include/linux/spi/spi_bitbang.h

#ifndef	__SPI_BITBANG_H
#define	__SPI_BITBANG_H

/*
 * Mix this utility code with some glue code to get one of several types of
 * simple SPI master driver.  Two do polled word-at-a-time I/O:
 *
 *   -	GPIO/parport bitbangers.  Provide chipselect() and txrx_word[](),
 *	expanding the per-word routines from the inline templates below.
 *
 *   -	Drivers for controllers resembling bare shift registers.  Provide
 *	chipselect() and txrx_word[](), with custom setup()/cleanup() methods
 *	that use your controller's clock and chipselect registers.
 *
 * Some hardware works well with requests at spi_transfer scope:
 *
 *   -	Drivers leveraging smarter hardware, with fifos or DMA; or for half
 *	duplex (MicroWire) controllers.  Provide chipslect() and txrx_bufs(),
 *	and custom setup()/cleanup() methods.
 */
struct spi_bitbang {
	struct workqueue_struct	*workqueue;
	struct work_struct	work;

	spinlock_t		lock;
	struct list_head	queue;
	u8			busy;
	u8			shutdown;
	u8			use_dma;

	struct spi_master	*master;

	/* setup_transfer() changes clock and/or wordsize to match settings
	 * for this transfer; zeroes restore defaults from spi_device.
	 */
	int	(*setup_transfer)(struct spi_device *spi,
			struct spi_transfer *t);

	void	(*chipselect)(struct spi_device *spi, int is_on);
#define	BITBANG_CS_ACTIVE	1	/* normally nCS, active low */
#define	BITBANG_CS_INACTIVE	0

	/* txrx_bufs() may handle dma mapping for transfers that don't
	 * already have one (transfer.{tx,rx}_dma is zero), or use PIO
	 */
	int	(*txrx_bufs)(struct spi_device *spi, struct spi_transfer *t);

	/* txrx_word[SPI_MODE_*]() just looks like a shift register */
	u32	(*txrx_word[4])(struct spi_device *spi,
			unsigned nsecs,
			u32 word, u8 bits);
};

/* you can call these default bitbang->master methods from your custom
 * methods, if you like.
 */
extern int spi_bitbang_setup(struct spi_device *spi);
extern void spi_bitbang_cleanup(const struct spi_device *spi);
extern int spi_bitbang_transfer(struct spi_device *spi, struct spi_message *m);

/* start or stop queue processing */
extern int spi_bitbang_start(struct spi_bitbang *spi);
extern int spi_bitbang_stop(struct spi_bitbang *spi);

#endif	/* __SPI_BITBANG_H */

/*-------------------------------------------------------------------------*/

#ifdef	EXPAND_BITBANG_TXRX

/*
 * The code that knows what GPIO pins do what should have declared four
 * functions, ideally as inlines, before #defining EXPAND_BITBANG_TXRX
 * and including this header:
 *
 *  void setsck(struct spi_device *, int is_on);
 *  void setmosi(struct spi_device *, int is_on);
 *  int getmiso(struct spi_device *);
 *  void spidelay(unsigned);
 *
 * A non-inlined routine would call bitbang_txrx_*() routines.  The
 * main loop could easily compile down to a handful of instructions,
 * especially if the delay is a NOP (to run at peak speed).
 *
 * Since this is software, the timings may not be exactly what your board's
 * chips need ... there may be several reasons you'd need to tweak timings
 * in these routines, not just make to make it faster or slower to match a
 * particular CPU clock rate.
 */

static inline u32
bitbang_txrx_be_cpha0(struct spi_device *spi,
		unsigned nsecs, unsigned cpol,
		u32 word, u8 bits)
{
	/* if (cpol == 0) this is SPI_MODE_0; else this is SPI_MODE_2 */

	/* clock starts at inactive polarity */
	for (word <<= (32 - bits); likely(bits); bits--) {

		/* setup MSB (to slave) on trailing edge */
		setmosi(spi, word & (1 << 31));
		spidelay(nsecs);	/* T(setup) */

		setsck(spi, !cpol);
		spidelay(nsecs);

		/* sample MSB (from slave) on leading edge */
		word <<= 1;
		word |= getmiso(spi);
		setsck(spi, cpol);
	}
	return word;
}

static inline u32
bitbang_txrx_be_cpha1(struct spi_device *spi,
		unsigned nsecs, unsigned cpol,
		u32 word, u8 bits)
{
	/* if (cpol == 0) this is SPI_MODE_1; else this is SPI_MODE_3 */

	/* clock starts at inactive polarity */
	for (word <<= (32 - bits); likely(bits); bits--) {

		/* setup MSB (to slave) on leading edge */
		setsck(spi, !cpol);
		setmosi(spi, word & (1 << 31));
		spidelay(nsecs); /* T(setup) */

		setsck(spi, cpol);
		spidelay(nsecs);

		/* sample MSB (from slave) on trailing edge */
		word <<= 1;
		word |= getmiso(spi);
	}
	return word;
}

#endif	/* EXPAND_BITBANG_TXRX */