[xdp-hints] Re: [PATCH bpf-next v3 2/3] net: stmmac: add Launch Time support to XDP ZC

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

Jesper Dangaard Brouer wrote:
> 
> 
> On 12/3/23 17:51, Song Yoong Siang wrote:
> > This patch enables Launch Time (Time-Based Scheduling) 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 ++
> 
> As requested before, I think we need to see another driver implementing 
> this.
> 
> I propose driver igc and chip i225.
> 
> The interesting thing for me is to see how the LaunchTime max 1 second
> into the future[1] is handled code wise. One suggestion is to add a 
> section to Documentation/networking/xsk-tx-metadata.rst per driver that 
> mentions/documents these different hardware limitations.  It is natural 
> that different types of hardware have limitations.  This is a close-to 
> hardware-level abstraction/API, and IMHO as long as we document the 
> limitations we can expose this API without too many limitations for more 
> capable hardware.

I would assume that the kfunc will fail when a value is passed that
cannot be programmed.

What is being implemented here already exists for qdiscs. The FQ
qdisc takes a horizon attribute and

    "
    when a packet is beyond the horizon
        at enqueue() time:
        - either drop the packet (default policy)
        - or cap its delivery time to the horizon.
    "
    commit 39d010504e6b ("net_sched: sch_fq: add horizon attribute")

Having the admin manually configure this on the qdisc based on
off-line knowledge of the device is more fragile than if the device
would somehow signal its limit to the stack.

But I don't think we should add enforcement of that as a requirement
for this xdp extension of pacing.