netdev - Re: [RFT PATCH 7/9] ethtool: prepare for larger netdev_features

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Message-ID: <1308604618.2701.189.camel@bwh-desktop>
Date:	Mon, 20 Jun 2011 22:16:58 +0100
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Michał Mirosław <mirq-linux@...e.qmqm.pl>
Cc:	netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>
Subject: Re: [RFT PATCH 7/9] ethtool: prepare for larger netdev_features_t
 type

On Mon, 2011-06-20 at 21:14 +0200, Michał Mirosław wrote:
[...]
> @@ -125,19 +131,26 @@ static int ethtool_set_features(struct net_device *dev, void __user *useraddr)
>  	if (copy_from_user(features, useraddr, sizeof(features)))
>  		return -EFAULT;
>  
> -	if (features[0].valid & ~NETIF_F_ETHTOOL_BITS)
> +	/* I wonder if the compiler will be smart enough to loop-unroll
> +	 * and optimize this... (no worries if not) --mq */
> +	for (i = ETHTOOL_DEV_FEATURE_WORDS; i-- > 0; ) {
> +		valid = (valid << 32)|features[i].valid;
> +		wanted = (wanted << 32)|features[i].requested;
> +	}
[...]

I don't know (or care) about optimisation of this, but I would expect
gcc to complain about shifting a 32-bit value by 32 bits.  I suggest you
write this as:

	for (i = 0; i < ETHTOOL_DEV_FEATURE_WORDS; ++i) {
		valid |= (netdev_features_t)features[i].valid << 32 *i;
		wanted |= (netdev_features_t)features[i].requested << 32 *i;
	}

Ben.

-- 
Ben Hutchings, Senior Software Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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