nic: Fix incorrect wrap handling of RX circular buffer

nic: move other nic functions into the dedicated nic files
uip: alloc buffer multiple of 8 bytes

Signed-off-by: João Silva <jgc3silva@gmail.com>

Makefile

@@ -103,6 +103,7 @@ SRCS += \
 	rtl837x_flash.c \
 	rtl837x_igmp.c \
 	rtl837x_init.c \
+	rtl837x_nic.c \
 	rtl837x_leds.c \
 	rtl837x_leds_dump.c \
 	rtl837x_phy.c \

boot.c

@@ -257,7 +257,7 @@ void nic_setup(void) __banked
 
 	// This sets the size of the RX buffer, the filling level is in 0x7874
 	// R7848-000004ff
-	REG_SET(RTL837X_REG_NIC_RXBUFF_RX, 0x4ff);
+	REG_SET(RTL837X_REG_NIC_RXBUFF_RX, RTL837X_NIC_RX_LAST_SLOT);
 
 	// R7844-000007fe
 	REG_SET(RTL837X_REG_NIC_BUFFSIZE_TX, 0x7fe);

doc/CpuPort.md

@@ -34,6 +34,21 @@ Next, transfer the actual packet over by repeating the SFR function with a
 pointer to the frame on the ASIC directly after the frame header and a
 length as given by the length in the frame header + 7, again divided by 8.
 
+The RX ring's inclusive last slot is configured as `0x4ff`, giving a byte
+range of `0x0000` through `0x27ff`. If the descriptor occupies the last slot
+at `0x27f8`, its payload starts at `0x0000`, not `0x2800`. The RX copy code
+normalizes that starting address before DMA. It relies on the hardware to
+wrap transfers that start inside the ring and cross its end, as observed
+on SWTGW218AS. Programming an initial address of `0x2800` does not wrap
+and can instead read zeros or stale TX data outside the RX ring.
+
+Frame lengths are checked against `UIP_CONF_BUFFER_SIZE` before copying.
+DMA works in whole eight-byte units. `UIP_CONF_BUFFER_ALLOC_SIZE` rounds
+the backing storage up to the next complete word: 1,560 bytes for the
+1,556-byte logical frame limit. This padding allows a maximum-size frame
+to be transferred directly without a separate tail copy. The logical
+frame limit and advertised MSS are unchanged.
+
 The received frame will have an RTL proprietary Ethernet frame type of
 0x8899 (RRPC) where normally the frame type 0x0800 for IPv4 would be located.
 Further 6 bytes follow describing the frame, before the normal IPv4 data

rtl837x_common.h

@@ -142,7 +142,7 @@ extern __xdata bool stp_enabled;
  * advertises it as the System Name TLV) read it from here. */
 extern __xdata char hostname[24];
 
-extern __xdata uint8_t uip_buf[UIP_CONF_BUFFER_SIZE+2];
+extern __xdata uint8_t uip_buf[UIP_CONF_BUFFER_ALLOC_SIZE];
 extern __xdata struct uip_eth_addr uip_ethaddr;
 
 // Headers for calls in the common code area (HOME/BANK0)

rtl837x_igmp.c

@@ -28,7 +28,7 @@ extern __xdata uint8_t igmpEnabled;
 extern __xdata uint8_t cpuPort;
 extern __xdata uint8_t sfr_data[4];
 extern __xdata struct machine_runtime machine_detected;
-extern __xdata uint8_t uip_buf[UIP_CONF_BUFFER_SIZE + 2];
+extern __xdata uint8_t uip_buf[UIP_CONF_BUFFER_ALLOC_SIZE];
 
 __xdata uint16_t idx;
 #define IGMP_IP_HDR		offsetof(struct igmp_pkt, hlen)

rtl837x_lacp.c

@@ -21,7 +21,7 @@ extern __code const struct machine machine;
 extern __xdata struct machine_runtime machine_detected;
 extern __xdata uint16_t management_vlan;
 extern __xdata struct uip_eth_addr uip_ethaddr;
-extern __xdata uint8_t uip_buf[UIP_CONF_BUFFER_SIZE + 2];
+extern __xdata uint8_t uip_buf[UIP_CONF_BUFFER_ALLOC_SIZE];
 
 /* 20-byte Actor/Partner information body (802.3ad 43.4.2.2), network byte order */
 struct lacp_info {

rtl837x_nic.c

@@ -0,0 +1,90 @@
+#include "rtl837x_common.h"
+#include "rtl837x_regs.h"
+#include "rtl837x_sfr.h"
+#include "rtl837x_nic.h"
+
+extern __xdata uint8_t rx_headers[16];
+
+
+#ifndef RTLP_HOST_TEST
+/* One RX DMA operation, starting inside the configured ring. The hardware
+ * wraps transfers that cross its end; the initial address must be normalized.
+ * Host tests supply a simulated DMA engine for this hardware boundary. */
+bool nic_rx_dma(__xdata uint8_t *buffer, uint16_t ring_ptr, uint8_t words) __reentrant
+{
+	uint16_t guard = 0;
+
+	SFR_NIC_DATA_U16LE = (uint16_t)buffer;
+	SFR_NIC_RING_U16LE = ring_ptr;
+	SFR_NIC_CTRL = words;
+	while (SFR_NIC_CTRL != 0) {
+		if (++guard == 0) {
+			print_string("NIC: RX transfer did not complete\n");
+			return false;
+		}
+	}
+	return true;
+}
+#endif
+
+static bool nic_rx_copy(__xdata uint8_t *buffer, uint16_t ring_ptr, uint8_t words) __reentrant
+{
+	if (ring_ptr > RTL837X_NIC_RX_RING_BYTES || (ring_ptr & 7))
+		return false;
+	if (ring_ptr == RTL837X_NIC_RX_RING_BYTES)
+		ring_ptr = 0;
+
+	return nic_rx_dma(buffer, ring_ptr, words);
+}
+
+
+bool nic_rx_header(uint16_t ring_ptr) __reentrant
+{
+	if (ring_ptr >= RTL837X_NIC_RX_RING_BYTES)
+		return false;
+	return nic_rx_copy(rx_headers, ring_ptr, 1);
+}
+
+
+bool nic_rx_packet(__xdata uint8_t *buffer, uint16_t ring_ptr) __reentrant
+{
+	uint16_t len = ((uint16_t)rx_headers[5] << 8) | rx_headers[4];
+
+	if (!len || len > UIP_CONF_BUFFER_SIZE)
+		return false;
+	return nic_rx_copy(buffer, ring_ptr, (len + 7) >> 3);
+}
+
+
+/*
+ * Transfers data in XMEM to the ASIC for transmission by the NIC
+ */
+void nic_tx_packet(__xdata uint8_t *buffer, uint16_t len, uint16_t ring_ptr)
+{
+	uint16_t guard = 0;
+
+	SFR_NIC_DATA_U16LE = (uint16_t)(uintptr_t)buffer;
+
+#ifdef RXTXDBG
+	print_string("TX: \n");
+	for (uint8_t i = 0; i < 100; i++) {
+		print_byte(uip_buf[i]);
+		write_char(' ');
+	}
+	write_char('\n');
+#endif
+
+	ring_ptr <<= 3;
+	ring_ptr |= 0x8000;
+	SFR_NIC_RING_U16LE = ring_ptr;
+
+	len += 0xf;
+	len >>= 3;
+	SFR_NIC_CTRL = len;
+	while (SFR_NIC_CTRL != 0) {
+		if (++guard == 0) {
+			print_string("NIC: TX transfer did not complete\n");
+			return;
+		}
+	}
+}

rtl837x_nic.h

@@ -0,0 +1,20 @@
+#ifndef RTL837X_NIC_H
+#define RTL837X_NIC_H
+
+#include <stdbool.h>
+#include <stdint.h>
+
+// #define RXTXDBG 1
+
+bool nic_rx_header(uint16_t ring_ptr) __reentrant;
+bool nic_rx_packet(__xdata uint8_t *buffer, uint16_t ring_ptr) __reentrant;
+
+/* buffer starts at the eight-byte NIC descriptor; len excludes that
+ * descriptor. ring_ptr is the ASIC TX position in eight-byte slots. */
+void nic_tx_packet(__xdata uint8_t *buffer, uint16_t len, uint16_t ring_ptr);
+
+/* RX DMA in eight-byte units. ring_ptr must start inside the configured
+ * RX ring; the hardware wraps transfers that cross its end. */
+bool nic_rx_dma(__xdata uint8_t *buffer, uint16_t ring_ptr, uint8_t words) __reentrant;
+
+#endif

rtl837x_regs.h

@@ -147,6 +147,9 @@
  */
 #define RTL837X_REG_NIC_BUFFSIZE_TX	0x7844
 #define RTL837X_REG_NIC_RXBUFF_RX	0x7848
+/* RX ring: inclusive final eight-byte slot, as programmed by nic_setup(). */
+#define RTL837X_NIC_RX_LAST_SLOT	0x04ff
+#define RTL837X_NIC_RX_RING_BYTES	((RTL837X_NIC_RX_LAST_SLOT + 1U) * 8U)
 #define RTL837X_REG_NIC_RXCMD		0x784c
 #define RTL837X_REG_NIC_TXCMD		0x7850
 #define RTL837X_REG_RX_CTRL		0x785c

rtl837x_stp.c

@@ -30,7 +30,7 @@ extern __xdata struct machine_runtime machine_detected;
 
 extern __xdata struct uip_eth_addr uip_ethaddr;
 
-extern __xdata uint8_t uip_buf[UIP_CONF_BUFFER_SIZE + 2];
+extern __xdata uint8_t uip_buf[UIP_CONF_BUFFER_ALLOC_SIZE];
 
 extern __xdata uint8_t cmd_buffer[CMD_BUF_SIZE];
 extern __xdata uint8_t cmd_words_len;

rtlplayground.c

@@ -2,7 +2,6 @@
 #include <stdint.h>
 
 // #define REGDBG 1
-// #define RXTXDBG 1
 
 #include "rtl837x_sfr.h"
 #include "rtl837x_regs.h"
@@ -12,6 +11,7 @@
 #include "rtl837x_phy.h"
 #include "rtl837x_port.h"
 #include "rtl837x_trap.h"
+#include "rtl837x_nic.h"
 #include "rtl837x_stp.h"
 #include "rtl837x_lacp.h"
 #include "rtl837x_igmp.h"
@@ -123,8 +123,8 @@ __code const uint8_t * __code const hex = "0123456789abcdef";
 __xdata uint8_t flash_buf[FLASH_BUF_SIZE];
 
 // NIC buffers for packet RX/TX
-__xdata uint8_t rx_headers[16]; // Packet header(s) on RX
-__xdata uint8_t uip_buf[UIP_CONF_BUFFER_SIZE+2];
+__xdata uint8_t rx_headers[16]; // RX descriptor
+__xdata uint8_t uip_buf[UIP_CONF_BUFFER_ALLOC_SIZE];
 
 __xdata uint16_t rx_packet_vlan;
 __xdata uint16_t management_vlan;
@@ -714,108 +714,6 @@ void get_random_32(void)
 }
 
 
-/*
- * Transfer Network Interface RX data from the ASIC to the 8051 XMEM
- * data will be stored in the rx_header structure
- * len is the length of data to be transferred
- */
-bool nic_rx_header(uint16_t ring_ptr)
-{
-	uint16_t buffer = (uint16_t) &rx_headers[0];
-	uint16_t guard = 0;
-
-	SFR_NIC_DATA_U16LE = buffer;
-	SFR_NIC_RING_U16LE = ring_ptr;
-	SFR_NIC_CTRL = 1;
-	while (SFR_NIC_CTRL != 0) {
-		if (++guard == 0) {
-			print_string("NIC: RX header transfer did not complete\n");
-			return false;
-		}
-	}
-	return true;
-}
-
-
-/*
- * Transfer Network Interface RX data from the ASIC to the 8051 XMEM
- * the description of the packet must be in the rx_headers data structure
- * data will be returned in the xmem buffer points to
- * ring_ptr is the current position of the RX Ring on the ASIC side
- */
-bool nic_rx_packet(uint16_t buffer, uint16_t ring_ptr)
-{
-	uint16_t guard = 0;
-
-	SFR_NIC_DATA_U16LE = buffer;
-	SFR_NIC_RING_U16LE = ring_ptr;
-
-	uint16_t len = (((uint16_t)rx_headers[5]) << 8) | rx_headers[4];
-	len += 7;
-	len >>= 3;
-#ifdef RXTXDBG
-	print_string(" len: ");
-	print_short(len);
-#endif
-	SFR_NIC_CTRL = len;
-	while (SFR_NIC_CTRL != 0) {
-		if (++guard == 0) {
-			print_string("NIC: RX transfer did not complete\n");
-			return false;
-		}
-	}
-	return true;
-}
-
-
-/*
- * Transfers data in XMEM to the ASIC for transmission by the nic
- */
-void nic_tx_packet(uint16_t ring_ptr)
-{
-	uint16_t len;
-	uint16_t guard = 0;
-
-	/* A frame that got a dot1Q tag was shifted forward over its padding, so it
-	 * starts at uip_buf and carries the q_frame layout. One that did not keeps
-	 * the padding in front and the nonq_frame layout, so the padding is skipped.
-	 */
-	if (frame_tagged) {
-		SFR_NIC_DATA_U16LE = (uint16_t) uip_buf;
-		len = FRAME_Q->len;
-		/*
-		(__xdata struct rtl_dot1q_frame *)uip_buf
-#define FRAME (((__xdata struct rtl_dot1q_frame *)&uip_buf[0]).nonq_frame)*/
-	} else {
-		SFR_NIC_DATA_U16LE = (uint16_t) uip_buf + VLAN_TAG_SIZE;
-		len = FRAME->len;
-	}
-
-#ifdef RXTXDBG
-	print_string("TX: \n");
-	for (uint8_t i = 0; i < 100; i++) {
-		print_byte(uip_buf[i]);
-		write_char(' ');
-	}
-	write_char('\n');
-#endif
-
-	ring_ptr <<= 3;
-	ring_ptr |= 0x8000;
-	SFR_NIC_RING_U16LE = ring_ptr;
-
-	len += 0xf;
-	len >>= 3;
-	SFR_NIC_CTRL = len;
-	while (SFR_NIC_CTRL != 0) {
-		if (++guard == 0) {
-			print_string("NIC: TX transfer did not complete\n");
-			return;
-		}
-	}
-}
-
-
 /* Read flash using the MMIO capabilities of the DW8051 core
  * Bank is < 0x3f and is the MSB
  * addr gives the address in the bank
@@ -999,8 +897,12 @@ void tcpip_output(void)
 	uint16_t ring_ptr = ((uint16_t)sfr_data[2]) << 8;
 	ring_ptr |= sfr_data[3];
 
-	// Move data over from xmem buffer to ASIC side using DMA
-	nic_tx_packet(ring_ptr);
+	// Move data over from xmem buffer to ASIC side using DMA. A VLAN-tagged
+	// frame starts at uip_buf; an untagged frame retains four bytes of padding.
+	if (frame_tagged)
+		nic_tx_packet(uip_buf, FRAME_Q->len, ring_ptr);
+	else
+		nic_tx_packet(uip_buf + VLAN_TAG_SIZE, FRAME->len, ring_ptr);
 
 	// New position of the ring-pointer on the NIC-side indicates number of bytes transmitted
 	reg_read_m(RTL837X_REG_NIC_TX_CURR_PKT);
@@ -1037,7 +939,7 @@ void handle_rx(void)
 			write_char(' ');
 		}
 #endif
-		if (!nic_rx_packet((uint16_t) &uip_buf[0], ring_ptr + 8)) {
+		if (!nic_rx_packet(uip_buf, ring_ptr + 8)) {
 			REG_SET(RTL837X_REG_NIC_RXCMD, 1);
 			return;
 		}

test/Makefile

@@ -29,6 +29,7 @@ TESTS = $(BUILDDIR)/test_cmd_editor \
         $(BUILDDIR)/test_page_json \
         $(BUILDDIR)/test_httpd_tx \
         $(BUILDDIR)/test_tick_gate \
+        $(BUILDDIR)/test_nic_rx \
         $(BUILDDIR)/test_lacp
 
 # The table tests link the register mock and one environment file; stub/
@@ -52,6 +53,11 @@ $(BUILDDIR)/test_port_tables: test_port_tables.c $(TABLE_ENV) hw_mock.h | $(BUIL
 $(BUILDDIR)/test_tick_gate: test_tick_gate.c support.c $(SRCROOT)/cmd_editor.c | $(BUILDDIR)
 	$(CC) $(CFLAGS) $^ -o $@ $(LDFLAGS)
 
+# Exercise the production RX ring reader; only the hardware DMA edge is mocked.
+# Discard unused TX functions so this RX fixture needs no TX register mocks.
+$(BUILDDIR)/test_nic_rx: test_nic_rx.c $(SRCROOT)/rtl837x_nic.c $(SRCROOT)/rtl837x_nic.h | $(BUILDDIR)
+	$(CC) $(CFLAGS) -ffunction-sections -fdata-sections $(filter %.c,$^) -o $@ $(LDFLAGS) -Wl,--gc-sections
+
 $(BUILDDIR)/test_page_json: test_page_json.c $(TABLE_ENV) hw_mock.h | $(BUILDDIR)
 	$(CC) $(CFLAGS) $(filter %.c,$^) -o $@ $(LDFLAGS)
 

test/test_httpd_tx.c

@@ -112,7 +112,7 @@ void read_reg_timer(uint32_t *tmr) { *tmr = 0; }
 
 /* uip-conf.h asks uIP not to define the packet buffer: on the switch it lives
  * at a fixed XDATA address, so the bench provides the storage itself. */
-u8_t uip_buf[UIP_BUFSIZE + 2];
+u8_t uip_buf[UIP_CONF_BUFFER_ALLOC_SIZE];
 
 uint16_t strlen_x(const char *s) { return (uint16_t)strlen(s); }
 

test/test_lacp.c

@@ -11,7 +11,7 @@
 
 /* ---------- platform mocks ---------- */
 
-uint8_t uip_buf[UIP_CONF_BUFFER_SIZE + 2];
+uint8_t uip_buf[UIP_CONF_BUFFER_ALLOC_SIZE];
 uint16_t uip_len;
 struct uip_eth_addr uip_ethaddr = { .addr = {0x02,0x11,0x22,0x33,0x44,0x55} };
 const struct machine machine = { .min_port = 0, .max_port = 7,

test/test_nic_rx.c

@@ -0,0 +1,277 @@
+#include <stdint.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <string.h>
+
+#include "rtl837x_common.h"
+#include "rtl837x_regs.h"
+#include "rtl837x_nic.h"
+
+uint8_t rx_headers[16];
+
+/* Model the DMA behavior reported on SWTGW218AS: a transfer starting inside
+ * the RX ring wraps at its end. An initial address outside the ring instead
+ * reads flat ASIC memory, including stale TX bytes at 0x2800. */
+static uint8_t asic[0x8000];
+static struct {
+	uint8_t before[16];
+	uint8_t data[UIP_CONF_BUFFER_ALLOC_SIZE];
+	uint8_t after[16];
+} destination;
+
+struct dma_request {
+	uint8_t *buffer;
+	uint16_t ring;
+	uint8_t words;
+};
+static struct dma_request requests[8];
+static unsigned request_count, fail_request;
+static int failures;
+
+static void check(bool condition, const char *what)
+{
+	if (!condition) {
+		printf("  FAIL: %s\n", what);
+		failures++;
+	}
+}
+
+bool nic_rx_dma(uint8_t *buffer, uint16_t ring, uint8_t words)
+{
+	size_t bytes = (size_t)words * 8;
+	unsigned call = ++request_count;
+
+	if (call <= sizeof(requests) / sizeof(requests[0]))
+		requests[call - 1] = (struct dma_request){ buffer, ring, words };
+	if (call == fail_request || !words || (size_t)ring + bytes > sizeof(asic))
+		return false;
+	if (ring < RTL837X_NIC_RX_RING_BYTES) {
+		for (size_t i = 0; i < bytes; i++)
+			buffer[i] = asic[((size_t)ring + i) % RTL837X_NIC_RX_RING_BYTES];
+	} else {
+		memcpy(buffer, asic + ring, bytes);
+	}
+	return true;
+}
+
+static void fixture_reset(void)
+{
+	memset(asic, 0xd7, sizeof(asic));
+	memset(&destination, 0xa5, sizeof(destination));
+	memset(rx_headers, 0x6c, sizeof(rx_headers));
+	memset(requests, 0, sizeof(requests));
+	request_count = 0;
+	fail_request = 0;
+}
+
+static bool filled(const uint8_t *data, size_t len, uint8_t value)
+{
+	while (len--)
+		if (*data++ != value)
+			return false;
+	return true;
+}
+
+static bool guards_intact(void)
+{
+	return filled(destination.before, sizeof(destination.before), 0xa5)
+		&& filled(destination.after, sizeof(destination.after), 0xa5);
+}
+
+static bool requests_start_in_rx_ring(void)
+{
+	for (unsigned i = 0; i < request_count; i++) {
+		if (i >= sizeof(requests) / sizeof(requests[0]) || !requests[i].words
+		    || (requests[i].ring & 7)
+		    || requests[i].ring >= RTL837X_NIC_RX_RING_BYTES)
+			return false;
+	}
+	return true;
+}
+
+static void ring_write(uint16_t offset, const uint8_t *data, size_t len)
+{
+	for (size_t i = 0; i < len; i++)
+		asic[((size_t)offset + i) % RTL837X_NIC_RX_RING_BYTES] = data[i];
+}
+
+static void put_descriptor(uint16_t offset, uint16_t len)
+{
+	uint8_t descriptor[8] = { 0x03, 0x00, 0xb0, 0xc8, len, len >> 8, 0, 0 };
+	ring_write(offset, descriptor, sizeof(descriptor));
+}
+
+static void expect_descriptor(uint16_t len)
+{
+	uint8_t descriptor[8] = { 0x03, 0x00, 0xb0, 0xc8, len, len >> 8, 0, 0 };
+	check(!memcmp(rx_headers, descriptor, sizeof(descriptor)), "the first eight descriptor bytes stay intact");
+}
+
+static void pattern(uint8_t *data, size_t len)
+{
+	for (size_t i = 0; i < len; i++)
+		data[i] = (uint8_t)(i * 37 + (i >> 8) + 0x13);
+}
+
+static void t_last_descriptor(void)
+{
+	static const uint8_t root_bpdu[68] = {
+		0x01, 0x80, 0xc2, 0, 0, 0, 0xd0, 0xea, 0x11, 0x12, 0x34, 0x56,
+		0x88, 0x99, 0x04, 0, 0, 0, 0, 8,
+		0, 0x27, 0x42, 0x42, 3, 0, 0, 2, 2, 0x3c,
+		0x10, 0, 0xd0, 0xea, 0x11, 0x12, 0x34, 0x56, 0, 0, 0, 0,
+		0x10, 0, 0xd0, 0xea, 0x11, 0x12, 0x34, 0x56, 0x80, 0x0a,
+		0, 0, 0x14, 0, 2, 0, 0x0f, 0, 0
+	};
+	uint8_t stale_tx[72] = { 0 };
+	uint16_t last = RTL837X_NIC_RX_RING_BYTES - 8;
+
+	printf("[test] the last RX descriptor wraps its BPDU payload before DMA\n");
+	fixture_reset();
+	put_descriptor(last, sizeof(root_bpdu));
+	ring_write(0, root_bpdu, sizeof(root_bpdu));
+	memcpy(stale_tx, root_bpdu, sizeof(root_bpdu));
+	stale_tx[17] = 0x20;
+	stale_tx[19] = 1; /* retained TX destination mask, not ingress port 8 */
+	memcpy(asic + RTL837X_NIC_RX_RING_BYTES, stale_tx, sizeof(stale_tx));
+	check(nic_rx_header(last), "the final descriptor is readable");
+	expect_descriptor(sizeof(root_bpdu));
+	check(nic_rx_packet(destination.data, last + 8), "a payload address at the RX ring limit wraps to zero");
+	check(!memcmp(destination.data, root_bpdu, sizeof(root_bpdu)), "the complete upstream BPDU is read instead of stale TX bytes");
+	check(destination.data[19] == 8, "the upstream ingress tag survives the ring boundary");
+	check(request_count == 2 && requests[0].ring == last && requests[0].words == 1
+	      && requests[1].ring == 0 && requests[1].words == 9,
+	      "descriptor and rounded payload DMA both start in the RX ring");
+	check(requests_start_in_rx_ring() && guards_intact(), "DMA starts inside the RX ring and preserves destination guards");
+	expect_descriptor(sizeof(root_bpdu));
+}
+
+static void t_payload_layouts(void)
+{
+	static const struct {
+		uint16_t descriptor, len;
+	} cases[] = {
+		{ 0x0100, 8 }, { 0x0100, 16 }, { 0x0100, 64 },
+		{ 0x0100, 68 }, { 0x0100, 72 },
+		{ RTL837X_NIC_RX_RING_BYTES - 16, 8 },
+		{ RTL837X_NIC_RX_RING_BYTES - 16, 16 },
+		{ RTL837X_NIC_RX_RING_BYTES - 16, 68 },
+		{ RTL837X_NIC_RX_RING_BYTES - 72, 64 },
+		{ RTL837X_NIC_RX_RING_BYTES - 48, 68 }
+	};
+	uint8_t frame[80];
+
+	printf("[test] payload DMA handles ordinary, exact-boundary, and crossing frames\n");
+	pattern(frame, sizeof(frame));
+	for (unsigned i = 0; i < sizeof(cases) / sizeof(cases[0]); i++) {
+		size_t rounded = (cases[i].len + 7u) & ~7u;
+		fixture_reset();
+		put_descriptor(cases[i].descriptor, cases[i].len);
+		ring_write(cases[i].descriptor + 8, frame, rounded);
+		check(nic_rx_header(cases[i].descriptor), "the descriptor transfer completes");
+		check(nic_rx_packet(destination.data, cases[i].descriptor + 8), "the payload transfer completes");
+		check(!memcmp(destination.data, frame, rounded), "every payload and permitted padding byte reaches the correct destination offset");
+		check(filled(destination.data + rounded, sizeof(destination.data) - rounded, 0xa5), "DMA does not write beyond the rounded frame");
+		check(request_count == 2, "the payload uses one DMA request even when crossing the ring end");
+		check(requests_start_in_rx_ring() && guards_intact(), "crossing DMA starts inside the RX ring and preserves destination guards");
+		expect_descriptor(cases[i].len);
+	}
+}
+
+static void t_maximum_frames(void)
+{
+	uint8_t frame[(UIP_CONF_BUFFER_SIZE + 7u) & ~7u];
+	static const uint16_t lengths[] = { 1, 7, 1549, 1550, 1551, 1552, 1553, 1554, 1555, 1556 };
+
+	printf("[test] rounded maximum frames fit without overwriting their descriptor\n");
+	pattern(frame, sizeof(frame));
+	for (unsigned layout = 0; layout < 3; layout++) {
+		uint16_t descriptor = layout == 0 ? 0x0100
+			: RTL837X_NIC_RX_RING_BYTES - (layout == 1 ? 16 : 8);
+		for (unsigned i = 0; i < sizeof(lengths) / sizeof(lengths[0]); i++) {
+			uint16_t len = lengths[i];
+			size_t rounded = (len + 7u) & ~7u;
+			fixture_reset();
+			put_descriptor(descriptor, len);
+			ring_write(descriptor + 8, frame, rounded);
+			check(nic_rx_header(descriptor), "the maximum-frame descriptor transfer completes");
+			check(nic_rx_packet(destination.data, descriptor + 8), "all supported frame lengths fit the destination");
+			check(!memcmp(destination.data, frame, rounded), "the full rounded frame is preserved");
+			check(filled(destination.data + rounded, sizeof(destination.data) - rounded, 0xa5), "rounded DMA writes no bytes beyond the available destination");
+			check(request_count == 2, "maximum frames use a single payload DMA request");
+			check(requests_start_in_rx_ring() && guards_intact(), "rounded DMA starts inside the RX ring and preserves destination guards");
+			expect_descriptor(len);
+		}
+	}
+}
+
+static void t_invalid_frames(void)
+{
+	static const uint16_t invalid_lengths[] = { 0, UIP_CONF_BUFFER_SIZE + 1, 0xffff };
+	static const uint16_t invalid_pointers[] = { 1, 7, RTL837X_NIC_RX_RING_BYTES + 8, 0xfff8 };
+
+	printf("[test] invalid lengths and DMA addresses are rejected before transfer\n");
+	for (unsigned i = 0; i < sizeof(invalid_lengths) / sizeof(invalid_lengths[0]); i++) {
+		fixture_reset();
+		rx_headers[4] = invalid_lengths[i];
+		rx_headers[5] = invalid_lengths[i] >> 8;
+		check(!nic_rx_packet(destination.data, 0x0100), "unsupported or overflowing lengths are rejected");
+		check(!request_count && filled(destination.data, sizeof(destination.data), 0xa5) && guards_intact(),
+		      "invalid frame lengths cause no DMA or destination changes");
+	}
+	for (unsigned i = 0; i < sizeof(invalid_pointers) / sizeof(invalid_pointers[0]); i++) {
+		fixture_reset();
+		rx_headers[4] = 68;
+		rx_headers[5] = 0;
+		check(!nic_rx_header(invalid_pointers[i]), "an invalid descriptor address is rejected");
+		check(!nic_rx_packet(destination.data, invalid_pointers[i]), "an invalid payload address is rejected");
+		check(!request_count && guards_intact(), "invalid addresses never reach DMA");
+	}
+	fixture_reset();
+	check(!nic_rx_header(RTL837X_NIC_RX_RING_BYTES) && !request_count,
+	      "a descriptor at the ring limit is rejected rather than wrapped");
+}
+
+static void t_dma_failure(void)
+{
+	uint8_t frame[UIP_CONF_BUFFER_ALLOC_SIZE];
+	static const uint16_t lengths[] = { 68, UIP_CONF_BUFFER_SIZE };
+
+	printf("[test] failed descriptor and single-payload DMA transfers propagate failure\n");
+	fixture_reset();
+	fail_request = 1;
+	check(!nic_rx_header(0x0100) && request_count == 1, "descriptor DMA failure reaches the caller");
+	pattern(frame, sizeof(frame));
+	for (unsigned layout = 0; layout < 3; layout++) {
+		uint16_t descriptor = layout == 0 ? 0x0100
+			: RTL837X_NIC_RX_RING_BYTES - (layout == 1 ? 16 : 8);
+		for (unsigned i = 0; i < sizeof(lengths) / sizeof(lengths[0]); i++) {
+			uint16_t len = lengths[i];
+			fixture_reset();
+			put_descriptor(descriptor, len);
+			ring_write(descriptor + 8, frame, (len + 7u) & ~7u);
+			check(nic_rx_header(descriptor), "the descriptor precedes the injected payload failure");
+			fail_request = 2;
+			check(!nic_rx_packet(destination.data, descriptor + 8), "payload DMA failure reaches the caller");
+			check(request_count == fail_request, "a failed payload DMA is not retried or split");
+			check(filled(destination.data, sizeof(destination.data), 0xa5) && guards_intact(),
+			      "an injected failure before DMA preserves the destination and guards");
+			expect_descriptor(len);
+		}
+	}
+}
+
+int main(void)
+{
+	t_last_descriptor();
+	t_payload_layouts();
+	t_maximum_frames();
+	t_invalid_frames();
+	t_dma_failure();
+	if (failures) {
+		printf("%d NIC RX check(s) failed\n", failures);
+		return 1;
+	}
+	puts("all NIC RX checks passed");
+	return 0;
+}

uip/uip-conf.h

@@ -132,6 +132,10 @@ typedef unsigned short uip_stats_t;
  */
 #define UIP_CONF_BUFFER_SIZE     1556
 
+/* Backing storage includes padding for the NIC's eight-byte DMA transfers.
+ * Keep UIP_CONF_BUFFER_SIZE as the logical frame/MSS limit. */
+#define UIP_CONF_BUFFER_ALLOC_SIZE ((UIP_CONF_BUFFER_SIZE + 7U) & ~7U)
+
 /**
  * Bytes of the buffer kept out of the advertised MSS.
  *

uip/uip.c

@@ -136,7 +136,7 @@ static __code const uip_ipaddr_t all_zeroes_addr =
 #endif /* UIP_CONF_IPV6 */
 
 #ifndef UIP_CONF_EXTERNAL_BUFFER
-u8_t uip_buf[UIP_BUFSIZE + 2];   /* The packet buffer that contains
+u8_t uip_buf[UIP_CONF_BUFFER_ALLOC_SIZE];   /* The packet buffer that contains
 				    incoming packets. */
 #endif /* UIP_CONF_EXTERNAL_BUFFER */
 

uip/uipopt.h

@@ -419,7 +419,7 @@
 #endif /* UIP_CONF_BUFFER_EXTRA */
 
 
-extern __xdata uint8_t uip_buf[UIP_CONF_BUFFER_SIZE+2];
+extern __xdata uint8_t uip_buf[UIP_CONF_BUFFER_ALLOC_SIZE];
 
 /**
  * Determines if statistics support should be compiled in.