Subject: RE: [PATCH iwl-next,v2 2/2] igc: Add Tx hardware timestamp request for AF_XDP zero-copy packet

From: John Fastabend <[email-masked]>
Date: Fri, 01 Mar 2024 09:54:34 -0800

Song Yoong Siang wrote:
> This patch adds support to per-packet Tx hardware timestamp request to
> AF_XDP zero-copy packet via XDP Tx metadata framework. Please note that
> user needs to enable Tx HW timestamp capability via igc_ioctl() with
> SIOCSHWTSTAMP cmd before sending xsk Tx hardware timestamp request.
> 
> Same as implementation in RX timestamp XDP hints kfunc metadata, Timer 0
> (adjustable clock) is used in xsk Tx hardware timestamp. i225/i226 have
> four sets of timestamping registers. Both *skb and *xsk_tx_buffer pointers
> are used to indicate whether the timestamping register is already occupied.
> 
> Furthermore, a boolean variable named xsk_pending_ts is used to hold the
> transmit completion until the tx hardware timestamp is ready. This is
> because, for i225/i226, the timestamp notification event comes some time
> after the transmit completion event. The driver will retrigger hardware irq
> to clean the packet after retrieve the tx hardware timestamp.
> 
> Besides, xsk_meta is added into struct igc_tx_timestamp_request as a hook
> to the metadata location of the transmit packet. When the Tx timestamp
> interrupt is fired, the interrupt handler will copy the value of Tx hwts
> into metadata location via xsk_tx_metadata_complete().
> 
> Co-developed-by: Lai Peter Jun Ann <[email-masked]>
> Signed-off-by: Lai Peter Jun Ann <[email-masked]>
> Signed-off-by: Song Yoong Siang <[email-masked]>
> ---

[...]

>  
> +static void igc_xsk_request_timestamp(void *_priv)
> +{
> +	struct igc_metadata_request *meta_req = _priv;
> +	struct igc_ring *tx_ring = meta_req->tx_ring;
> +	struct igc_tx_timestamp_request *tstamp;
> +	u32 tx_flags = IGC_TX_FLAGS_TSTAMP;
> +	struct igc_adapter *adapter;
> +	unsigned long lock_flags;
> +	bool found = false;
> +	int i;
> +
> +	if (test_bit(IGC_RING_FLAG_TX_HWTSTAMP, &tx_ring->flags)) {
> +		adapter = netdev_priv(tx_ring->netdev);
> +
> +		spin_lock_irqsave(&adapter->ptp_tx_lock, lock_flags);
> +
> +		/* Search for available tstamp regs */
> +		for (i = 0; i < IGC_MAX_TX_TSTAMP_REGS; i++) {
> +			tstamp = &adapter->tx_tstamp[i];
> +
> +			if (tstamp->skb)
> +				continue;
> +
> +			found = true;
> +			break;

Not how I would have written this loop construct seems a bit odd
to default break but it works.

> +		}
> +
> +		/* Return if no available tstamp regs */
> +		if (!found) {
> +			adapter->tx_hwtstamp_skipped++;
> +			spin_unlock_irqrestore(&adapter->ptp_tx_lock,
> +					       lock_flags);
> +			return;
> +		}

[...]

>  
> +static void igc_ptp_free_tx_buffer(struct igc_adapter *adapter,
> +				   struct igc_tx_timestamp_request *tstamp)
> +{
> +	if (tstamp->buffer_type == IGC_TX_BUFFER_TYPE_XSK) {
> +		/* Release the transmit completion */
> +		tstamp->xsk_tx_buffer->xsk_pending_ts = false;
> +		tstamp->xsk_tx_buffer = NULL;
> +		tstamp->buffer_type = 0;
> +
> +		/* Trigger txrx interrupt for transmit completion */
> +		igc_xsk_wakeup(adapter->netdev, tstamp->xsk_queue_index, 0);

Just curious because I didn't find it. Fairly sure I just need to look more,
but don't you want to still 'tstamp->skb = NULL' in this path somewhere?
It looks like triggering the tx interrupt again with buffer_type == 0 wouldn't
do the null.

I suspect I just missed it.