[xdp-hints] Re: [PATCH bpf-next v2 2/3] net: stmmac: Add txtime support to XDP ZC

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

Song, Yoong Siang wrote:
> On Friday, December 1, 2023 11:02 PM, Jesper Dangaard Brouer wrote:
> >On 12/1/23 07:24, Song Yoong Siang wrote:
> >> This patch enables txtime support to XDP zero copy via XDP Tx
> >> metadata framework.
> >>
> >> Signed-off-by: Song Yoong Siang<yoong.siang.song@intel.com>
> >> ---
> >>   drivers/net/ethernet/stmicro/stmmac/stmmac.h      |  2 ++
> >>   drivers/net/ethernet/stmicro/stmmac/stmmac_main.c | 13 +++++++++++++
> >>   2 files changed, 15 insertions(+)
> >
> >I think we need to see other drivers using this new feature to evaluate
> >if API is sane.
> >
> >I suggest implementing this for igc driver (chip i225) and also for igb
> >(i210 chip) that both support this kind of LaunchTime feature in HW.
> >
> >The API and stmmac driver takes a u64 as time.
> >I'm wondering how this applies to i210 that[1] have 25-bit for
> >LaunchTime (with 32 nanosec granularity) limiting LaunchTime max 0.5
> >second into the future.
> >And i225 that [1] have 30-bit max 1 second into the future.
> >
> >
> >[1]
> >https://github.com/xdp-project/xdp-
> >project/blob/master/areas/tsn/code01_follow_qdisc_TSN_offload.org
> 
> I am using u64 for launch time because existing EDT framework is using it.
> Refer to struct sk_buff below. Both u64 and ktime_t can be used as launch time.
> I choose u64 because ktime_t often requires additional type conversion and
> we didn't expect negative value of time.
> 
> include/linux/skbuff.h-744- *   @tstamp: Time we arrived/left
> include/linux/skbuff.h:745- *   @skb_mstamp_ns: (aka @tstamp) earliest departure time; start point
> include/linux/skbuff.h-746- *           for retransmit timer
> --
> include/linux/skbuff.h-880-     union {
> include/linux/skbuff.h-881-             ktime_t         tstamp;
> include/linux/skbuff.h:882-             u64             skb_mstamp_ns; /* earliest departure time */
> include/linux/skbuff.h-883-     };
> 
> tstamp/skb_mstamp_ns are used by various drivers for launch time support
> on normal packet, so I think u64 should be "friendly" to all the drivers. For an
> example, igc driver will take launch time from tstamp and recalculate it 
> accordingly (i225 expect user to program "delta time" instead of "time" into
> HW register).
> 
> drivers/net/ethernet/intel/igc/igc_main.c-1602- txtime = skb->tstamp;
> drivers/net/ethernet/intel/igc/igc_main.c-1603- skb->tstamp = ktime_set(0, 0);
> drivers/net/ethernet/intel/igc/igc_main.c:1604- launch_time = igc_tx_launchtime(tx_ring, txtime, &first_flag, &insert_empty);
> 
> Do you think this is enough to say the API is sane?

u64 nsec sounds sane to be. It must be made explicit with clock source
it is against.

Some applications could want to do the conversion from a clock source
to raw NIC cycle counter in userspace or BPF and program the raw
value. So it may be worthwhile to add an clock source argument -- even
if initially only CLOCK_MONOTONIC is supported.

See tools/testing/selftests/net/so_txtime.sh for how the FQ and ETF
qdiscs already disagree on the clock source that they use.