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/*
* 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 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;
}
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