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

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

On Monday, December 4, 2023 10:55 PM, Willem de Bruijn wrote:
>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.

Sure. I will include igc patches in next version.

>>
>> 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.

Sure. I will try to add hardware limitations in documentation. 

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

In current design, the xsk_tx_metadata_request() dint got return value. 
So user won't know if their request is fail. 
It is complex to inform user which request is failing. 
Therefore, IMHO, it is good that we let driver handle the error silently. 

>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.