Re: [PATCH v3 3/4] rt2x00: Reorganize L2 padding inserting function.

[Ivo van Doorn <ivdoorn@gmail.com>]

On Monday 30 November 2009, Gertjan van Wingerde wrote:
> Simplify the rt2x00queue_insert_l2pad function by handling the alignment
> operations one by one. Do not special case special circumstances.
> Basically first perform header alignment, and then perform payload alignment
> (if any payload does exist). This results in a properly aligned skb.
> 
> The end result is better readable code, with better results, as now L2 padding
> is inserted only when a payload is actually present in the frame.
> 
> Signed-off-by: Gertjan van Wingerde <gwingerde@gmail.com>
> ---
>  drivers/net/wireless/rt2x00/rt2x00queue.c |   60 +++++++++++++----------------
>  1 files changed, 27 insertions(+), 33 deletions(-)
> 
> diff --git a/drivers/net/wireless/rt2x00/rt2x00queue.c b/drivers/net/wireless/rt2x00/rt2x00queue.c
> index 719f4ae..7452fa8 100644
> --- a/drivers/net/wireless/rt2x00/rt2x00queue.c
> +++ b/drivers/net/wireless/rt2x00/rt2x00queue.c
> @@ -177,45 +177,38 @@ void rt2x00queue_align_payload(struct sk_buff *skb, unsigned int header_length)
>  
>  void rt2x00queue_insert_l2pad(struct sk_buff *skb, unsigned int header_length)
>  {
> -	unsigned int frame_length = skb->len;
> +	unsigned int payload_length = skb->len - header_length;
>  	unsigned int header_align = ALIGN_SIZE(skb, 0);
>  	unsigned int payload_align = ALIGN_SIZE(skb, header_length);
>  	unsigned int l2pad = L2PAD_SIZE(header_length);
>  
> -	if (header_align == payload_align) {
> -		/*
> -		 * Both header and payload must be moved the same
> -		 * amount of bytes to align them properly. This means
> -		 * we don't use the L2 padding but just move the entire
> -		 * frame.
> -		 */
> -		rt2x00queue_align_frame(skb);
> -	} else if (!payload_align) {
> -		/*
> -		 * Simple L2 padding, only the header needs to be moved,
> -		 * the payload is already properly aligned.
> -		 */
> -		skb_push(skb, header_align);
> -		memmove(skb->data, skb->data + header_align, header_length);
> -	} else {
> -		/*
> -		 *
> -		 * Complicated L2 padding, both header and payload need
> -		 * to be moved. By default we only move to the start
> -		 * of the buffer, so our header alignment needs to be
> -		 * increased if there is not enough room for the header
> -		 * to be moved.
> -		 */
> -		if (payload_align > header_align)
> -			header_align += 4;
> +	/*
> +	 * Adjust the header alignment if the payload needs to be moved more
> +	 * than the header.
> +	 */
> +	if (payload_align > header_align)
> +		header_align += 4;
> +
> +	/* There is nothing to do if no alignment is needed */
> +	if (!header_align)
> +		return;
>  
> -		skb_push(skb, header_align);
> -		memmove(skb->data, skb->data + header_align, header_length);
> +	/* Reserve the amount of space needed in front of the frame */
> +	skb_push(skb, header_align);
> +
> +	/*
> +	 * Move the header.
> +	 */
> +	memmove(skb->data, skb->data + header_align, header_length);
> +
> +	/* Move the payload, if present and if required */
> +	if (payload_length && payload_align)
>  		memmove(skb->data + header_length + l2pad,
>  			skb->data + header_length + l2pad + payload_align,
> -			frame_length - header_length);
> -		skb_trim(skb, frame_length + l2pad);
> -	}
> +			payload_length);
> +
> +	/* Trim the skb to the correct size */
> +	skb_trim(skb, header_length + l2pad + payload_length);
>  }
>  
>  void rt2x00queue_remove_l2pad(struct sk_buff *skb, unsigned int header_length)
> @@ -343,7 +336,8 @@ static void rt2x00queue_create_tx_descriptor(struct queue_entry *entry,
>  	 * Header and alignment information.
>  	 */
>  	txdesc->header_length = ieee80211_get_hdrlen_from_skb(entry->skb);
> -	if (test_bit(DRIVER_REQUIRE_L2PAD, &rt2x00dev->flags))
> +	if (test_bit(DRIVER_REQUIRE_L2PAD, &rt2x00dev->flags) &&
> +	    (entry->skb->len > txdesc->header_length))
>  		txdesc->l2pad = L2PAD_SIZE(txdesc->header_length);

This check is a bit unclear, are you not simply checking for ieee80211_is_data() or
ieee80211_is_data_present()?

Ivo