Re: [PATCH net-next v5 3/3] net: stmmac: add tc mqprio hw queue priority offload

[Daniel Thompson <daniel@riscstar.com>]

On Tue, Oct 06, 2026 at 12:46:27PM +0200, Lorenzo Bianconi wrote:
> Implement the offload of the tc mqprio hw queue priority in the stmmac
> driver. When the mqprio qdisc is configured in channel mode, the MTL TX
> scheduler is switched to strict priority, and the PSTQX/PSTC priority
> bitmask of each TX queue is programmed from the set of frame priorities
> mapped to the owning traffic class (qopt->prio_tc_map).
>
> The channel mode offload requires the DCB hw feature and a 1:1 TC to TX
> queue mapping, with a single queue per TC. Configurations with AVB queues
> are rejected, since forcing strict priority conflicts with the CBS
> algorithm. The CBS offload is refused while the strict-priority scheduler
> is active, and so is a channel count change that would enable an AVB
> queue.

I don't think these CBS offload checks are necessary. It's quite reasonable
to mix strict priority on low priority queues with CBS on higher
priority queues.

In fact on the hardware I have (TC956x) the lower priority queues don't
implement CBS so mixing AVB and DCB queuing (whether SP or ETS) isn't
just reasonable, it's pretty much unavoidable when using CBS!

As a concrete example of combining mqprio and CBS, the following works well on
TC956x (when I tested an older version of your patch):

--- cut here ---
tc qdisc replace dev eth0 handle 100 parent root mqprio num_tc 4 \
    map  0  0  0  0  0  1  2  3  0  0  0  0  0  0  0  0 \
    queues 1@0 1@1 1@2 1@3 mode channel hw 1

# No CBS for TxQ1 means we can't offload this (h/ware stays in SP mode)
tc qdisc replace dev eth0 parent 100:2 handle 102: cbs \
    idleslope 800000 sendslope -1700000 hicredit 494 locredit -1049 \
    offload 0

tc qdisc replace dev eth0 parent 100:3 handle 103: cbs \
    idleslope 400000 sendslope -2100000 hicredit 247 locredit -1296 \
    offload 1

tc qdisc replace dev eth0 parent 100:4 handle 104: cbs \
    idleslope 200000 sendslope -2300000 hicredit 124 locredit -1419 \
    offload 1
--- cut here ---


Daniel.