I assume your OMAP4 linux uses one of arch/arm/boot/dts/{omap4.dtsi,am33xx.dtsi} device-tree, thus it compiles drivers/spi/spi-omap2-mcspi.c (if you don't know about device-tree, read this). Then:

• the SPI master driver is done,
• it (most probably) registers with Linux SPI core framework drivers/spi/spi.c,
• it (probably) works fine on your OMAP4.

You actually don't need to care about the master driver to write your slave device driver. How do I know spi-omap2-mcspi.c is a master driver? It calls spi_register_master().

SPI master, SPI slave ?

Please refer to Documentation/spi/spi_summary. The doc refers to Controller driver (master) and Protocol drivers (slave). From your description, I understand you want to write a Protocol/Device driver.

SPI protocol ?

To understand that, you need your slave device datasheet, it shall tell you:

• the SPI mode understood by your device,
• the protocol it expects on the bus.

Contrary to i2c, SPI does not define a protocol or handshake, SPI chips manufacturers have to define their own. So check the datasheet.

SPI mode

From include/linux/spi/spi.h:

 * @mode: The spi mode defines how data is clocked out and in.
 *  This may be changed by the device's driver.
 *  The "active low" default for chipselect mode can be overridden
 *  (by specifying SPI_CS_HIGH) as can the "MSB first" default for
 *  each word in a transfer (by specifying SPI_LSB_FIRST).

Again, check your SPI device datasheet.

An example SPI device driver?

To give you a relevant example, I need to know your SPI device type. You would understand that a SPI flash device driver is different from a SPI FPGA device driver. Unfortunately there are not so many SPI device drivers out there. To find them:

$ cd linux 
$ git grep "spi_new_device\|spi_add_device"