docs: record v1.1.0 low-RAM TCP backpressure design

App/tcp_client.c

@@ -17,6 +17,8 @@ typedef struct {
     uint8_t rx_ring[TCP_CLIENT_RX_BUFFER_SIZE];
     uint16_t rx_head;
     uint16_t rx_tail;
+    struct pbuf *hold_pbuf;
+    uint16_t hold_offset;
     uint32_t next_retry_ms;
     uint8_t index;
     tcp_client_instance_config_t config;
@@ -30,10 +32,65 @@ static uint16_t ring_free(uint16_t head, uint16_t tail, uint16_t size)
     return (head >= tail) ? (uint16_t)(size - head + tail - 1u) : (uint16_t)(tail - head - 1u);
 }
 
+static uint16_t ring_used(uint16_t head, uint16_t tail, uint16_t size)
+{
+    return (head >= tail) ? (uint16_t)(head - tail) : (uint16_t)(size - tail + head);
+}
+
+static void tcp_client_reset_rx_state(tcp_client_ctx_t *ctx)
+{
+    if (ctx == NULL) {
+        return;
+    }
+    if (ctx->hold_pbuf != NULL) {
+        pbuf_free(ctx->hold_pbuf);
+        ctx->hold_pbuf = NULL;
+    }
+    ctx->hold_offset = 0u;
+    ctx->rx_head = 0u;
+    ctx->rx_tail = 0u;
+}
+
+static void tcp_client_fill_ring_from_pbuf(tcp_client_ctx_t *ctx)
+{
+    struct pbuf *q;
+    uint16_t offset;
+
+    if (ctx == NULL || ctx->hold_pbuf == NULL) {
+        return;
+    }
+
+    q = ctx->hold_pbuf;
+    offset = ctx->hold_offset;
+    while (q != NULL && offset >= q->len) {
+        offset = (uint16_t)(offset - q->len);
+        q = q->next;
+    }
+
+    while (q != NULL) {
+        const uint8_t *src = (const uint8_t *)q->payload;
+        for (uint16_t i = offset; i < q->len; ++i) {
+            if (ring_free(ctx->rx_head, ctx->rx_tail, TCP_CLIENT_RX_BUFFER_SIZE) == 0u) {
+                ctx->hold_offset = (uint16_t)(ctx->hold_offset + i - offset);
+                return;
+            }
+            ctx->rx_ring[ctx->rx_head] = src[i];
+            ctx->rx_head = (uint16_t)((ctx->rx_head + 1u) % TCP_CLIENT_RX_BUFFER_SIZE);
+            ctx->status.rx_bytes++;
+        }
+        ctx->hold_offset = (uint16_t)(ctx->hold_offset + q->len - offset);
+        offset = 0u;
+        q = q->next;
+    }
+
+    pbuf_free(ctx->hold_pbuf);
+    ctx->hold_pbuf = NULL;
+    ctx->hold_offset = 0u;
+}
+
 static err_t tcp_client_on_recv(void *arg, struct tcp_pcb *pcb, struct pbuf *p, err_t err)
 {
     tcp_client_ctx_t *ctx = (tcp_client_ctx_t *)arg;
-    struct pbuf *q;
 
     if (ctx == NULL) {
         if (p != NULL) {
@@ -59,21 +116,16 @@ static err_t tcp_client_on_recv(void *arg, struct tcp_pcb *pcb, struct pbuf *p,
         return ERR_ABRT;
     }
 
-    for (q = p; q != NULL; q = q->next) {
+    if (ctx->hold_pbuf != NULL) {
-        const uint8_t *src = (const uint8_t *)q->payload;
-        for (uint16_t i = 0; i < q->len; ++i) {
-            if (ring_free(ctx->rx_head, ctx->rx_tail, TCP_CLIENT_RX_BUFFER_SIZE) == 0u) {
         ctx->status.errors++;
-                break;
+        return ERR_MEM;
-            }
-            ctx->rx_ring[ctx->rx_head] = src[i];
-            ctx->rx_head = (uint16_t)((ctx->rx_head + 1u) % TCP_CLIENT_RX_BUFFER_SIZE);
-            ctx->status.rx_bytes++;
-        }
     }
 
-    tcp_recved(pcb, p->tot_len);
+    pbuf_ref(p);
+    ctx->hold_pbuf = p;
+    ctx->hold_offset = 0u;
     pbuf_free(p);
+    tcp_client_fill_ring_from_pbuf(ctx);
     return ERR_OK;
 }
 
@@ -93,6 +145,7 @@ static void tcp_client_on_err(void *arg, err_t err)
     if (ctx == NULL) {
         return;
     }
+    tcp_client_reset_rx_state(ctx);
     ctx->pcb = NULL;
     ctx->status.state = TCP_CLIENT_STATE_DISCONNECTED;
     ctx->status.errors++;
@@ -213,6 +266,7 @@ int tcp_client_disconnect(uint8_t instance)
     }
     ctx = &g_clients[instance];
     if (ctx->pcb != NULL) {
+        tcp_client_reset_rx_state(ctx);
         tcp_arg(ctx->pcb, NULL);
         tcp_recv(ctx->pcb, NULL);
         tcp_sent(ctx->pcb, NULL);
@@ -221,8 +275,7 @@ int tcp_client_disconnect(uint8_t instance)
         ctx->pcb = NULL;
     }
     ctx->status.state = TCP_CLIENT_STATE_DISCONNECTED;
-    ctx->rx_head = 0u;
+    tcp_client_reset_rx_state(ctx);
-    ctx->rx_tail = 0u;
     return 0;
 }
 
@@ -272,13 +325,66 @@ int tcp_client_recv(uint8_t instance, uint8_t *data, uint16_t max_len)
         return -1;
     }
     ctx = &g_clients[instance];
+    tcp_client_fill_ring_from_pbuf(ctx);
     while (copied < max_len && ctx->rx_tail != ctx->rx_head) {
         data[copied++] = ctx->rx_ring[ctx->rx_tail];
         ctx->rx_tail = (uint16_t)((ctx->rx_tail + 1u) % TCP_CLIENT_RX_BUFFER_SIZE);
     }
+    if (copied > 0u && ctx->pcb != NULL) {
+        tcp_recved(ctx->pcb, copied);
+    }
     return (int)copied;
 }
 
+uint16_t tcp_client_rx_available(uint8_t instance)
+{
+    if (instance >= TCP_CLIENT_INSTANCE_COUNT) {
+        return 0u;
+    }
+    tcp_client_fill_ring_from_pbuf(&g_clients[instance]);
+    return ring_used(g_clients[instance].rx_head, g_clients[instance].rx_tail, TCP_CLIENT_RX_BUFFER_SIZE);
+}
+
+uint16_t tcp_client_peek(uint8_t instance, uint8_t *data, uint16_t max_len)
+{
+    uint16_t copied = 0u;
+    uint16_t tail;
+    tcp_client_ctx_t *ctx;
+
+    if (instance >= TCP_CLIENT_INSTANCE_COUNT || data == NULL || max_len == 0u) {
+        return 0u;
+    }
+
+    ctx = &g_clients[instance];
+    tcp_client_fill_ring_from_pbuf(ctx);
+    tail = ctx->rx_tail;
+    while (copied < max_len && tail != ctx->rx_head) {
+        data[copied++] = ctx->rx_ring[tail];
+        tail = (uint16_t)((tail + 1u) % TCP_CLIENT_RX_BUFFER_SIZE);
+    }
+    return copied;
+}
+
+void tcp_client_drop(uint8_t instance, uint16_t len)
+{
+    tcp_client_ctx_t *ctx;
+    uint16_t dropped = 0u;
+
+    if (instance >= TCP_CLIENT_INSTANCE_COUNT || len == 0u) {
+        return;
+    }
+
+    ctx = &g_clients[instance];
+    while (dropped < len && ctx->rx_tail != ctx->rx_head) {
+        ctx->rx_tail = (uint16_t)((ctx->rx_tail + 1u) % TCP_CLIENT_RX_BUFFER_SIZE);
+        dropped++;
+    }
+    if (dropped > 0u && ctx->pcb != NULL) {
+        tcp_recved(ctx->pcb, dropped);
+    }
+    tcp_client_fill_ring_from_pbuf(ctx);
+}
+
 bool tcp_client_is_connected(uint8_t instance)
 {
     return (instance < TCP_CLIENT_INSTANCE_COUNT) &&
@@ -299,6 +405,7 @@ void tcp_client_poll(void)
 
     for (uint8_t i = 0; i < TCP_CLIENT_INSTANCE_COUNT; ++i) {
         tcp_client_ctx_t *ctx = &g_clients[i];
+        tcp_client_fill_ring_from_pbuf(ctx);
         if (!ctx->config.enabled || !ctx->config.auto_reconnect || tcp_client_is_connected(i)) {
             continue;
         }

App/tcp_client.h

@@ -14,7 +14,7 @@ extern "C" {
 #endif
 
 #define TCP_CLIENT_INSTANCE_COUNT      2u
-#define TCP_CLIENT_RX_BUFFER_SIZE      512u
+#define TCP_CLIENT_RX_BUFFER_SIZE      480u
 #define TCP_CLIENT_RECONNECT_DELAY_MS  3000u
 
 typedef enum {
@@ -48,6 +48,9 @@ int tcp_client_connect(uint8_t instance);
 int tcp_client_disconnect(uint8_t instance);
 int tcp_client_send(uint8_t instance, const uint8_t *data, uint16_t len);
 int tcp_client_recv(uint8_t instance, uint8_t *data, uint16_t max_len);
+uint16_t tcp_client_rx_available(uint8_t instance);
+uint16_t tcp_client_peek(uint8_t instance, uint8_t *data, uint16_t max_len);
+void tcp_client_drop(uint8_t instance, uint16_t len);
 bool tcp_client_is_connected(uint8_t instance);
 void tcp_client_get_status(uint8_t instance, tcp_client_status_t *status);
 void tcp_client_poll(void);

App/tcp_server.c

@@ -18,6 +18,8 @@ typedef struct {
     uint8_t rx_ring[TCP_SERVER_RX_BUFFER_SIZE];
     uint16_t rx_head;
     uint16_t rx_tail;
+    struct pbuf *hold_pbuf;
+    uint16_t hold_offset;
     uint8_t index;
     tcp_server_instance_config_t config;
     tcp_server_status_t status;
@@ -30,10 +32,65 @@ static uint16_t ring_free(uint16_t head, uint16_t tail, uint16_t size)
     return (head >= tail) ? (uint16_t)(size - head + tail - 1u) : (uint16_t)(tail - head - 1u);
 }
 
+static uint16_t ring_used(uint16_t head, uint16_t tail, uint16_t size)
+{
+    return (head >= tail) ? (uint16_t)(head - tail) : (uint16_t)(size - tail + head);
+}
+
+static void tcp_server_reset_rx_state(tcp_server_ctx_t *ctx)
+{
+    if (ctx == NULL) {
+        return;
+    }
+    if (ctx->hold_pbuf != NULL) {
+        pbuf_free(ctx->hold_pbuf);
+        ctx->hold_pbuf = NULL;
+    }
+    ctx->hold_offset = 0u;
+    ctx->rx_head = 0u;
+    ctx->rx_tail = 0u;
+}
+
+static void tcp_server_fill_ring_from_pbuf(tcp_server_ctx_t *ctx)
+{
+    struct pbuf *q;
+    uint16_t offset;
+
+    if (ctx == NULL || ctx->hold_pbuf == NULL) {
+        return;
+    }
+
+    q = ctx->hold_pbuf;
+    offset = ctx->hold_offset;
+    while (q != NULL && offset >= q->len) {
+        offset = (uint16_t)(offset - q->len);
+        q = q->next;
+    }
+
+    while (q != NULL) {
+        const uint8_t *src = (const uint8_t *)q->payload;
+        for (uint16_t i = offset; i < q->len; ++i) {
+            if (ring_free(ctx->rx_head, ctx->rx_tail, TCP_SERVER_RX_BUFFER_SIZE) == 0u) {
+                ctx->hold_offset = (uint16_t)(ctx->hold_offset + i - offset);
+                return;
+            }
+            ctx->rx_ring[ctx->rx_head] = src[i];
+            ctx->rx_head = (uint16_t)((ctx->rx_head + 1u) % TCP_SERVER_RX_BUFFER_SIZE);
+            ctx->status.rx_bytes++;
+        }
+        ctx->hold_offset = (uint16_t)(ctx->hold_offset + q->len - offset);
+        offset = 0u;
+        q = q->next;
+    }
+
+    pbuf_free(ctx->hold_pbuf);
+    ctx->hold_pbuf = NULL;
+    ctx->hold_offset = 0u;
+}
+
 static err_t tcp_server_on_recv(void *arg, struct tcp_pcb *pcb, struct pbuf *p, err_t err)
 {
     tcp_server_ctx_t *ctx = (tcp_server_ctx_t *)arg;
-    struct pbuf *q;
 
     if (ctx == NULL) {
         if (p != NULL) {
@@ -58,21 +115,16 @@ static err_t tcp_server_on_recv(void *arg, struct tcp_pcb *pcb, struct pbuf *p,
         return ERR_ABRT;
     }
 
-    for (q = p; q != NULL; q = q->next) {
+    if (ctx->hold_pbuf != NULL) {
-        const uint8_t *src = (const uint8_t *)q->payload;
-        for (uint16_t i = 0; i < q->len; ++i) {
-            if (ring_free(ctx->rx_head, ctx->rx_tail, TCP_SERVER_RX_BUFFER_SIZE) == 0u) {
         ctx->status.errors++;
-                break;
+        return ERR_MEM;
-            }
-            ctx->rx_ring[ctx->rx_head] = src[i];
-            ctx->rx_head = (uint16_t)((ctx->rx_head + 1u) % TCP_SERVER_RX_BUFFER_SIZE);
-            ctx->status.rx_bytes++;
-        }
     }
 
-    tcp_recved(pcb, p->tot_len);
+    pbuf_ref(p);
+    ctx->hold_pbuf = p;
+    ctx->hold_offset = 0u;
     pbuf_free(p);
+    tcp_server_fill_ring_from_pbuf(ctx);
     return ERR_OK;
 }
 
@@ -92,6 +144,7 @@ static void tcp_server_on_err(void *arg, err_t err)
     if (ctx == NULL) {
         return;
     }
+    tcp_server_reset_rx_state(ctx);
     ctx->client_pcb = NULL;
     ctx->status.state = ctx->config.enabled ? TCP_SERVER_STATE_LISTENING : TCP_SERVER_STATE_IDLE;
     ctx->status.errors++;
@@ -193,6 +246,7 @@ int tcp_server_stop(uint8_t instance)
     ctx = &g_servers[instance];
 
     if (ctx->client_pcb != NULL) {
+        tcp_server_reset_rx_state(ctx);
         tcp_arg(ctx->client_pcb, NULL);
         tcp_recv(ctx->client_pcb, NULL);
         tcp_sent(ctx->client_pcb, NULL);
@@ -210,8 +264,7 @@ int tcp_server_stop(uint8_t instance)
     }
 
     ctx->status.state = TCP_SERVER_STATE_IDLE;
-    ctx->rx_head = 0u;
+    tcp_server_reset_rx_state(ctx);
-    ctx->rx_tail = 0u;
     return 0;
 }
 
@@ -262,13 +315,66 @@ int tcp_server_recv(uint8_t instance, uint8_t *data, uint16_t max_len)
         return -1;
     }
     ctx = &g_servers[instance];
+    tcp_server_fill_ring_from_pbuf(ctx);
     while (copied < max_len && ctx->rx_tail != ctx->rx_head) {
         data[copied++] = ctx->rx_ring[ctx->rx_tail];
         ctx->rx_tail = (uint16_t)((ctx->rx_tail + 1u) % TCP_SERVER_RX_BUFFER_SIZE);
     }
+    if (copied > 0u && ctx->client_pcb != NULL) {
+        tcp_recved(ctx->client_pcb, copied);
+    }
     return (int)copied;
 }
 
+uint16_t tcp_server_rx_available(uint8_t instance)
+{
+    if (instance >= TCP_SERVER_INSTANCE_COUNT) {
+        return 0u;
+    }
+    tcp_server_fill_ring_from_pbuf(&g_servers[instance]);
+    return ring_used(g_servers[instance].rx_head, g_servers[instance].rx_tail, TCP_SERVER_RX_BUFFER_SIZE);
+}
+
+uint16_t tcp_server_peek(uint8_t instance, uint8_t *data, uint16_t max_len)
+{
+    uint16_t copied = 0u;
+    uint16_t tail;
+    tcp_server_ctx_t *ctx;
+
+    if (instance >= TCP_SERVER_INSTANCE_COUNT || data == NULL || max_len == 0u) {
+        return 0u;
+    }
+
+    ctx = &g_servers[instance];
+    tcp_server_fill_ring_from_pbuf(ctx);
+    tail = ctx->rx_tail;
+    while (copied < max_len && tail != ctx->rx_head) {
+        data[copied++] = ctx->rx_ring[tail];
+        tail = (uint16_t)((tail + 1u) % TCP_SERVER_RX_BUFFER_SIZE);
+    }
+    return copied;
+}
+
+void tcp_server_drop(uint8_t instance, uint16_t len)
+{
+    tcp_server_ctx_t *ctx;
+    uint16_t dropped = 0u;
+
+    if (instance >= TCP_SERVER_INSTANCE_COUNT || len == 0u) {
+        return;
+    }
+
+    ctx = &g_servers[instance];
+    while (dropped < len && ctx->rx_tail != ctx->rx_head) {
+        ctx->rx_tail = (uint16_t)((ctx->rx_tail + 1u) % TCP_SERVER_RX_BUFFER_SIZE);
+        dropped++;
+    }
+    if (dropped > 0u && ctx->client_pcb != NULL) {
+        tcp_recved(ctx->client_pcb, dropped);
+    }
+    tcp_server_fill_ring_from_pbuf(ctx);
+}
+
 bool tcp_server_is_connected(uint8_t instance)
 {
     return (instance < TCP_SERVER_INSTANCE_COUNT) && (g_servers[instance].client_pcb != NULL);
@@ -280,3 +386,10 @@ void tcp_server_get_status(uint8_t instance, tcp_server_status_t *status)
         *status = g_servers[instance].status;
     }
 }
+
+void tcp_server_poll(void)
+{
+    for (uint8_t i = 0; i < TCP_SERVER_INSTANCE_COUNT; ++i) {
+        tcp_server_fill_ring_from_pbuf(&g_servers[i]);
+    }
+}

App/tcp_server.h

@@ -14,7 +14,7 @@ extern "C" {
 #endif
 
 #define TCP_SERVER_INSTANCE_COUNT    2u
-#define TCP_SERVER_RX_BUFFER_SIZE    512u
+#define TCP_SERVER_RX_BUFFER_SIZE    480u
 
 typedef enum {
     TCP_SERVER_STATE_IDLE = 0,
@@ -42,8 +42,12 @@ int tcp_server_start(uint8_t instance);
 int tcp_server_stop(uint8_t instance);
 int tcp_server_send(uint8_t instance, const uint8_t *data, uint16_t len);
 int tcp_server_recv(uint8_t instance, uint8_t *data, uint16_t max_len);
+uint16_t tcp_server_rx_available(uint8_t instance);
+uint16_t tcp_server_peek(uint8_t instance, uint8_t *data, uint16_t max_len);
+void tcp_server_drop(uint8_t instance, uint16_t len);
 bool tcp_server_is_connected(uint8_t instance);
 void tcp_server_get_status(uint8_t instance, tcp_server_status_t *status);
+void tcp_server_poll(void);
 
 #ifdef __cplusplus
 }

App/uart_trans.c

@@ -281,6 +281,14 @@ uint16_t uart_trans_write(uart_channel_t channel, const uint8_t *data, uint16_t
     return written;
 }
 
+uint16_t uart_trans_tx_free(uart_channel_t channel)
+{
+    if (channel >= UART_CHANNEL_MAX) {
+        return 0u;
+    }
+    return ring_free(g_channels[channel].tx_head, g_channels[channel].tx_tail, UART_TX_RING_BUFFER_SIZE);
+}
+
 void uart_trans_get_stats(uart_channel_t channel, uart_stats_t *stats)
 {
     if (channel < UART_CHANNEL_MAX && stats != NULL) {

App/uart_trans.h

@@ -55,6 +55,7 @@ void uart_trans_poll(void);
 uint16_t uart_trans_rx_available(uart_channel_t channel);
 uint16_t uart_trans_read(uart_channel_t channel, uint8_t *data, uint16_t max_len);
 uint16_t uart_trans_write(uart_channel_t channel, const uint8_t *data, uint16_t len);
+uint16_t uart_trans_tx_free(uart_channel_t channel);
 void uart_trans_get_stats(uart_channel_t channel, uart_stats_t *stats);
 void uart_trans_reset_stats(uart_channel_t channel);
 void uart_trans_idle_handler(uart_channel_t channel);

Core/Src/main.c

@@ -38,6 +38,7 @@
 #define LED_PIN                     GPIO_PIN_13
 #define LED_PORT                    GPIOC
 #define APP_ROUTE_BUFFER_SIZE       256u
+#define APP_TCP_TO_UART_CHUNK_SIZE  128u
 #define STACK_GUARD_WORD            0xA5A5A5A5u
 #define APP_HEALTH_CHECK_INTERVAL_MS 5000u
 /* USER CODE END PD */
@@ -67,6 +68,8 @@ static void App_RouteTcpTraffic(void);
 static void StackGuard_Init(void);
 static void StackGuard_Check(void);
 static bool App_SendToUart(uint8_t uart_index, uint8_t src_id, uint8_t dst_mask, const uint8_t *data, uint16_t len);
+static uint16_t App_SendTcpPayloadToUartRaw(uint8_t uart_index, const uint8_t *data, uint16_t len);
+static bool App_SendTcpPayloadToUartMux(uint8_t uart_index, uint8_t src_id, uint8_t dst_mask, const uint8_t *data, uint16_t len);
 static bool App_SendTcpServerPayload(uint8_t instance, const uint8_t *data, uint16_t len);
 static bool App_SendTcpClientPayload(uint8_t instance, const uint8_t *data, uint16_t len);
 /* USER CODE END PFP */
@@ -296,36 +299,160 @@ static bool App_SendToUart(uint8_t uart_index, uint8_t src_id, uint8_t dst_mask,
     }
 }
 
+static uint16_t App_SendTcpPayloadToUartRaw(uint8_t uart_index, const uint8_t *data, uint16_t len)
+{
+    uart_channel_t channel = (uart_index == LINK_UART_U1) ? UART_CHANNEL_U1 : UART_CHANNEL_U0;
+    return uart_trans_write(channel, data, len);
+}
+
+static bool App_SendTcpPayloadToUartMux(uint8_t uart_index, uint8_t src_id, uint8_t dst_mask, const uint8_t *data, uint16_t len)
+{
+    uart_channel_t channel = (uart_index == LINK_UART_U1) ? UART_CHANNEL_U1 : UART_CHANNEL_U0;
+    uint8_t frame[APP_TCP_TO_UART_CHUNK_SIZE + 6u];
+    uint16_t frame_len = 0u;
+
+    if (len == 0u || len > APP_TCP_TO_UART_CHUNK_SIZE) {
+        return false;
+    }
+    if (uart_trans_tx_free(channel) < (uint16_t)(len + 6u)) {
+        return false;
+    }
+    if (!uart_mux_encode_frame(src_id, dst_mask, data, len, frame, &frame_len, sizeof(frame))) {
+        return false;
+    }
+    return uart_trans_write(channel, frame, frame_len) == frame_len;
+}
+
 static void App_RouteTcpTraffic(void)
 {
     const device_config_t *cfg = config_get();
-    uint8_t buffer[APP_ROUTE_BUFFER_SIZE];
+    uint8_t buffer[APP_TCP_TO_UART_CHUNK_SIZE];
 
     for (uint8_t i = 0; i < TCP_SERVER_INSTANCE_COUNT; ++i) {
-        int rc = tcp_server_recv(i, buffer, sizeof(buffer));
+        uint16_t available = tcp_server_rx_available(i);
-        if (rc > 0) {
+        if (available > 0u) {
             uint8_t link_index = (i == 0u) ? CONFIG_LINK_S1 : CONFIG_LINK_S2;
-            if (!App_SendToUart(cfg->links[link_index].uart,
+            uint8_t uart_index = cfg->links[link_index].uart;
-                                config_link_index_to_endpoint(link_index),
+            uint8_t src_id = config_link_index_to_endpoint(link_index);
-                                config_uart_index_to_endpoint(cfg->links[link_index].uart),
+            uint8_t dst_mask = config_uart_index_to_endpoint(uart_index);
-                                buffer,
+            uart_channel_t channel = (uart_index == LINK_UART_U1) ? UART_CHANNEL_U1 : UART_CHANNEL_U0;
-                                (uint16_t)rc)) {
+
+            if (cfg->mux_mode == MUX_MODE_FRAME) {
+                uint16_t tx_free = uart_trans_tx_free(channel);
+                uint16_t payload_len;
+                if (tx_free <= 6u) {
                     return;
                 }
+                payload_len = available;
+                if (payload_len > APP_TCP_TO_UART_CHUNK_SIZE) {
+                    payload_len = APP_TCP_TO_UART_CHUNK_SIZE;
+                }
+                if (payload_len > (uint16_t)(tx_free - 6u)) {
+                    payload_len = (uint16_t)(tx_free - 6u);
+                }
+                if (payload_len == 0u) {
+                    return;
+                }
+                payload_len = tcp_server_peek(i, buffer, payload_len);
+                if (payload_len == 0u) {
+                    continue;
+                }
+                if (!App_SendTcpPayloadToUartMux(uart_index, src_id, dst_mask, buffer, payload_len)) {
+                    return;
+                }
+                tcp_server_drop(i, payload_len);
+            } else {
+                uint16_t chunk = available;
+                uint16_t tx_free = uart_trans_tx_free(channel);
+                uint16_t written;
+                if (tx_free == 0u) {
+                    return;
+                }
+                if (chunk > APP_TCP_TO_UART_CHUNK_SIZE) {
+                    chunk = APP_TCP_TO_UART_CHUNK_SIZE;
+                }
+                if (chunk > tx_free) {
+                    chunk = tx_free;
+                }
+                if (chunk == 0u) {
+                    return;
+                }
+                chunk = tcp_server_peek(i, buffer, chunk);
+                if (chunk == 0u) {
+                    continue;
+                }
+                written = App_SendTcpPayloadToUartRaw(uart_index, buffer, chunk);
+                if (written > 0u) {
+                    tcp_server_drop(i, written);
+                }
+                if (written < chunk) {
+                    return;
+                }
+            }
         }
     }
 
     for (uint8_t i = 0; i < TCP_CLIENT_INSTANCE_COUNT; ++i) {
-        int rc = tcp_client_recv(i, buffer, sizeof(buffer));
+        uint16_t available = tcp_client_rx_available(i);
-        if (rc > 0) {
+        if (available > 0u) {
             uint8_t link_index = (i == 0u) ? CONFIG_LINK_C1 : CONFIG_LINK_C2;
-            if (!App_SendToUart(cfg->links[link_index].uart,
+            uint8_t uart_index = cfg->links[link_index].uart;
-                                config_link_index_to_endpoint(link_index),
+            uint8_t src_id = config_link_index_to_endpoint(link_index);
-                                config_uart_index_to_endpoint(cfg->links[link_index].uart),
+            uint8_t dst_mask = config_uart_index_to_endpoint(uart_index);
-                                buffer,
+            uart_channel_t channel = (uart_index == LINK_UART_U1) ? UART_CHANNEL_U1 : UART_CHANNEL_U0;
-                                (uint16_t)rc)) {
+
+            if (cfg->mux_mode == MUX_MODE_FRAME) {
+                uint16_t tx_free = uart_trans_tx_free(channel);
+                uint16_t payload_len;
+                if (tx_free <= 6u) {
                     return;
                 }
+                payload_len = available;
+                if (payload_len > APP_TCP_TO_UART_CHUNK_SIZE) {
+                    payload_len = APP_TCP_TO_UART_CHUNK_SIZE;
+                }
+                if (payload_len > (uint16_t)(tx_free - 6u)) {
+                    payload_len = (uint16_t)(tx_free - 6u);
+                }
+                if (payload_len == 0u) {
+                    return;
+                }
+                payload_len = tcp_client_peek(i, buffer, payload_len);
+                if (payload_len == 0u) {
+                    continue;
+                }
+                if (!App_SendTcpPayloadToUartMux(uart_index, src_id, dst_mask, buffer, payload_len)) {
+                    return;
+                }
+                tcp_client_drop(i, payload_len);
+            } else {
+                uint16_t chunk = available;
+                uint16_t tx_free = uart_trans_tx_free(channel);
+                uint16_t written;
+                if (tx_free == 0u) {
+                    return;
+                }
+                if (chunk > APP_TCP_TO_UART_CHUNK_SIZE) {
+                    chunk = APP_TCP_TO_UART_CHUNK_SIZE;
+                }
+                if (chunk > tx_free) {
+                    chunk = tx_free;
+                }
+                if (chunk == 0u) {
+                    return;
+                }
+                chunk = tcp_client_peek(i, buffer, chunk);
+                if (chunk == 0u) {
+                    continue;
+                }
+                written = App_SendTcpPayloadToUartRaw(uart_index, buffer, chunk);
+                if (written > 0u) {
+                    tcp_client_drop(i, written);
+                }
+                if (written < chunk) {
+                    return;
+                }
+            }
         }
     }
 }
@@ -510,6 +637,7 @@ static void App_Poll(void)
     sys_check_timeouts();
     App_StopLinksIfNeeded();
     App_StartLinksIfNeeded();
+    tcp_server_poll();
     tcp_client_poll();
     uart_trans_poll();
     StackGuard_Check();

MDK-ARM/keil-build-viewer-record.txt

@@ -17,7 +17,7 @@
        778          0          0          2          0          0   ip4.o
         46          0          4          0          0          0   ip4_addr.o
         44          0          0          0         12          0   iwdg.o
-      2842          0        185         12        272          0   main.o
+      3212          0        185         12        272          0   main.o
        828          0          0         12       4115          0   mem.o
        196          0        244         32       6464          0   memp.o
        582          0          0         12          0          0   netif.o
@@ -43,13 +43,13 @@
        490          0          0          0          0          0   stm32f1xx_it.o
          2          0         24          4          0          0   system_stm32f1xx.o
       3474          0        193         32          0          0   tcp.o
-      1232          0          0          0       1120          0   tcp_client.o
+      1556          0          0          0       1072          0   tcp_client.o
       3684          0          0         36         20          0   tcp_in.o
       3862          0          0          0          0          0   tcp_out.o
-       986          0          0          0       1104          0   tcp_server.o
+      1346          0          0          0       1048          0   tcp_server.o
        164          0          0          0         72          0   tim.o
        374          0         16         12          0          0   timeouts.o
-      1538          0          0          0       2936          0   uart_trans.o
+      1590          0          0          0       2936          0   uart_trans.o
        816          0          0          0        624          0   usart.o
 Object Totals
 
@@ -57,8 +57,8 @@ Memory Map of the image
 
 	Load Region LR_IROM1 
 
-		Execution Region ER_IROM1 (Exec base: 0x08000000, Size: 0x0000D72C, Max: 0x00010000, END)
+		Execution Region ER_IROM1 (Exec base: 0x08000000, Size: 0x0000DB7C, Max: 0x00010000, END)
 
-		Execution Region RW_IRAM1 (Exec base: 0x20000000, Size: 0x00005000, Max: 0x00005000, END)
+		Execution Region RW_IRAM1 (Exec base: 0x20000000, Size: 0x00004F98, Max: 0x00005000, END)
 
 Image component sizes

项目技术实现.md

@@ -184,6 +184,68 @@ EN,LPORT,RIP,RPORT,UART
 2. 统一受 `LINK[idx]` 配置驱动
 3. 由调度层决定实例与 UART 的数据交换路径
 
+### 6.4 `v1.1.0` 低 RAM TCP 背压修复
+
+`v1.1.0` 起，`TCP -> UART` 路径补充如下实现约束，用于解决“TCP 接收过快、UART 发送过慢时本地缓存被冲垮”的问题，同时尽量不新增静态 RAM：
+
+1. 继续复用 `tcp_server` / `tcp_client` 现有 `RX ring`，不为每个连接新增独立的大块 pending payload 缓冲。
+2. `tcp_server_on_recv()` / `tcp_client_on_recv()` 不再在回调内立即 `tcp_recved()`。
+3. lwIP 交来的 `pbuf` 在回调中通过 `pbuf_ref()` 转为应用持有，再释放回调上下文的原始引用；后续由应用在主循环中继续把数据泵入 `RX ring`，最终在消费完成后释放。
+4. 当 `RX ring` 暂时装不下时，剩余数据保留在 `hold_pbuf + hold_offset` 中，等待主循环下一轮继续搬运。
+5. 只有当数据真正从 `TCP RX ring` 被 `drop` 掉，也就是已经被下游 `UART` 接收进入发送路径时，才调用 `tcp_recved()` 释放 TCP 接收窗口。
+
+这样做的效果是：
+
+1. `UART` 慢时，TCP 窗口不会继续无条件放大。
+2. 对端发送速度会被 lwIP 接收窗口自然压制。
+3. 修复点建立在已有 ring 与主循环调度之上，不引入 `FreeRTOS` 或新的大块静态缓存。
+
+#### RAW 与 MUX 的分流规则
+
+在 `v1.1.0` 中，`TCP` 侧拿到的都是纯 payload，因此 `TCP` 背压逻辑在 `RAW` 与 `MUX` 两种模式下共用到 `UART commit` 之前：
+
+1. `RAW` 模式：
+   - 主循环先查看 `uart_trans_tx_free()`
+   - 再按 `min(tcp_available, tx_free, APP_TCP_TO_UART_CHUNK_SIZE)` 从 TCP ring `peek`
+   - `uart_trans_write()` 实际写入多少，就 `drop + tcp_recved` 多少
+2. `MUX` 模式：
+   - `TCP` payload 本身不带帧头尾
+   - 只有当 `UART TX free >= payload_len + 6` 时，才在栈上临时编码一帧并一次性写入 `UART TX ring`
+   - 只有整帧成功入队后，才按原始 payload 长度执行 `drop + tcp_recved`
+
+该设计保证：
+
+1. `RAW` 模式允许流式逐步提交
+2. `MUX` 模式保持“单个 UART 输出帧必须完整入队”的语义
+3. `TCP` 接收窗口始终以真实下游消费进度为准，而不是以“回调里已经 memcpy 到本地”作为提交点
+
+#### RAM 与 chunk 策略
+
+为给新增的 `hold_pbuf / hold_offset` 状态字段让位，并进一步降低单轮转发压力，`v1.1.0` 同步采用以下策略：
+
+1. 新增 `APP_TCP_TO_UART_CHUNK_SIZE = 128`
+2. `TCP_SERVER_RX_BUFFER_SIZE` 从 `512` 调整为 `480`
+3. `TCP_CLIENT_RX_BUFFER_SIZE` 从 `512` 调整为 `480`
+
+设计意图：
+
+1. 利用更小的单次转发块提升主循环调度颗粒度
+2. 让 `MUX` 模式下 `payload + 6` 更容易完整进入 `UART TX ring`
+3. 在静态 RAM 已接近上限时，为少量新状态字段回收空间
+
+#### 构建基线
+
+`v1.1.0` 以 `MDK-ARM/TCP2UART.uvprojx` 的 `TCP2UART` Target 为构建验收基线。
+
+当前一次通过的参考结果：
+
+1. `errors = 0`
+2. `warnings = 0`
+3. `flash_bytes = 56544`
+4. `ram_bytes = 20376`
+
+该结果说明修复后工程仍满足 `STM32F103R8T6` 的 `20KB RAM` 上限，但余量已经很小；后续若继续增加功能，应优先考虑复用现有缓冲与状态，而不是增加新的静态大数组。
+
 ## 七、主循环实现方向
 
 主循环仍保持裸机轮询风格：