netdev - Re: [PATCH net-next v2 6/7] drivers: net: smc911x: Fix cast from pointer to integer of different size

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Message-ID: <20201105144711.40a2f8f6@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Thu, 5 Nov 2020 14:47:11 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     netdev <netdev@...r.kernel.org>, Nicolas Pitre <nico@...xnic.net>,
        David Laight <David.Laight@...LAB.COM>,
        Lee Jones <lee.jones@...aro.org>
Subject: Re: [PATCH net-next v2 6/7] drivers: net: smc911x: Fix cast from
 pointer to integer of different size

On Wed,  4 Nov 2020 16:48:57 +0100 Andrew Lunn wrote:
> -	buf = (char*)((u32)skb->data & ~0x3);
> -	len = (skb->len + 3 + ((u32)skb->data & 3)) & ~0x3;
> -	cmdA = (((u32)skb->data & 0x3) << 16) |
> +	offset = (unsigned long)skb->data & 3;
> +	buf = skb->data - offset;
> +	len = skb->len + offset;
> +	cmdA = (offset << 16) |

The len calculation is wrong, you trusted people on the mailing list 
too much ;)

This is better:

-	len = (skb->len + 3 + ((u32)skb->data & 3)) & ~0x3;
+	len = round_up(skb->len, 4);

You can also do PTR_ALIGN_DOWN() for the skb->data if you want.
And give the same treatment to the other side of the #if statement.