Re: [PATCH v4 net-next 12/15] net/sched: taprio: pass mqprio queue configuration to ndo_setup_tc()

[Simon Horman <simon.horman@corigine.com>]

On Mon, Jan 30, 2023 at 07:31:42PM +0200, Vladimir Oltean wrote:
> The taprio offload does not currently pass the mqprio queue configuration
> down to the offloading device driver. So the driver cannot act upon the
> TXQ counts/offsets per TC, or upon the prio->tc map. It was probably
> assumed that the driver only wants to offload num_tc (see
> TC_MQPRIO_HW_OFFLOAD_TCS), which it can get from netdev_get_num_tc(),
> but there's clearly more to the mqprio configuration than that.
> 
> To remedy that, we need to actually reconstruct a struct
> tc_mqprio_qopt_offload to pass as part of the tc_taprio_qopt_offload.
> The problem is that taprio doesn't keep a persistent reference to the
> mqprio queue structure in its own struct taprio_sched, instead it just
> applies the contents of that to the netdev state (prio:tc map, per-TC
> TXQ counts and offsets, num_tc etc). Maybe it's easier to understand
> why, when we look at the size of struct tc_mqprio_qopt_offload: 352
> bytes on arm64. Keeping such a large structure would throw off the
> memory accesses in struct taprio_sched no matter where we put it.
> So we prefer to dynamically reconstruct the mqprio offload structure
> based on netdev information, rather than saving a copy of it.
> 
> Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>

Reviewed-by: Simon Horman <simon.horman@corigine.com>