[PATCH 7/7] net: mctp: usblib: Add test for failing append

[James Lee <james@codeconstruct.com.au>]

Add test ensuring that network device stats are updated appropriately
when a previously pushed packet fails to send during a subsequent push.

Signed-off-by: James Lee <james@codeconstruct.com.au>
---
 drivers/net/mctp/mctp-usblib-test.c | 42 +++++++++++++++++++++++++++++++++++++
 1 file changed, 42 insertions(+)

diff --git a/drivers/net/mctp/mctp-usblib-test.c b/drivers/net/mctp/mctp-usblib-test.c
index 7dc382caffc17644dd616f78df332201d412f709..b3a2cc5cbd99defde308cf23a1b559dbdc73ca22 100644
--- a/drivers/net/mctp/mctp-usblib-test.c
+++ b/drivers/net/mctp/mctp-usblib-test.c
@@ -6,6 +6,7 @@
  * Copyright (C) 2026 Code Construct Pty Ltd
  */
 
+#include "linux/percpu-defs.h"
 #include <linux/array_size.h>
 #include <uapi/linux/netdevice.h>
 #include <linux/netdevice.h>
@@ -871,6 +872,46 @@ static void mctp_usblib_test_tx_multi_push(struct kunit *test)
 	KUNIT_EXPECT_EQ(test, i, ARRAY_SIZE(sends));
 }
 
+static void mctp_usblib_test_tx_overflow(struct kunit *test)
+{
+	struct mctp_usblib_test_ctx *ctx;
+	struct mctp_usblib_tx_ops ops;
+	struct pcpu_dstats *dstats;
+	struct mctp_usblib_tx *tx;
+	struct net_device *ndev;
+	struct sk_buff *skb;
+	unsigned int start;
+	size_t len, i;
+	u64 stats;
+	u8 *buf;
+	int rc;
+
+	len = 200;
+
+	ctx = mctp_usblib_test_init(test);
+	ndev = ctx->dev->ndev;
+
+	ops.send = mctp_usblib_test_tx_send_fail;
+
+	tx = mctp_usblib_test_tx_init(test, &ops, ctx, false);
+	buf = mctp_usblib_test_init_buf(test, len);
+
+	for (i = 0; i < 3; i++) {
+		skb = mctp_usblib_test_init_skb(test, len, ndev, buf);
+
+		rc = mctp_usblib_tx_push(ndev, tx, skb, i != 2);
+		KUNIT_EXPECT_EQ(test, rc, 0);
+	}
+
+	dstats = get_cpu_ptr(ndev->dstats);
+	do {
+		start = u64_stats_fetch_begin(&dstats->syncp);
+		stats = u64_stats_read(&dstats->tx_drops);
+	} while (u64_stats_fetch_retry(&dstats->syncp, start));
+
+	KUNIT_EXPECT_EQ(test, stats, 3);
+}
+
 static struct kunit_case mctp_usblib_test_cases[] = {
 	KUNIT_CASE(mctp_usblib_test_rx_single),
 	KUNIT_CASE_PARAM(mctp_usblib_test_rx_pkt_span,
@@ -883,6 +924,7 @@ static struct kunit_case mctp_usblib_test_cases[] = {
 	KUNIT_CASE(mctp_usblib_test_tx_pkt_span),
 	KUNIT_CASE(mctp_usblib_test_tx_multi_push),
 	KUNIT_CASE(mctp_usblib_test_tx_failing_send),
+	KUNIT_CASE(mctp_usblib_test_tx_overflow),
 	{}
 };
 

-- 
2.47.3