[PATCH net-next v3 5/9] bnxt_en: set the netmem TX frag size limit to 32KB

[Stanislav Fomichev <sdf.kernel@gmail.com>]

The TX BD length field is 16 bits wide. Set the netmem TX frag size
limit to the largest power of 2 which fits, which keeps 4K aligned
frags 4K aligned.

With 4K pages this raises the PAGE_SIZE default. With 64K pages it
lowers it, 64KB frags would otherwise wrap the BD length to 0 with BIG
TCP.

Tested on a 50Gb/s NIC: devmem TX from a 2MB hugepage-backed udmabuf
runs at line rate with 2-3 frags per 64KB skb, none above 32KB.

kperf output (sender side, 1 flow):

  client: == Source <sender>
  client:   Tx 46.358 Gbps (174483046400 bytes in 30110590 usec)
  client:   Rx  0.000 Gbps (0 bytes in 30110590 usec)
  client:   TCP retrans reord rtt rttvar d_ce snd_wnd cwnd
  client:           729   153 599     48    0 13893632 3147
  client:   net CPU 71: usr: 0.03% sys: 0.06% idle:67.62% iow: 0.00% irq: 1.12% sirq:31.14%
  client:   app CPU 72: usr: 0.16% sys: 6.01% idle:90.66% iow: 0.00% irq: 0.83% sirq: 2.32%

Signed-off-by: Stanislav Fomichev <sdf@fomichev.me>
Reviewed-by: Mina Almasry <almasrymina@google.com>
Reviewed-by: Michael Chan <michael.chan@broadcom.com>
Reviewed-by: Dragos Tatulea <dtatulea@nvidia.com>
---
 drivers/net/ethernet/broadcom/bnxt/bnxt.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/net/ethernet/broadcom/bnxt/bnxt.c b/drivers/net/ethernet/broadcom/bnxt/bnxt.c
index e7a4190a8c78..c8a887facc20 100644
--- a/drivers/net/ethernet/broadcom/bnxt/bnxt.c
+++ b/drivers/net/ethernet/broadcom/bnxt/bnxt.c
@@ -17439,6 +17439,7 @@ static int bnxt_init_one(struct pci_dev *pdev, const struct pci_device_id *ent)
 	if (BNXT_SUPPORTS_QUEUE_API(bp))
 		dev->queue_mgmt_ops = &bnxt_queue_mgmt_ops;
 	dev->netmem_tx = NETMEM_TX_DMA;
+	dev->netmem_tx_max_frag_size = SZ_32K;
 
 	rc = register_netdev(dev);
 	if (rc)
-- 
2.53.0-Meta