Re: [PATCH net-next v5] net: airoha: add HW GRO offload support

[Lorenzo Bianconi <lorenzo@kernel.org>]

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On Sep 03, Paolo Abeni wrote:
> On 9/3/26 2:35 PM, Lorenzo Bianconi wrote:
> >> On 8/31/26 8:34 AM, Lorenzo Bianconi wrote:
> >>> Add hardware GRO offload support to the airoha_eth driver, leveraging
> >>> the EN7581/AN7583 SoC's 8 dedicated LRO hardware queues mapped to RX
> >>> queues 24-31. HW GRO offloading does not support Scatter-Gather (SG) so
> >>> it is required to increase the page_pool allocation order to 2 for RX
> >>> queues 24-31 (LRO queues).
> >>> Since HW GRO is configured per-QDMA and shared across all devices using
> >>> it, HW GRO is mutually exclusive with multiple devices bound to the
> >>> same QDMA block. Call airoha_update_netdev_features() in
> >>> airoha_dev_set_qdma() so that NETIF_F_GRO_HW availability is re-evaluated
> >>> whenever the QDMA user count changes (device registration and runtime QDMA
> >>> migration).
> >>> Set CHECKSUM_PARTIAL with pseudo-header checksum on aggregated packets
> >>> so that L3-forwarded traffic is correctly handled by the GSO/TSO path
> >>> on the egress device.
> >>> The HW does not report the per-segment MSS (msg3[31:16] only reports
> >>> the max aggregated size), so the gso_size of an aggregated packet is
> >>> just an approximation computed as DIV_ROUND_UP(data_len, agg_count).
> >>
> >> What is the <max aggregated size> exactly? I read it as the maximum size
> >> of the aggregated segments, am I correct?
> > 
> > Hi Paolo,
> > 
> > with "max aggregated size" I refer to the length of the aggregated TCP
> > packet (composed by multiple segments). This value is reported via
> > QDMA_DESC_LEN_MASK field in the DMA descriptor. Moreover, the hw reports
> > the exact number of the aggregated segments via QDMA_ETH_RXMSG_AGG_COUNT_MASK
> > field.
> > 
> >>
> >> Also any more details on how the aggregation engine works? i.e. can it
> >> aggregate "random" segment sizes (i.e. 200 - 300 - 400) or does it
> >> respect HW_GRO layout? (i.e. all segments except the last one must have
> >> equal size, the can be smaller).
> > 
> > The hw engine, for each LRO queue, is configured with:
> > 
> > - CDM_LRO_AGG_NUM_MASK: max number of segments for each aggregated TCP packet
> >                         (64 in the current configuration).
> > - CDM_LRO_AGG_SIZE_MASK: max size of the aggregated TCP packet (composed by
> >                          multiple segments). 16KB in the current configuration.
> > - CDM_LRO_AGG_TIME_MASK: max timeout to compose the aggregated TCP packet.
> > 
> > In order to validate the scenario, I run the following test:
> > 
> > TCP client:
> > ------------
> > - disable TSO/GSO
> > - set MSS to 256 bytes (iperf3 -M option)
> > 
> > I can see multiple 310 bytes TCP segments on the wire
> > 
> > TCP server: (where rx-gro-hw is enabled):
> > -------------------------------------------
> > - the engine aggregates ~64 segments in a ~16Kbyes TCP packet
> > 
> >   so gso_size ~ 16KB / 64 ~ 256B
> > 
> > I guess this would be the behaviour even if the original packets
> > have different size (not sure if it is a real use-case).
> Lacking more details from the vendor, I think you need to use a
> pktdrill-like sender, explicitly sets the segment lengths to some not
> mergeable (like, i.e. 200-300-400) but otherwise fitting a GRO packet
> (i.e. same hdr except for the sequence number and push flag allowed only
> in the last packet), ensure that the aggregation timeout lasts long
> enough to receive all of them, and check if the engine really aggregates
> them or not.

I think in the example you pointed out (length 200,300,400) the hw engine will
create a single TCP packet composed by 3 segments (with gso_size = 300) while
sw GRO will just push sigle skbs. I agree this is just a GRO approximation.
If it is not enough I am fine to switch back to LRO implementation.

Regards,
Lorenzo

> 
> /P
> 

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