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

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Date:   Fri, 3 Jan 2020 15:59:11 +0100
From:   Michal Kubecek <mkubecek@...e.cz>
To:     netdev@...r.kernel.org
Cc:     Matthew Wilcox <willy@...radead.org>, yu kuai <yukuai3@...wei.com>,
        klassert@...nel.org, davem@...emloft.net, hkallweit1@...il.com,
        jakub.kicinski@...ronome.com, hslester96@...il.com, mst@...hat.com,
        yang.wei9@....com.cn, 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 06:46:23AM -0800, Matthew Wilcox wrote:
> 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.

Removing the call may be wrong (and certainly isn't obviously correct)
but the warning makes sense, IMHO: if the return value is not used
anywhere, there is no point assigning it to a variable.

Michal Kubecek