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[net-next,0/5] net: lan966x: Add support for FDMA

Message ID 20220317185159.1661469-1-horatiu.vultur@microchip.com
Headers show
Series net: lan966x: Add support for FDMA | expand

Message

Horatiu Vultur March 17, 2022, 6:51 p.m. UTC 
Currently when injecting or extracting a frame from CPU, the frame
is given to the HW each word at a time. There is another way to
inject/extract frames from CPU using FDMA(Frame Direct Memory Access).
In this way the entire frame is given to the HW. This improves both
RX and TX bitrate.

Horatiu Vultur (5):
  dt-bindings: net: lan966x: Extend with FDMA interrupt
  net: lan966x: Add registers that are used for FDMA.
  net: lan966x: Expose functions that are needed by FDMA
  net: lan966x: Add FDMA functionality
  net: lan96x: Update FDMA to change MTU.

 .../net/microchip,lan966x-switch.yaml         |   2 +
 .../net/ethernet/microchip/lan966x/Makefile   |   2 +-
 .../ethernet/microchip/lan966x/lan966x_fdma.c | 772 ++++++++++++++++++
 .../ethernet/microchip/lan966x/lan966x_main.c |  44 +-
 .../ethernet/microchip/lan966x/lan966x_main.h | 120 +++
 .../ethernet/microchip/lan966x/lan966x_port.c |   3 +
 .../ethernet/microchip/lan966x/lan966x_regs.h | 106 +++
 7 files changed, 1037 insertions(+), 12 deletions(-)
 create mode 100644 drivers/net/ethernet/microchip/lan966x/lan966x_fdma.c

Comments

Michael Walle March 18, 2022, 11:07 a.m. UTC | #1 
Hi Horatiu,

> Currently when injecting or extracting a frame from CPU, the frame
> is given to the HW each word at a time. There is another way to
> inject/extract frames from CPU using FDMA(Frame Direct Memory Access).
> In this way the entire frame is given to the HW. This improves both
> RX and TX bitrate.

I wanted to test this. ping and such works fine and I'm also
seeing fdma interrupts. But as soon as I try iperf3 I get a skb_panic
(due to frame size?). Hope that splash below helps.

-michael

[  159.778850] skbuff: skb_over_panic: text:c07960c0 len:106 put:4 head:c8cc6e00 data:c8cc6e9a tail:0xc8cc6f04 end:0xc8cc6f00 dev:eth0
[  159.788067] ------------[ cut here ]------------
[  159.792575] kernel BUG at net/core/skbuff.c:113!
..
[  160.620995] Backtrace:
[  160.623426] [<c0a62350>] (skb_panic) from [<c08876f8>] (skb_put+0x54/0x58)
[  160.630284] [<c08876a4>] (skb_put) from [<c07960c0>] (lan966x_fdma_xmit+0x108/0x504)
[  160.638011]  r5:00000000 r4:00000000
[  160.641566] [<c0795fb8>] (lan966x_fdma_xmit) from [<c078cdf8>] (lan966x_port_xmit+0x1b0/0x450)
[  160.650171]  r10:c1960160 r9:c1960040 r8:00000001 r7:0000001c r6:c196b000 r5:c891a780
[  160.657973]  r4:00000000
[  160.660488] [<c078cc48>] (lan966x_port_xmit) from [<c08a3e24>] (dev_hard_start_xmit+0x114/0x248)
[  160.669267]  r10:c0f016e8 r9:c0f060cc r8:c0e8d350 r7:c196ba00 r6:00000000 r5:c196b000
[  160.677069]  r4:c891a780
[  160.679584] [<c08a3d10>] (dev_hard_start_xmit) from [<c0904728>] (sch_direct_xmit+0x11c/0x31c)
[  160.688188]  r10:2a01000a r9:00000000 r8:00000000 r7:c196b000 r6:c196ba00 r5:c891a780
[  160.695991]  r4:c1e57400
[  160.698507] [<c090460c>] (sch_direct_xmit) from [<c08a4520>] (__dev_queue_xmit+0x508/0xaac)
[  160.706850]  r9:c8cc6f00 r8:0000004a r7:00000001 r6:00000000 r5:c1e57400 r4:c891a780
[  160.714565] [<c08a4018>] (__dev_queue_xmit) from [<c08a4ad8>] (dev_queue_xmit+0x14/0x18)
[  160.722648]  r10:2a01000a r9:00000010 r8:0000004a r7:c1e7038c r6:00000000 r5:c891a780
[  160.730451]  r4:c1e70300
[  160.732966] [<c08a4ac4>] (dev_queue_xmit) from [<c092b0c0>] (ip_finish_output2+0x270/0x5dc)
[  160.741300] [<c092ae50>] (ip_finish_output2) from [<c092ceb8>] (__ip_finish_output+0x9c/0x144)
[  160.749903]  r9:c8cdef00 r8:c10833c0 r7:000005dc r6:c8924000 r5:c891a780 r4:c891a780
[  160.757618] [<c092ce1c>] (__ip_finish_output) from [<c092d01c>] (ip_output+0xbc/0xc4)
[  160.765441]  r10:2a01000a r9:c8cdef00 r8:c8cc6e00 r7:c196b000 r6:c8924000 r5:c10833c0
[  160.773244]  r4:c891a780
[  160.775759] [<c092cf60>] (ip_output) from [<c092ab10>] (ip_local_out+0x60/0x6c)
[  160.783058]  r7:00000000 r6:c8924000 r5:c10833c0 r4:c891a780
[  160.788692] [<c092aab0>] (ip_local_out) from [<c092ad74>] (ip_build_and_send_pkt+0x110/0x1ec)
[  160.797206]  r7:00000000 r6:c8d40000 r5:000000c4 r4:c891a780
[  160.802841] [<c092ac64>] (ip_build_and_send_pkt) from [<c0952bb8>] (tcp_v4_send_synack+0xec/0x138)
[  160.811792]  r10:c8cdef00 r9:c0b9b614 r8:c10833c0 r7:8b01000a r6:2a01000a r5:c8d40000
[  160.819595]  r4:c8924000
[  160.822110] [<c0952acc>] (tcp_v4_send_synack) from [<c0941520>] (tcp_conn_request+0x6cc/0x984)
[  160.830711]  r7:c891ab40 r6:c0952acc r5:c8d40000 r4:c8924000
[  160.836345] [<c0940e54>] (tcp_conn_request) from [<c095299c>] (tcp_v4_conn_request+0x38/0x84)
[  160.844863]  r10:c892902a r9:c892903e r8:c10833c0 r7:00000000 r6:00000000 r5:c891ab40
[  160.852666]  r4:c8d40000
[  160.855181] [<c0952964>] (tcp_v4_conn_request) from [<c09d05a0>] (tcp_v6_conn_request+0x12c/0x138)
[  160.864122] [<c09d0474>] (tcp_v6_conn_request) from [<c0948870>] (tcp_rcv_state_process+0x29c/0xf44)
[  160.873240]  r5:c891ab40 r4:c8d40000
[  160.876794] [<c09485d4>] (tcp_rcv_state_process) from [<c0953374>] (tcp_v4_do_rcv+0x110/0x22c)
[  160.885398]  r10:c892902a r9:c892903e r8:c10833c0 r7:00000000 r6:c8d40000 r5:c891ab40
[  160.893201]  r4:c8d40000
[  160.895716] [<c0953264>] (tcp_v4_do_rcv) from [<c0955158>] (tcp_v4_rcv+0xb08/0xba8)
[  160.903362]  r7:00000000 r6:c8d40000 r5:00000000 r4:c891ab40
[  160.908998] [<c0954650>] (tcp_v4_rcv) from [<c09273f4>] (ip_protocol_deliver_rcu+0x34/0x1d8)
[  160.917428]  r10:c10833c0 r9:c891ab40 r8:00000000 r7:c10833c0 r6:c0f06f64 r5:c891ab40
[  160.925230]  r4:00000006
[  160.927746] [<c09273c0>] (ip_protocol_deliver_rcu) from [<c0927640>] (ip_local_deliver+0xa8/0xf0)
[  160.936609]  r8:c0f01c0c r7:c0f01c0c r6:00000000 r5:c0f01bc4 r4:c891ab40
[  160.943283] [<c0927598>] (ip_local_deliver) from [<c0926794>] (ip_sublist_rcv_finish+0x44/0x58)
[  160.951967]  r5:c0f01bc4 r4:c0f01bc4
[  160.955522] [<c0926750>] (ip_sublist_rcv_finish) from [<c0926d58>] (ip_list_rcv_finish.constprop.0+0x10c/0x134)
[  160.965598]  r7:c0f01c0c r6:c0f01bc4 r5:c0f01c0c r4:c0f01c0c
[  160.971232] [<c0926c4c>] (ip_list_rcv_finish.constprop.0) from [<c09277cc>] (ip_list_rcv+0xec/0x114)
[  160.980358]  r10:c0f01c5c r9:c10833c0 r8:c0f01c0c r7:c0f01c5c r6:c10833c0 r5:c196b000
[  160.988160]  r4:c891ab40
[  160.990676] [<c09276e0>] (ip_list_rcv) from [<c08a6328>] (__netif_receive_skb_list_core+0x90/0x214)
[  160.999714]  r10:c196b000 r9:c196b000 r8:00000000 r7:c0f01c5c r6:c0f07450 r5:c891acc0
[  161.007517]  r4:c09276e0
[  161.010032] [<c08a6298>] (__netif_receive_skb_list_core) from [<c08a6670>] (netif_receive_skb_list_internal+0x1c4/0x2c4)
[  161.020893]  r10:c10833b8 r9:c0f08a00 r8:00000000 r7:c0f01d44 r6:c0f01d44 r5:c0f01d44
[  161.028696]  r4:00000000
[  161.031211] [<c08a64ac>] (netif_receive_skb_list_internal) from [<c08a67a8>] (netif_receive_skb_list+0x38/0xe4)
[  161.041291]  r10:c1963de0 r9:f08527d0 r8:f08527c0 r7:00000003 r6:c1960040 r5:c103d920
[  161.049094]  r4:c0f01d44
[  161.051609] [<c08a6770>] (netif_receive_skb_list) from [<c0795af4>] (lan966x_fdma_napi_poll+0x3ec/0x490)
[  161.061081]  r9:f08527d0 r8:f08527c0 r7:00000003 r6:c1960040 r5:c1962de0 r4:00000040
[  161.068796] [<c0795708>] (lan966x_fdma_napi_poll) from [<c08a6ee4>] (__napi_poll+0x34/0x1dc)
[  161.077226]  r10:ef7e6080 r9:c0f01db4 r8:c0f03d00 r7:c0f01db3 r6:00000040 r5:c1963de0
[  161.085029]  r4:00000001
[  161.087545] [<c08a6eb0>] (__napi_poll) from [<c08a7290>] (net_rx_action+0xec/0x288)
[  161.095190]  r7:2e957000 r6:ffffc936 r5:0000012c r4:c1963de0
[  161.100825] [<c08a71a4>] (net_rx_action) from [<c010143c>] (__do_softirq+0x13c/0x384)
[  161.108647]  r10:c0f08a00 r9:00000008 r8:00000100 r7:c1037d40 r6:00000003 r5:00000004
[  161.116451]  r4:c0f0308c
[  161.118966] [<c0101300>] (__do_softirq) from [<c013bc9c>] (irq_exit+0xac/0xdc)
[  161.126182]  r10:c0f04f78 r9:c0f08a00 r8:00000000 r7:c0f01ed4 r6:00000000 r5:c0f01ea0
[  161.133985]  r4:c0f08a00
[  161.136500] [<c013bbf0>] (irq_exit) from [<c0a62c84>] (generic_handle_arch_irq+0x48/0x4c)
[  161.144663]  r5:c0f01ea0 r4:c0e8d358
[  161.148218] [<c0a62c3c>] (generic_handle_arch_irq) from [<c0100be8>] (__irq_svc+0x88/0xb0)
Horatiu Vultur March 18, 2022, 12:10 p.m. UTC | #2 
The 03/18/2022 12:07, Michael Walle wrote:
> 
> Hi Horatiu,

Hi Michael,

> 
> > Currently when injecting or extracting a frame from CPU, the frame
> > is given to the HW each word at a time. There is another way to
> > inject/extract frames from CPU using FDMA(Frame Direct Memory Access).
> > In this way the entire frame is given to the HW. This improves both
> > RX and TX bitrate.
> 
> I wanted to test this. ping and such works fine and I'm also
> seeing fdma interrupts. 

Thanks for testing this also on your board.

> But as soon as I try iperf3 I get a skb_panic
> (due to frame size?). Hope that splash below helps.

I have not seen this issue. But it looks like it is a problem that there
is no more space to add the FCS.
Can you tell me how you run iperf3 so I can also try it?

Also I have a small diff that might fix the issue:
---
--- a/drivers/net/ethernet/microchip/lan966x/lan966x_fdma.c
+++ b/drivers/net/ethernet/microchip/lan966x/lan966x_fdma.c
@@ -534,6 +534,8 @@ int lan966x_fdma_xmit(struct sk_buff *skb, __be32 *ifh, struct net_device *dev)
        struct lan966x_tx_dcb *next_dcb, *dcb;
        struct lan966x_tx *tx = &lan966x->tx;
        struct lan966x_db *next_db;
+       int needed_headroom;
+       int needed_tailroom;
        dma_addr_t dma_addr;
        int next_to_use;
        int err;
@@ -554,10 +556,11 @@ int lan966x_fdma_xmit(struct sk_buff *skb, __be32 *ifh, struct net_device *dev)

        /* skb processing */
        skb_tx_timestamp(skb);
-       if (skb_headroom(skb) < IFH_LEN * sizeof(u32)) {
-               err = pskb_expand_head(skb,
-                                      IFH_LEN * sizeof(u32) - skb_headroom(skb),
-                                      0, GFP_ATOMIC);
+       needed_headroom = max_t(int, IFH_LEN * sizeof(u32) - skb_headroom(skb), 0);
+       needed_tailroom = max_t(int, ETH_FCS_LEN - skb_tailroom(skb), 0);
+       if (needed_headroom || needed_tailroom) {
+               err = pskb_expand_head(skb, needed_headroom, needed_tailroom,
+                                      GFP_ATOMIC);
                if (unlikely(err)) {
                        dev->stats.tx_dropped++;
                        err = NETDEV_TX_OK;
---

> 
> -michael
> 
> [  159.778850] skbuff: skb_over_panic: text:c07960c0 len:106 put:4 head:c8cc6e00 data:c8cc6e9a tail:0xc8cc6f04 end:0xc8cc6f00 dev:eth0
> [  159.788067] ------------[ cut here ]------------
> [  159.792575] kernel BUG at net/core/skbuff.c:113!
> ..
> [  160.620995] Backtrace:
> [  160.623426] [<c0a62350>] (skb_panic) from [<c08876f8>] (skb_put+0x54/0x58)
> [  160.630284] [<c08876a4>] (skb_put) from [<c07960c0>] (lan966x_fdma_xmit+0x108/0x504)
> [  160.638011]  r5:00000000 r4:00000000
> [  160.641566] [<c0795fb8>] (lan966x_fdma_xmit) from [<c078cdf8>] (lan966x_port_xmit+0x1b0/0x450)
> [  160.650171]  r10:c1960160 r9:c1960040 r8:00000001 r7:0000001c r6:c196b000 r5:c891a780
> [  160.657973]  r4:00000000
> [  160.660488] [<c078cc48>] (lan966x_port_xmit) from [<c08a3e24>] (dev_hard_start_xmit+0x114/0x248)
> [  160.669267]  r10:c0f016e8 r9:c0f060cc r8:c0e8d350 r7:c196ba00 r6:00000000 r5:c196b000
> [  160.677069]  r4:c891a780
> [  160.679584] [<c08a3d10>] (dev_hard_start_xmit) from [<c0904728>] (sch_direct_xmit+0x11c/0x31c)
> [  160.688188]  r10:2a01000a r9:00000000 r8:00000000 r7:c196b000 r6:c196ba00 r5:c891a780
> [  160.695991]  r4:c1e57400
> [  160.698507] [<c090460c>] (sch_direct_xmit) from [<c08a4520>] (__dev_queue_xmit+0x508/0xaac)
> [  160.706850]  r9:c8cc6f00 r8:0000004a r7:00000001 r6:00000000 r5:c1e57400 r4:c891a780
> [  160.714565] [<c08a4018>] (__dev_queue_xmit) from [<c08a4ad8>] (dev_queue_xmit+0x14/0x18)
> [  160.722648]  r10:2a01000a r9:00000010 r8:0000004a r7:c1e7038c r6:00000000 r5:c891a780
> [  160.730451]  r4:c1e70300
> [  160.732966] [<c08a4ac4>] (dev_queue_xmit) from [<c092b0c0>] (ip_finish_output2+0x270/0x5dc)
> [  160.741300] [<c092ae50>] (ip_finish_output2) from [<c092ceb8>] (__ip_finish_output+0x9c/0x144)
> [  160.749903]  r9:c8cdef00 r8:c10833c0 r7:000005dc r6:c8924000 r5:c891a780 r4:c891a780
> [  160.757618] [<c092ce1c>] (__ip_finish_output) from [<c092d01c>] (ip_output+0xbc/0xc4)
> [  160.765441]  r10:2a01000a r9:c8cdef00 r8:c8cc6e00 r7:c196b000 r6:c8924000 r5:c10833c0
> [  160.773244]  r4:c891a780
> [  160.775759] [<c092cf60>] (ip_output) from [<c092ab10>] (ip_local_out+0x60/0x6c)
> [  160.783058]  r7:00000000 r6:c8924000 r5:c10833c0 r4:c891a780
> [  160.788692] [<c092aab0>] (ip_local_out) from [<c092ad74>] (ip_build_and_send_pkt+0x110/0x1ec)
> [  160.797206]  r7:00000000 r6:c8d40000 r5:000000c4 r4:c891a780
> [  160.802841] [<c092ac64>] (ip_build_and_send_pkt) from [<c0952bb8>] (tcp_v4_send_synack+0xec/0x138)
> [  160.811792]  r10:c8cdef00 r9:c0b9b614 r8:c10833c0 r7:8b01000a r6:2a01000a r5:c8d40000
> [  160.819595]  r4:c8924000
> [  160.822110] [<c0952acc>] (tcp_v4_send_synack) from [<c0941520>] (tcp_conn_request+0x6cc/0x984)
> [  160.830711]  r7:c891ab40 r6:c0952acc r5:c8d40000 r4:c8924000
> [  160.836345] [<c0940e54>] (tcp_conn_request) from [<c095299c>] (tcp_v4_conn_request+0x38/0x84)
> [  160.844863]  r10:c892902a r9:c892903e r8:c10833c0 r7:00000000 r6:00000000 r5:c891ab40
> [  160.852666]  r4:c8d40000
> [  160.855181] [<c0952964>] (tcp_v4_conn_request) from [<c09d05a0>] (tcp_v6_conn_request+0x12c/0x138)
> [  160.864122] [<c09d0474>] (tcp_v6_conn_request) from [<c0948870>] (tcp_rcv_state_process+0x29c/0xf44)
> [  160.873240]  r5:c891ab40 r4:c8d40000
> [  160.876794] [<c09485d4>] (tcp_rcv_state_process) from [<c0953374>] (tcp_v4_do_rcv+0x110/0x22c)
> [  160.885398]  r10:c892902a r9:c892903e r8:c10833c0 r7:00000000 r6:c8d40000 r5:c891ab40
> [  160.893201]  r4:c8d40000
> [  160.895716] [<c0953264>] (tcp_v4_do_rcv) from [<c0955158>] (tcp_v4_rcv+0xb08/0xba8)
> [  160.903362]  r7:00000000 r6:c8d40000 r5:00000000 r4:c891ab40
> [  160.908998] [<c0954650>] (tcp_v4_rcv) from [<c09273f4>] (ip_protocol_deliver_rcu+0x34/0x1d8)
> [  160.917428]  r10:c10833c0 r9:c891ab40 r8:00000000 r7:c10833c0 r6:c0f06f64 r5:c891ab40
> [  160.925230]  r4:00000006
> [  160.927746] [<c09273c0>] (ip_protocol_deliver_rcu) from [<c0927640>] (ip_local_deliver+0xa8/0xf0)
> [  160.936609]  r8:c0f01c0c r7:c0f01c0c r6:00000000 r5:c0f01bc4 r4:c891ab40
> [  160.943283] [<c0927598>] (ip_local_deliver) from [<c0926794>] (ip_sublist_rcv_finish+0x44/0x58)
> [  160.951967]  r5:c0f01bc4 r4:c0f01bc4
> [  160.955522] [<c0926750>] (ip_sublist_rcv_finish) from [<c0926d58>] (ip_list_rcv_finish.constprop.0+0x10c/0x134)
> [  160.965598]  r7:c0f01c0c r6:c0f01bc4 r5:c0f01c0c r4:c0f01c0c
> [  160.971232] [<c0926c4c>] (ip_list_rcv_finish.constprop.0) from [<c09277cc>] (ip_list_rcv+0xec/0x114)
> [  160.980358]  r10:c0f01c5c r9:c10833c0 r8:c0f01c0c r7:c0f01c5c r6:c10833c0 r5:c196b000
> [  160.988160]  r4:c891ab40
> [  160.990676] [<c09276e0>] (ip_list_rcv) from [<c08a6328>] (__netif_receive_skb_list_core+0x90/0x214)
> [  160.999714]  r10:c196b000 r9:c196b000 r8:00000000 r7:c0f01c5c r6:c0f07450 r5:c891acc0
> [  161.007517]  r4:c09276e0
> [  161.010032] [<c08a6298>] (__netif_receive_skb_list_core) from [<c08a6670>] (netif_receive_skb_list_internal+0x1c4/0x2c4)
> [  161.020893]  r10:c10833b8 r9:c0f08a00 r8:00000000 r7:c0f01d44 r6:c0f01d44 r5:c0f01d44
> [  161.028696]  r4:00000000
> [  161.031211] [<c08a64ac>] (netif_receive_skb_list_internal) from [<c08a67a8>] (netif_receive_skb_list+0x38/0xe4)
> [  161.041291]  r10:c1963de0 r9:f08527d0 r8:f08527c0 r7:00000003 r6:c1960040 r5:c103d920
> [  161.049094]  r4:c0f01d44
> [  161.051609] [<c08a6770>] (netif_receive_skb_list) from [<c0795af4>] (lan966x_fdma_napi_poll+0x3ec/0x490)
> [  161.061081]  r9:f08527d0 r8:f08527c0 r7:00000003 r6:c1960040 r5:c1962de0 r4:00000040
> [  161.068796] [<c0795708>] (lan966x_fdma_napi_poll) from [<c08a6ee4>] (__napi_poll+0x34/0x1dc)
> [  161.077226]  r10:ef7e6080 r9:c0f01db4 r8:c0f03d00 r7:c0f01db3 r6:00000040 r5:c1963de0
> [  161.085029]  r4:00000001
> [  161.087545] [<c08a6eb0>] (__napi_poll) from [<c08a7290>] (net_rx_action+0xec/0x288)
> [  161.095190]  r7:2e957000 r6:ffffc936 r5:0000012c r4:c1963de0
> [  161.100825] [<c08a71a4>] (net_rx_action) from [<c010143c>] (__do_softirq+0x13c/0x384)
> [  161.108647]  r10:c0f08a00 r9:00000008 r8:00000100 r7:c1037d40 r6:00000003 r5:00000004
> [  161.116451]  r4:c0f0308c
> [  161.118966] [<c0101300>] (__do_softirq) from [<c013bc9c>] (irq_exit+0xac/0xdc)
> [  161.126182]  r10:c0f04f78 r9:c0f08a00 r8:00000000 r7:c0f01ed4 r6:00000000 r5:c0f01ea0
> [  161.133985]  r4:c0f08a00
> [  161.136500] [<c013bbf0>] (irq_exit) from [<c0a62c84>] (generic_handle_arch_irq+0x48/0x4c)
> [  161.144663]  r5:c0f01ea0 r4:c0e8d358
> [  161.148218] [<c0a62c3c>] (generic_handle_arch_irq) from [<c0100be8>] (__irq_svc+0x88/0xb0)
Michael Walle March 18, 2022, 12:42 p.m. UTC | #3 
Am 2022-03-18 13:10, schrieb Horatiu Vultur:
> The 03/18/2022 12:07, Michael Walle wrote:
>> > Currently when injecting or extracting a frame from CPU, the frame
>> > is given to the HW each word at a time. There is another way to
>> > inject/extract frames from CPU using FDMA(Frame Direct Memory Access).
>> > In this way the entire frame is given to the HW. This improves both
>> > RX and TX bitrate.
>> 
>> I wanted to test this. ping and such works fine and I'm also
>> seeing fdma interrupts.
> 
> Thanks for testing this also on your board.
> 
>> But as soon as I try iperf3 I get a skb_panic
>> (due to frame size?). Hope that splash below helps.
> 
> I have not seen this issue. But it looks like it is a problem that 
> there
> is no more space to add the FCS.
> Can you tell me how you run iperf3 so I can also try it?

oh, I forgot to include the commandline.

# on the remote computer
$ iperf3 --version
iperf 3.6 (cJSON 1.5.2)
Linux eddie01 4.19.0-18-686-pae #1 SMP Debian 4.19.208-1 (2021-09-29) 
i686
Optional features available: CPU affinity setting, IPv6 flow label, 
SCTP, TCP congestion algorithm setting, sendfile / zerocopy, socket 
pacing, authentication
$ iperf3 -s

# on the board
$ iperf3 --version
iperf 3.10.1 (cJSON 1.7.13)
Linux buildroot 5.17.0-rc8-next-20220316-00058-gc6cb0628f2a6-dirty #385 
SMP Fri Mar 18 13:34:26 CET 2022 armv7l
Optional features available: CPU affinity setting, IPv6 flow label, TCP 
congestion algorithm setting, sendfile / zerocopy, socket pacing, bind 
to device, support IPv4 don't fragment
$ iperf3 -c eddie01

> Also I have a small diff that might fix the issue:
> ---
> --- a/drivers/net/ethernet/microchip/lan966x/lan966x_fdma.c
> +++ b/drivers/net/ethernet/microchip/lan966x/lan966x_fdma.c
> @@ -534,6 +534,8 @@ int lan966x_fdma_xmit(struct sk_buff *skb, __be32
> *ifh, struct net_device *dev)
>         struct lan966x_tx_dcb *next_dcb, *dcb;
>         struct lan966x_tx *tx = &lan966x->tx;
>         struct lan966x_db *next_db;
> +       int needed_headroom;
> +       int needed_tailroom;
>         dma_addr_t dma_addr;
>         int next_to_use;
>         int err;
> @@ -554,10 +556,11 @@ int lan966x_fdma_xmit(struct sk_buff *skb,
> __be32 *ifh, struct net_device *dev)
> 
>         /* skb processing */
>         skb_tx_timestamp(skb);

btw. skb_tx_timestamp() should be as close to the handover
of the frame to the hardware as possible, no?

> -       if (skb_headroom(skb) < IFH_LEN * sizeof(u32)) {
> -               err = pskb_expand_head(skb,
> -                                      IFH_LEN * sizeof(u32) -
> skb_headroom(skb),
> -                                      0, GFP_ATOMIC);
> +       needed_headroom = max_t(int, IFH_LEN * sizeof(u32) -
> skb_headroom(skb), 0);
> +       needed_tailroom = max_t(int, ETH_FCS_LEN - skb_tailroom(skb), 
> 0);
> +       if (needed_headroom || needed_tailroom) {
> +               err = pskb_expand_head(skb, needed_headroom, 
> needed_tailroom,
> +                                      GFP_ATOMIC);
>                 if (unlikely(err)) {
>                         dev->stats.tx_dropped++;
>                         err = NETDEV_TX_OK;

Indeed this will fix the issue:

# iperf3 -c eddie01
Connecting to host eddie01, port 5201
[  5] local 10.0.1.143 port 55342 connected to 10.0.1.42 port 5201
[ ID] Interval           Transfer     Bitrate         Retr  Cwnd
[  5]   0.00-1.01   sec  43.8 MBytes   364 Mbits/sec    0    245 KBytes
[  5]   1.01-2.02   sec  43.8 MBytes   364 Mbits/sec    0    246 KBytes
[  5]   2.02-3.03   sec  43.8 MBytes   364 Mbits/sec    0    259 KBytes

# iperf3 -R -c eddie01
Connecting to host eddie01, port 5201
Reverse mode, remote host eddie01 is sending
[  5] local 10.0.1.143 port 55346 connected to 10.0.1.42 port 5201
[ ID] Interval           Transfer     Bitrate
[  5]   0.00-1.00   sec  28.6 MBytes   240 Mbits/sec
[  5]   1.00-2.00   sec  28.9 MBytes   242 Mbits/sec
[  5]   2.00-3.00   sec  28.7 MBytes   241 Mbits/sec

-michael