linux-kernel - Re: [PATCH] usbnet: smsc95xx: simplify tx

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Message-Id: <20181005.142412.601607260441380535.davem@davemloft.net>
Date:   Fri, 05 Oct 2018 14:24:12 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     ben.dooks@...ethink.co.uk
Cc:     netdev@...r.kernel.org, David.Laight@...LAB.COM, oneukum@...e.com,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-kernel@...ts.codethink.co.uk
Subject: Re: [PATCH] usbnet: smsc95xx: simplify tx_fixup code

From: Ben Dooks <ben.dooks@...ethink.co.uk>
Date: Tue,  2 Oct 2018 17:56:02 +0100

> -	memcpy(skb->data, &tx_cmd_a, 4);
> +	ptr = skb_push(skb, 8);
> +	tx_cmd_a = cpu_to_le32(tx_cmd_a);
> +	tx_cmd_b = cpu_to_le32(tx_cmd_b);
> +	memcpy(ptr, &tx_cmd_a, 4);
> +	memcpy(ptr+4, &tx_cmd_b, 4);

Even a memcpy() through a void pointer does not guarantee that gcc will
not emit word sized loads and stores.

You must use the get_unaligned()/put_unaligned() facilities to do this
properly.

I also agree that making a proper type and structure instead of using
a void pointer would be better.