[xdp-hints] Re: [PATCH bpf-next v9 4/5] igc: Refactor empty frame insertion for launch time support

[Maciej Fijalkowski <maciej.fijalkowski@intel.com>]

On Thu, Feb 06, 2025 at 02:04:07PM +0800, Song Yoong Siang wrote:
> Refactor the code for inserting an empty frame into a new function
> igc_insert_empty_frame(). This change extracts the logic for inserting
> an empty packet from igc_xmit_frame_ring() into a separate function,
> allowing it to be reused in future implementations, such as the XDP
> zero copy transmit function.
> 
> Remove the igc_desc_unused() checking in igc_init_tx_empty_descriptor()
> because the number of descriptors needed is guaranteed.
> 
> Ensure that skb allocation and DMA mapping work for the empty frame,
> before proceeding to fill in igc_tx_buffer info, context descriptor,
> and data descriptor.
> 
> Rate limit the error messages for skb allocation and DMA mapping failures.
> 
> Update the comment to indicate that the 2 descriptors needed by the empty
> frame are already taken into consideration in igc_xmit_frame_ring().
> 
> Handle the case where the insertion of an empty frame fails and explain
> the reason behind this handling.
> 
> Signed-off-by: Song Yoong Siang <yoong.siang.song@intel.com>

Reviewed-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>

with one nit below.

> ---
>  drivers/net/ethernet/intel/igc/igc_main.c | 84 ++++++++++++++---------
>  1 file changed, 52 insertions(+), 32 deletions(-)
> 
> diff --git a/drivers/net/ethernet/intel/igc/igc_main.c b/drivers/net/ethernet/intel/igc/igc_main.c
> index 84307bb7313e..3df608601a4b 100644
> --- a/drivers/net/ethernet/intel/igc/igc_main.c
> +++ b/drivers/net/ethernet/intel/igc/igc_main.c
> @@ -1092,7 +1092,9 @@ static int igc_init_empty_frame(struct igc_ring *ring,
>  
>  	dma = dma_map_single(ring->dev, skb->data, size, DMA_TO_DEVICE);
>  	if (dma_mapping_error(ring->dev, dma)) {
> -		netdev_err_once(ring->netdev, "Failed to map DMA for TX\n");
> +		if (net_ratelimit())
> +			netdev_err(ring->netdev,
> +				   "DMA mapping error for empty frame\n");

This is open-coded net_err_ratelimited(), no?

>  		return -ENOMEM;
>  	}
>  
> @@ -1108,20 +1110,12 @@ static int igc_init_empty_frame(struct igc_ring *ring,
>  	return 0;
>  }
>  
> -static int igc_init_tx_empty_descriptor(struct igc_ring *ring,
> -					struct sk_buff *skb,
> -					struct igc_tx_buffer *first)
> +static void igc_init_tx_empty_descriptor(struct igc_ring *ring,
> +					 struct sk_buff *skb,
> +					 struct igc_tx_buffer *first)
>  {
>  	union igc_adv_tx_desc *desc;
>  	u32 cmd_type, olinfo_status;
> -	int err;
> -
> -	if (!igc_desc_unused(ring))
> -		return -EBUSY;
> -
> -	err = igc_init_empty_frame(ring, first, skb);
> -	if (err)
> -		return err;
>  
>  	cmd_type = IGC_ADVTXD_DTYP_DATA | IGC_ADVTXD_DCMD_DEXT |
>  		   IGC_ADVTXD_DCMD_IFCS | IGC_TXD_DCMD |
> @@ -1140,8 +1134,6 @@ static int igc_init_tx_empty_descriptor(struct igc_ring *ring,
>  	ring->next_to_use++;
>  	if (ring->next_to_use == ring->count)
>  		ring->next_to_use = 0;
> -
> -	return 0;
>  }
>  
>  #define IGC_EMPTY_FRAME_SIZE 60
> @@ -1567,6 +1559,41 @@ static bool igc_request_tx_tstamp(struct igc_adapter *adapter, struct sk_buff *s
>  	return false;
>  }
>  
> +static int igc_insert_empty_frame(struct igc_ring *tx_ring)
> +{
> +	struct igc_tx_buffer *empty_info;
> +	struct sk_buff *empty_skb;
> +	void *data;
> +	int ret;
> +
> +	empty_info = &tx_ring->tx_buffer_info[tx_ring->next_to_use];
> +	empty_skb = alloc_skb(IGC_EMPTY_FRAME_SIZE, GFP_ATOMIC);
> +	if (unlikely(!empty_skb)) {
> +		if (net_ratelimit())
> +			netdev_err(tx_ring->netdev,
> +				   "skb alloc error for empty frame\n");

ditto

> +		return -ENOMEM;
> +	}
> +
> +	data = skb_put(empty_skb, IGC_EMPTY_FRAME_SIZE);
> +	memset(data, 0, IGC_EMPTY_FRAME_SIZE);
> +
> +	/* Prepare DMA mapping and Tx buffer information */
> +	ret = igc_init_empty_frame(tx_ring, empty_info, empty_skb);
> +	if (unlikely(ret)) {
> +		dev_kfree_skb_any(empty_skb);
> +		return ret;
> +	}
> +
> +	/* Prepare advanced context descriptor for empty packet */
> +	igc_tx_ctxtdesc(tx_ring, 0, false, 0, 0, 0);
> +
> +	/* Prepare advanced data descriptor for empty packet */
> +	igc_init_tx_empty_descriptor(tx_ring, empty_skb, empty_info);
> +
> +	return 0;
> +}
> +
>  static netdev_tx_t igc_xmit_frame_ring(struct sk_buff *skb,
>  				       struct igc_ring *tx_ring)
>  {
> @@ -1586,6 +1613,7 @@ static netdev_tx_t igc_xmit_frame_ring(struct sk_buff *skb,
>  	 *	+ 1 desc for skb_headlen/IGC_MAX_DATA_PER_TXD,
>  	 *	+ 2 desc gap to keep tail from touching head,
>  	 *	+ 1 desc for context descriptor,
> +	 *	+ 2 desc for inserting an empty packet for launch time,
>  	 * otherwise try next time
>  	 */
>  	for (f = 0; f < skb_shinfo(skb)->nr_frags; f++)
> @@ -1605,24 +1633,16 @@ static netdev_tx_t igc_xmit_frame_ring(struct sk_buff *skb,
>  	launch_time = igc_tx_launchtime(tx_ring, txtime, &first_flag, &insert_empty);
>  
>  	if (insert_empty) {
> -		struct igc_tx_buffer *empty_info;
> -		struct sk_buff *empty;
> -		void *data;
> -
> -		empty_info = &tx_ring->tx_buffer_info[tx_ring->next_to_use];
> -		empty = alloc_skb(IGC_EMPTY_FRAME_SIZE, GFP_ATOMIC);
> -		if (!empty)
> -			goto done;
> -
> -		data = skb_put(empty, IGC_EMPTY_FRAME_SIZE);
> -		memset(data, 0, IGC_EMPTY_FRAME_SIZE);
> -
> -		igc_tx_ctxtdesc(tx_ring, 0, false, 0, 0, 0);
> -
> -		if (igc_init_tx_empty_descriptor(tx_ring,
> -						 empty,
> -						 empty_info) < 0)
> -			dev_kfree_skb_any(empty);
> +		/* Reset the launch time if the required empty frame fails to
> +		 * be inserted. However, this packet is not dropped, so it
> +		 * "dirties" the current Qbv cycle. This ensures that the
> +		 * upcoming packet, which is scheduled in the next Qbv cycle,
> +		 * does not require an empty frame. This way, the launch time
> +		 * continues to function correctly despite the current failure
> +		 * to insert the empty frame.
> +		 */
> +		if (igc_insert_empty_frame(tx_ring))
> +			launch_time = 0;
>  	}
>  
>  done:
> -- 
> 2.34.1
>