[PATCH net-next v6 0/9] net: bcmgenet: support larger MTUs

[Nicolai Buchwitz]

This is v6 of my genet series which adds larger MTU / jumbo packet support.

Larger MTU aka. jumbo frames are requested often by Raspberry Pi users, but
the hardware has its quirks with larger frames. Dave Stevenson's PoC [1] had
the receive side working, and thanks to Justin, who checked the RTL for what
the hardware does with long frames, I could add the missing puzzle pieces.

GENET never sets dev->max_mtu and the packet ready thresholds stay at their
2048 byte reset default, so long frames were dropped on receive and never
sent on transmit.

Patches 1 to 5 prepare the driver, patch 8 sets the MTU, as far as one
descriptor reaches (3564 bytes with 4K pages). Patch 9 reassembles what the
hardware hands up above that, which gets the ceiling to 16347 [2]. Patch 6
splits a frame ending just past the packet ready threshold, which otherwise
stops the transmitter [3]. Patch 7 builds the new rings before dropping the
old ones, so a failed MTU change leaves the interface alone.

Patch 8 needs bcmgenet_fini_dma() to clear rx_cbs and tx_cbs, which landed
in net as 1c1f51c3ee9c.

Tested on a Raspberry Pi CM4 (GENET v5), peer at the same MTU:
- MTU 1500: iperf3 TCP 941/941 Mbit/s, UDP 956 Mbit/s, unchanged
- MTU 9000: iperf3 TCP 990/989 Mbit/s, UDP 993 Mbit/s, no loss
- MTU 1500 to 16347 with traffic and ifdown/ifup's
- pktgen at line rate, 3858/3872/3886 byte frames mixed with 64 byte ones

Pierre-Marin Leclercq tested v5 on a Raspberry Pi 4.

[1] https://github.com/raspberrypi/linux/pull/7614
[2] https://github.com/raspberrypi/linux/pull/7617#issuecomment-5669564355
[3] https://github.com/raspberrypi/linux/pull/7617#issuecomment-5735557324

Signed-off-by: Nicolai Buchwitz <nb@xxxxxxxxxxx>
---
Changes in v6:
- Patch 6 splits the frame instead of padding it (Jakub)
- New patch 7 builds the rings before freeing the running ones (Jakub)
- Patch 8 drops the rollback and the datapath_up flag (Jakub)
- Dropped Pierre-Marin's Tested-by on patches 6 and 8, both changed

v5: https://patch.msgid.link/20261013-nb-genet-mtu-nn-v2-v5-0-076b6fc240be@xxxxxxxxxxx
v4: https://patch.msgid.link/20261012-nb-genet-mtu-nn-v2-v4-0-2e2e3bfaf8cc@xxxxxxxxxxx
v3: https://patch.msgid.link/20261007-nb-genet-mtu-nn-v2-v3-0-74a796c019ce@xxxxxxxxxxx
v2: https://patch.msgid.link/20261006-nb-genet-mtu-nn-v2-v2-0-c4be21151912@xxxxxxxxxxx
v1: https://patch.msgid.link/20261002-nb-genet-mtu-nn-v2-v1-0-96dc6d54cbee@xxxxxxxxxxx

To: Doug Berger <opendmb@xxxxxxxxx>
To: Florian Fainelli <florian.fainelli@xxxxxxxxxxxx>
To: Nicolai Buchwitz <nb@xxxxxxxxxxx>
To: Broadcom internal kernel review list <bcm-kernel-feedback-list@xxxxxxxxxxxx>
To: Andrew Lunn <andrew+netdev@xxxxxxx>
To: "David S. Miller" <davem@xxxxxxxxxxxxx>
To: Eric Dumazet <edumazet@xxxxxxxxxx>
To: Jakub Kicinski <kuba@xxxxxxxxxx>
To: Paolo Abeni <pabeni@xxxxxxxxxx>
Cc: Florian Fainelli <f.fainelli@xxxxxxxxx>
Cc: netdev@xxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx

---
Nicolai Buchwitz (9):
      net: bcmgenet: ring the doorbell when the last frame of a batch is dropped
      net: bcmgenet: let the caller decide whether to start the PHY
      net: bcmgenet: allow a continuation descriptor without the alignment pad
      net: bcmgenet: rename ENET_MAX_MTU_SIZE to ENET_MAX_FRAME_LEN
      net: bcmgenet: derive the receive buffer length from the MTU
      net: bcmgenet: split frames that end just past the transmit threshold
      net: bcmgenet: build the DMA rings before giving up the running ones
      net: bcmgenet: allow the MTU to be changed
      net: bcmgenet: reassemble jumbo frames from status block fragments

 drivers/net/ethernet/broadcom/genet/bcmgenet.c | 727 +++++++++++++++++++------
 drivers/net/ethernet/broadcom/genet/bcmgenet.h |  34 +-
 2 files changed, 585 insertions(+), 176 deletions(-)
---
base-commit: 4dd16e31135ea8437a720222df34e8745f385d6d
change-id: 20261002-nb-genet-mtu-nn-v2-402b5e6f306f

Best regards,
--  
Nicolai Buchwitz <nb@xxxxxxxxxxx>