spi: Fix Rx/Tx buffer logic & simplify

* tx/rx: Rename wrap_addr to start_addr for clarity
* Fix RX logic, when advancing the tail pointer, not retrieving the stored RX buffer start address but the default value instead
* Fix TX logic, when advancing the head pointer for next transmission, not retrieving the stored TX buffer end address but the default value instead
* Rename SPI-related const's to match the datasheet: RERXDATA→RRXDATA, WEGPDATA→WGPDATA
* Remove useless const's for SRAM default addresses for TX buffer
* Simplify code

src/lib.rs

@@ -96,7 +96,7 @@ impl <SPI: Transfer<u8>,
 
     pub fn init_rxbuf(&mut self) -> Result<(), Error> {
         // Set ERXST pointer
-        self.spi_port.write_reg_16b(spi::addrs::ERXST, self.rx_buf.get_wrap_addr())?;
+        self.spi_port.write_reg_16b(spi::addrs::ERXST, self.rx_buf.get_start_addr())?;
         // Set ERXTAIL pointer
         self.spi_port.write_reg_16b(spi::addrs::ERXTAIL, self.rx_buf.get_tail_addr())?;
         // Set MAMXFL to maximum number of bytes in each accepted packet
@@ -180,8 +180,11 @@ impl <SPI: Transfer<u8>,
         self.spi_port.read_rxdat(&mut frame_buf, rx_packet.get_frame_length())?;
         rx_packet.copy_frame_from(&frame_buf[1..]);
         // Set ERXTAIL pointer to (next_addr - 2)
-        if self.rx_buf.get_next_addr() > rx::ERXST_DEFAULT {
+        // * Assume head, tail, next and wrap addresses are word-aligned (even)
+        // - If next_addr is at least (start_addr+2), then set tail pointer to the word right before next_addr
+        if self.rx_buf.get_next_addr() > self.rx_buf.get_start_addr() {
             self.spi_port.write_reg_16b(spi::addrs::ERXTAIL, self.rx_buf.get_next_addr() - 2)?;
+        // - Otherwise, next_addr will wrap, so set tail pointer to the last word address of RX buffer
         } else {
             self.spi_port.write_reg_16b(spi::addrs::ERXTAIL, rx::RX_MAX_ADDRESS - 1)?;
         }
@@ -216,8 +219,9 @@ impl <SPI: Transfer<u8>,
         // TODO: Read ETXSTAT to understand Ethernet transmission status
         // (See: Register 9-2, ENC424J600 Data Sheet)
         // Update TX buffer start address
+        // * Assume TX buffer consumes the entire general-purpose SRAM block
         self.tx_buf.set_next_addr((self.tx_buf.get_next_addr() + packet.get_frame_length() as u16) %
-            tx::GPBUFEN_DEFAULT);
+            self.rx_buf.get_start_addr() - self.tx_buf.get_start_addr());
         Ok(())
     }
 }

src/rx.rs

@@ -11,7 +11,7 @@ pub const RSV_LENGTH: usize = 6;
 /// Struct for RX Buffer on the hardware
 /// TODO: Should be a singleton
 pub struct RxBuffer {
-    wrap_addr: u16,
+    start_addr: u16,
     next_addr: u16,
     tail_addr: u16
 }
@@ -19,17 +19,17 @@ pub struct RxBuffer {
 impl RxBuffer {
     pub fn new() -> Self {
         RxBuffer {
-            wrap_addr: ERXST_DEFAULT,
+            start_addr: ERXST_DEFAULT,
             next_addr: ERXST_DEFAULT,
             tail_addr: ERXTAIL_DEFAULT
         }
     }
 
-    pub fn set_wrap_addr(&mut self, addr: u16) {
-        self.wrap_addr = addr;
+    pub fn set_start_addr(&mut self, addr: u16) {
+        self.start_addr = addr;
     }
-    pub fn get_wrap_addr(& self) -> u16{
-        self.wrap_addr
+    pub fn get_start_addr(& self) -> u16{
+        self.start_addr
     }
 
     pub fn set_next_addr(&mut self, addr: u16) {

src/spi.rs

@@ -18,8 +18,8 @@ pub mod opcodes {
     /// SPI Opcodes
     pub const RCRU: u8 = 0b0010_0000;
     pub const WCRU: u8 = 0b0010_0010;
-    pub const RERXDATA: u8 = 0b0010_1100;   // 8-bit opcode followed by data
-    pub const WEGPDATA: u8 = 0b0010_1010;   // 8-bit opcode followed by data
+    pub const RRXDATA: u8 = 0b0010_1100;    // 8-bit opcode followed by data
+    pub const WGPDATA: u8 = 0b0010_1010;    // 8-bit opcode followed by data
 }
 
 pub mod addrs {
@@ -94,16 +94,14 @@ impl <SPI: Transfer<u8>,
     // Currently requires manual slicing (buf[1..]) for the data read back
     pub fn read_rxdat<'a>(&mut self, buf: &'a mut [u8], data_length: usize)
                          -> Result<(), Error> {
-        let r_valid = self.r_n(buf, opcodes::RERXDATA, data_length)?;
-        Ok(r_valid)
+        self.r_n(buf, opcodes::RRXDATA, data_length)
     }
 
-    // Currenly requires actual data to be stored in buf[1..] instead of buf[0..]
+    // Currently requires actual data to be stored in buf[1..] instead of buf[0..]
     // TODO: Maybe better naming?
     pub fn write_txdat<'a>(&mut self, buf: &'a mut [u8], data_length: usize)
                           -> Result<(), Error> {
-        let w_valid = self.w_n(buf, opcodes::WEGPDATA, data_length)?;
-        Ok(w_valid)
+        self.w_n(buf, opcodes::WGPDATA, data_length)
     }
 
     pub fn write_reg_8b(&mut self, addr: u8, data: u8) -> Result<(), Error> {

src/tx.rs

@@ -1,13 +1,9 @@
 use crate::RAW_FRAME_LENGTH_MAX;
 
-/// SRAM Addresses
-pub const GPBUFST_DEFAULT: u16 = 0x0000;    // Start of General-Purpose SRAM Buffer
-pub const GPBUFEN_DEFAULT: u16 = 0x5340;    // End of General-Purpose SRAM Buffer == ERXST default
-
 /// Struct for TX Buffer on the hardware
 /// TODO: Should be a singleton
 pub struct TxBuffer {
-    wrap_addr: u16,
+    start_addr: u16,
     // The following two fields are controlled by firmware
     next_addr: u16,
     tail_addr: u16
@@ -16,17 +12,17 @@ pub struct TxBuffer {
 impl TxBuffer {
     pub fn new() -> Self {
         TxBuffer {
-            wrap_addr: GPBUFST_DEFAULT,
-            next_addr: GPBUFST_DEFAULT + 1,
-            tail_addr: GPBUFST_DEFAULT
+            start_addr: 0x0000,
+            next_addr: 0x0001,
+            tail_addr: 0x0000
         }
     }
 
-    pub fn set_wrap_addr(&mut self, addr: u16) {
-        self.wrap_addr = addr;
+    pub fn set_start_addr(&mut self, addr: u16) {
+        self.start_addr = addr;
     }
-    pub fn get_wrap_addr(& self) -> u16{
-        self.wrap_addr
+    pub fn get_start_addr(& self) -> u16{
+        self.start_addr
     }
 
     pub fn set_next_addr(&mut self, addr: u16) {