gate lwIP throughput tuning + iperf on MCU SRAM tier

The bigger TCP_WND/PBUF_POOL/MEMP_NUM_TCP_SEG and the always-on iperf
overflowed SRAM on stm32c0/f1/wb, lpc11/13, samd11. Add LWIP_HIGH_THROUGHPUT
gate (defined in tusb_config.h, consumed by lwipopts.h) so RAM-tight MCUs
keep the original modest buffers and skip iperf, while RAM-rich targets
(max32*/stm32f2/f4/f7/h5/h7/h7rs/u5/n6, rp2040, mimxrt1xxx, nrf5x) keep
the throughput tuning needed for the iperf HIL test.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

2 files changed, 40 insertions, 7 deletions

diff --git a/examples/device/net_lwip_webserver/src/lwipopts.h b/examples/device/net_lwip_webserver/src/lwipopts.h
index 6682d2903..350120423 100644
--- a/examples/device/net_lwip_webserver/src/lwipopts.h
+++ b/examples/device/net_lwip_webserver/src/lwipopts.h
@@ -32,6 +32,14 @@
 #ifndef LWIPOPTS_H__
 #define LWIPOPTS_H__
 
+// Pulls in tusb_option.h → tusb_config.h, which defines LWIP_HIGH_THROUGHPUT
+// based on the target MCU's SRAM tier.
+#include "tusb_option.h"
+
+#ifndef LWIP_HIGH_THROUGHPUT
+  #define LWIP_HIGH_THROUGHPUT          0
+#endif
+
 /* Prevent having to link sys_arch.c (we don't test the API layers in unit tests) */
 #define NO_SYS                          1
 #define MEM_ALIGNMENT                   4
@@ -49,7 +57,15 @@
 
 #define TCP_MSS                         (1500 /*mtu*/ - 20 /*iphdr*/ - 20 /*tcphhr*/)
 #define TCP_SND_BUF                     (4 * TCP_MSS)
-#define TCP_WND                         (8 * TCP_MSS)
+#if LWIP_HIGH_THROUGHPUT
+  #define TCP_WND                       (8 * TCP_MSS)
+  #define PBUF_POOL_SIZE                8
+  // Must grow in step with TCP_SND_BUF (default MEMP_NUM_TCP_SEG=16 caps TCP_SND_BUF at 4*MSS).
+  #define MEMP_NUM_TCP_SEG              16
+#else
+  #define TCP_WND                       (4 * TCP_MSS)
+  #define PBUF_POOL_SIZE                4
+#endif
 
 #define ETHARP_SUPPORT_STATIC_ENTRIES   1
 
@@ -60,10 +76,6 @@
 #define LWIP_SINGLE_NETIF               1
 #define LWIP_NETIF_LINK_CALLBACK        1
 
-#define PBUF_POOL_SIZE                  8
-// Must grow in step with TCP_SND_BUF (default MEMP_NUM_TCP_SEG=16 caps TCP_SND_BUF at 4*MSS).
-#define MEMP_NUM_TCP_SEG                16
-
 #define HTTPD_USE_CUSTOM_FSDATA         0
 
 #define LWIP_MULTICAST_PING             1
diff --git a/examples/device/net_lwip_webserver/src/tusb_config.h b/examples/device/net_lwip_webserver/src/tusb_config.h
index 6809a3fae..24082fe25 100644
--- a/examples/device/net_lwip_webserver/src/tusb_config.h
+++ b/examples/device/net_lwip_webserver/src/tusb_config.h
@@ -97,7 +97,24 @@ extern "C" {
 #endif
 #endif
 
-#ifndef INCLUDE_IPERF
+// MCU SRAM tier — drives the bigger lwIP buffers in lwipopts.h, the larger
+// NCM OUT NTB size below, and whether iperf is built. Small-RAM MCUs
+// (stm32c0/f1/wb, lpc11/13, samd11) keep modest defaults to fit.
+#ifndef LWIP_HIGH_THROUGHPUT
+  #if TU_CHECK_MCU(OPT_MCU_MAX32650, OPT_MCU_MAX32666, OPT_MCU_MAX32690, OPT_MCU_MAX78002) || \
+      TU_CHECK_MCU(OPT_MCU_STM32F2,  OPT_MCU_STM32F4,  OPT_MCU_STM32F7) || \
+      TU_CHECK_MCU(OPT_MCU_STM32H5,  OPT_MCU_STM32H7,  OPT_MCU_STM32H7RS) || \
+      TU_CHECK_MCU(OPT_MCU_STM32U5,  OPT_MCU_STM32N6) || \
+      TU_CHECK_MCU(OPT_MCU_RP2040) || \
+      TU_CHECK_MCU(OPT_MCU_MIMXRT1XXX) || \
+      TU_CHECK_MCU(OPT_MCU_NRF5X)
+    #define LWIP_HIGH_THROUGHPUT 1
+  #else
+    #define LWIP_HIGH_THROUGHPUT 0
+  #endif
+#endif
+
+#if LWIP_HIGH_THROUGHPUT && !defined(INCLUDE_IPERF)
   #define INCLUDE_IPERF
 #endif
 
@@ -111,7 +128,11 @@ extern "C" {
 
 // Must be >> MTU
 // Can be set to smaller values if wNtbOutMaxDatagrams==1
-#define CFG_TUD_NCM_OUT_NTB_MAX_SIZE (3 * TCP_MSS + 100)
+#if LWIP_HIGH_THROUGHPUT
+  #define CFG_TUD_NCM_OUT_NTB_MAX_SIZE (3 * TCP_MSS + 100)
+#else
+  #define CFG_TUD_NCM_OUT_NTB_MAX_SIZE (2 * TCP_MSS + 100)
+#endif
 
 // Number of NCM transfer blocks for reception side
 #ifndef CFG_TUD_NCM_OUT_NTB_N