[xdp-hints] Re: [PATCH bpf-next v7 4/5] igc: Refactor empty packet insertion into a reusable function

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

On Tue, Feb 04, 2025 at 12:07:21PM +0100, Song, Yoong Siang wrote:
> On Tuesday, February 4, 2025 5:50 PM, Fijalkowski, Maciej <maciej.fijalkowski@intel.com> wrote:
> >On Tue, Feb 04, 2025 at 08:49:06AM +0800, Song Yoong Siang wrote:
> >> Refactor the code for inserting an empty packet into a new function
> >> igc_insert_empty_packet(). 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.
> >>
> >> This patch introduces no functional changes.
> >>
> >> Signed-off-by: Song Yoong Siang <yoong.siang.song@intel.com>
> >> ---
> >>  drivers/net/ethernet/intel/igc/igc_main.c | 42 ++++++++++++-----------
> >>  1 file changed, 22 insertions(+), 20 deletions(-)
> >>
> >> diff --git a/drivers/net/ethernet/intel/igc/igc_main.c
> >b/drivers/net/ethernet/intel/igc/igc_main.c
> >> index 56a35d58e7a6..c3edd8bcf633 100644
> >> --- a/drivers/net/ethernet/intel/igc/igc_main.c
> >> +++ b/drivers/net/ethernet/intel/igc/igc_main.c
> >> @@ -1566,6 +1566,26 @@ static bool igc_request_tx_tstamp(struct igc_adapter
> >*adapter, struct sk_buff *s
> >>  	return false;
> >>  }
> >>
> >> +static void igc_insert_empty_packet(struct igc_ring *tx_ring)
> >> +{
> >> +	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)
> >> +		return;
> >> +
> >> +	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);
> >> +}
> >> +
> >>  static netdev_tx_t igc_xmit_frame_ring(struct sk_buff *skb,
> >>  				       struct igc_ring *tx_ring)
> >>  {
> >> @@ -1603,26 +1623,8 @@ static netdev_tx_t igc_xmit_frame_ring(struct
> >sk_buff *skb,
> >>  	skb->tstamp = ktime_set(0, 0);
> >>  	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;
> >
> >shouldn't this be 'goto drop' from day 1? pretty weird to silently ignore
> >allocation error.
> >
> 
> Hi Fijalkowski Maciej,
> 
> Thanks for your comments.
> 
> "insert an empty packet" is a launch time trick to send a packet in
> next Qbv cycle. The design is, the driver will still sending the
> packet, even the empty packet insertion trick is fail (unable to
> allocate). The intention of this patch set is to enable launch time
> on XDP zero-copy data path, so I try not to change the original
> behavior of launch time.
> 
> btw, do you think driver should drop the packet if something went
> wrong with the launch time, like launch time offload not enabled,
> launch time over horizon, empty packet insertion fail, etc?
> If yes, then maybe i can submit another patch to change the behavior
> of launch time and we can continue to discuss there.

That's rather a question to you since I am no TSN expert here :P
the alloc skbs failures would rather be a minor thing but anyways it
didn't look correct from a first glance to silently ignore this behavior
if rest of the logic relies on this. I won't be insisting on any changes
here but it's something you could consider to change maybe.

The real question is in 5/5, regarding the cleaning of these empty descs
from ZC path.

> 
> >> -
> >> -		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);
> >
> >ditto
> >
> 
> ditto
> 
> >> -	}
> >> +	if (insert_empty)
> >> +		igc_insert_empty_packet(tx_ring);
> >>
> >>  done:
> >>  	/* record the location of the first descriptor for this packet */
> >> --
> >> 2.34.1
> >>
> 
> Thanks & Regards
> Siang