spi: Add function to allocate a new SPI slave

At present it is difficult to extend the SPI structure since all
drivers allocate it themselves, and few of them zero all fields. Add
a new function spi_alloc_slave() which can be used by SPI drivers
to perform this allocation, and thus ensure that all drivers can
better cope with SPI structure changes.

Signed-off-by: Simon Glass <sjg@chromium.org>

include/spi.h

@@ -62,6 +62,47 @@ struct spi_slave {
  */
 void spi_init(void);
 
+/**
+ * spi_do_alloc_slave - Allocate a new SPI slave (internal)
+ *
+ * Allocate and zero all fields in the spi slave, and set the bus/chip
+ * select. Use the helper macro spi_alloc_slave() to call this.
+ *
+ * @offset: Offset of struct spi_slave within slave structure
+ * @size: Size of slave structure
+ * @bus: Bus ID of the slave chip.
+ * @cs: Chip select ID of the slave chip on the specified bus.
+ */
+void *spi_do_alloc_slave(int offset, int size, unsigned int bus,
+			 unsigned int cs);
+
+/**
+ * spi_alloc_slave - Allocate a new SPI slave
+ *
+ * Allocate and zero all fields in the spi slave, and set the bus/chip
+ * select.
+ *
+ * @_struct: Name of structure to allocate (e.g. struct tegra_spi). This
+ *	structure must contain a member 'struct spi_slave *slave'.
+ * @bus: Bus ID of the slave chip.
+ * @cs: Chip select ID of the slave chip on the specified bus.
+ */
+#define spi_alloc_slave(_struct, bus, cs) \
+	spi_do_alloc_slave(offsetof(_struct, slave), \
+			    sizeof(_struct), bus, cs)
+
+/**
+ * spi_alloc_slave_base - Allocate a new SPI slave with no private data
+ *
+ * Allocate and zero all fields in the spi slave, and set the bus/chip
+ * select.
+ *
+ * @bus: Bus ID of the slave chip.
+ * @cs: Chip select ID of the slave chip on the specified bus.
+ */
+#define spi_alloc_slave_base(bus, cs) \
+	spi_do_alloc_slave(0, sizeof(struct spi_slave), bus, cs)
+
 /*-----------------------------------------------------------------------
  * Set up communications parameters for a SPI slave.
  *