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

["Song, Yoong Siang" <yoong.siang.song@intel.com>]

On Sunday, March 3, 2024 9:59 AM, John Fastabend <john.fastabend@gmail.com> wrote:
>Song, Yoong Siang wrote:
>> On Saturday, March 2, 2024 1:55 AM, John Fastabend <john.fastabend@gmail.com> wrote:
>> >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 <jun.ann.lai@intel.com>
>> >> Signed-off-by: Lai Peter Jun Ann <jun.ann.lai@intel.com>
>> >> Signed-off-by: Song Yoong Siang <yoong.siang.song@intel.com>
>> >> ---
>> >
>> >[...]
>> >
>> >>
>> >> +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.
>>
>> Hi John,
>> First of all, thank you for reviewing the patch.
>> I agree that we can make the loop better.
>> How about I change the loop to below:
>
>That is more natural to me, but whatever reads best for you
>is probably ok.
>

I am ok with both versions.
I will change it in next version for better readability and clarity.

>>
>> for (i = 0; i < IGC_MAX_TX_TSTAMP_REGS; i++) {
>> 	tstamp = &adapter->tx_tstamp[i];
>>
>> 	if (!tstamp->skb) {
>> 		found = true;
>> 		break;
>> 	}
>> }
>>
>> >
>> >> +		}
>> >> +
>> >> +		/* 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.
>>
>> Since the timestamp register will only be used by either skb or xsk,
>> So we make tstamp->xsk_tx_buffer and tstamp->skb as union,
>> Then based on tstamp->buffer_type to decide whether
>> tstamp->xsk_tx_buffer or tstamp->skb should be used.
>>
>> My thought is, by setting tstamp->xsk_tx_buffer=NULL,
>> tstamp->skb will become NULL as well, and vice versa.
>
>Seems good to me. Maybe a comment though? Otherwise I suspect next
>person to read the code will have to spend the extra time to track
>it down as well.
>

Sure. I will add comment in next version.

>>
>> Thanks & Regards
>> Siang
>
>Also feel free to carry my ack into the v2 if you make the couple
>small nitpick changes.
>
>Acked-by: John Fastabend <john.fastabend@gmail.com>
>
Thank you for your acknowledgment.