linux/drivers/spi/spi-bitbang-txrx.h

/* SPDX-License-Identifier: GPL-2.0 */
/*
 * 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 chipselect() and txrx_bufs(),
 *	and custom setup()/cleanup() methods.
 */

/*
 * The code that knows what GPIO pins do what should have declared four
 * functions, ideally as inlines, before 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);
 *
 * setsck()'s is_on parameter is a zero/nonzero boolean.
 *
 * setmosi()'s is_on parameter is a zero/nonzero boolean.
 *
 * getmiso() is required to return 0 or 1 only. Any other value is invalid
 * and will result in improper operation.
 *
 * 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 to make it faster or slower to match a
 * particular CPU clock rate.
 *
 * ToDo: Maybe the bitrev macros can be used to improve the code?
 */

static inline u32
bitbang_txrx_be_cpha0(struct spi_device *spi,
		unsigned nsecs, unsigned cpol, unsigned flags,
		u32 word, u8 bits)
{ … }

static inline u32
bitbang_txrx_be_cpha1(struct spi_device *spi,
		unsigned nsecs, unsigned cpol, unsigned flags,
		u32 word, u8 bits)
{ … }

static inline u32
bitbang_txrx_le_cpha0(struct spi_device *spi,
		unsigned int nsecs, unsigned int cpol, unsigned int flags,
		u32 word, u8 bits)
{ … }

static inline u32
bitbang_txrx_le_cpha1(struct spi_device *spi,
		unsigned int nsecs, unsigned int cpol, unsigned int flags,
		u32 word, u8 bits)
{ … }