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

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Date:   Mon, 8 Oct 2018 08:41:49 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'David Miller' <davem@...emloft.net>,
        "ben.dooks@...ethink.co.uk" <ben.dooks@...ethink.co.uk>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "oneukum@...e.com" <oneukum@...e.com>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-kernel@...ts.codethink.co.uk" 
        <linux-kernel@...ts.codethink.co.uk>
Subject: RE: [PATCH] usbnet: smsc95xx: simplify tx_fixup code

From: David Miller
> Sent: 05 October 2018 22:24
> 
> 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.

True, but only if gcc can 'see' something that would require the
pointer be aligned.
In this case the void pointer comes from an external function
so is fine.

> 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.

The structure would need to be marked 'packed' - since its alignment
isn't guaranteed.
Then you don't need to use put_unaligned().

If it wasn't 'packed' then gcc would implement
memcpy(&hdr->tx_cmd_a, &tx_cmd_a, 4) using an aligned write.

	David

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