Generalised the SPI abstraction layer to also be able to handle interfaces

that don't provide the full flexibility of a bit-bashing interface.

Temporarily hacked rtl8139.c to use the new interface.

src/drivers/nvs/threewire.c

@@ -17,8 +17,7 @@
  */
 
 #include <stddef.h>
-#include <byteswap.h>
-#include <gpxe/spi.h>
+#include <assert.h>
 #include <gpxe/threewire.h>
 
 /** @file
@@ -27,31 +26,23 @@
  *
  */
 
-/**
- * Read from a three-wire device
+/** Read data from three-wire device
  *
- * @v three	Three-wire device
- * @v address	Address
- * @ret data	Data
+ * @v device		SPI device
+ * @v address		Address from which to read
+ * @v data		Data buffer
+ * @v len		Length of data to read, in @b words
+ * @ret rc		Return status code
  */
-unsigned long threewire_read ( struct threewire_device *three,
-			       unsigned long address ) {
-	struct spi_interface *spi = three->spi;
-	uint32_t data;
+int threewire_read ( struct spi_device *device, unsigned int address,
+		     void *data, unsigned int len ) {
+	struct spi_bus *bus = device->bus;
 
-	/* Activate chip select line */
-	spi->select_slave ( spi, three->slave );
+	assert ( bus->mode == SPI_MODE_THREEWIRE );
 
-	/* Send command and address */
-	data = cpu_to_le32 ( threewire_cmd_read ( three, address ) );
-	spi->transfer ( spi, &data, NULL, threewire_cmd_len ( three ) );
-	
-	/* Read back data */
-	data = 0;
-	spi->transfer ( spi, NULL, &data, three->datasize );
+	DBG ( "3wire %p reading words [%04x,%04x)\n", device,
+	      address, ( address + len ) );
 
-	/* Deactivate chip select line */
-	spi->deselect_slave ( spi );
-
-	return le32_to_cpu ( data );;
+	return bus->rw ( bus, device, THREEWIRE_READ, address,
+			 NULL, data, len );
 }