w5100: use SPI transfer buf

Performance increase as suggested by @jayzakk here:
arduino-libraries#145

Author: sophec

src/utility/w5100.cpp

@@ -412,9 +412,14 @@ uint16_t W5100Class::read(uint16_t addr, uint8_t *buf, uint16_t len)
 		cmd[1] = addr & 0xFF;
 		cmd[2] = (len >> 8) & 0x7F;
 		cmd[3] = len & 0xFF;
+#ifdef SPI_HAS_TRANSFER_BUF
+		_spibus.transfer(cmd, nullptr, 4);
+		_spibus.transfer(nullptr, buf, len);
+#else
 		_spibus.transfer(cmd, 4);
 		memset(buf, 0, len);
 		_spibus.transfer(buf, len);
+#endif
 		resetSS();
 	} else { // chip == 55
 		setSS();
@@ -456,9 +461,14 @@ uint16_t W5100Class::read(uint16_t addr, uint8_t *buf, uint16_t len)
 			cmd[2] = ((addr >> 6) & 0xE0) | 0x18; // 2K buffers
 			#endif
 		}
+#ifdef SPI_HAS_TRANSFER_BUF
+		_spibus.transfer(cmd, nullptr, 3);
+		_spibus.transfer(nullptr, buf, len);
+#else
 		_spibus.transfer(cmd, 3);
 		memset(buf, 0, len);
 		_spibus.transfer(buf, len);
+#endif
 		resetSS();
 	}
 	return len;

src/utility/w5100.h

@@ -18,14 +18,14 @@
 #include <SPI.h>
 
 // Safe for all chips
-#define SPI_ETHERNET_SETTINGS SPISettings(14000000, MSBFIRST, SPI_MODE0)
+//#define SPI_ETHERNET_SETTINGS SPISettings(14000000, MSBFIRST, SPI_MODE0)
 
 // Safe for W5200 and W5500, but too fast for W5100
 // Uncomment this if you know you'll never need W5100 support.
 //  Higher SPI clock only results in faster transfer to hosts on a LAN
 //  or with very low packet latency.  With ordinary internet latency,
 //  the TCP window size & packet loss determine your overall speed.
-//#define SPI_ETHERNET_SETTINGS SPISettings(30000000, MSBFIRST, SPI_MODE0)
+#define SPI_ETHERNET_SETTINGS SPISettings(30000000, MSBFIRST, SPI_MODE0)
 
 
 // Require Ethernet.h, because we need MAX_SOCK_NUM