, hkallweit1@...il.com,
        jakub.kicinski@...ronome.com, hslester96@...il.com, mst@...hat.com,
        yang.wei9@....com.cn, netdev@...r.kernel.org,
        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 Fri, Jan 03, 2020 at 08:19:07PM +0800, yu kuai wrote:
> Fixes gcc '-Wunused-but-set-variable' warning:
> 
> drivers/net/ethernet/3com/3c59x.c: In function ‘vortex_up’:
> drivers/net/ethernet/3com/3c59x.c:1551:9: warning: variable
> ‘mii_reg1’ set but not used [-Wunused-but-set-variable]
> 
> It is never used, and so can be removed.
...
>  	if (dev->if_port == XCVR_MII || dev->if_port == XCVR_NWAY) {
> -		mii_reg1 = mdio_read(dev, vp->phys[0], MII_BMSR);
>  		mii_reg5 = mdio_read(dev, vp->phys[0], MII_LPA);

I know nothing about the MII interface, but in general this is not
a safe thing to do.  You're removing a read from a device register.
Register reads can have side effects.  I'm actually quite surprised
that GCC emits this warning, since there should be some kind of
volatile cast in mdio_read() to let GCC know that something unusual
is going on here.