linux/drivers/spi/spi-bitbang-txrx.h
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   1/*
   2 * Mix this utility code with some glue code to get one of several types of
   3 * simple SPI master driver.  Two do polled word-at-a-time I/O:
   4 *
   5 *   -  GPIO/parport bitbangers.  Provide chipselect() and txrx_word[](),
   6 *      expanding the per-word routines from the inline templates below.
   7 *
   8 *   -  Drivers for controllers resembling bare shift registers.  Provide
   9 *      chipselect() and txrx_word[](), with custom setup()/cleanup() methods
  10 *      that use your controller's clock and chipselect registers.
  11 *
  12 * Some hardware works well with requests at spi_transfer scope:
  13 *
  14 *   -  Drivers leveraging smarter hardware, with fifos or DMA; or for half
  15 *      duplex (MicroWire) controllers.  Provide chipselect() and txrx_bufs(),
  16 *      and custom setup()/cleanup() methods.
  17 */
  18
  19/*
  20 * The code that knows what GPIO pins do what should have declared four
  21 * functions, ideally as inlines, before including this header:
  22 *
  23 *  void setsck(struct spi_device *, int is_on);
  24 *  void setmosi(struct spi_device *, int is_on);
  25 *  int getmiso(struct spi_device *);
  26 *  void spidelay(unsigned);
  27 *
  28 * setsck()'s is_on parameter is a zero/nonzero boolean.
  29 *
  30 * setmosi()'s is_on parameter is a zero/nonzero boolean.
  31 *
  32 * getmiso() is required to return 0 or 1 only. Any other value is invalid
  33 * and will result in improper operation.
  34 *
  35 * A non-inlined routine would call bitbang_txrx_*() routines.  The
  36 * main loop could easily compile down to a handful of instructions,
  37 * especially if the delay is a NOP (to run at peak speed).
  38 *
  39 * Since this is software, the timings may not be exactly what your board's
  40 * chips need ... there may be several reasons you'd need to tweak timings
  41 * in these routines, not just make to make it faster or slower to match a
  42 * particular CPU clock rate.
  43 */
  44
  45static inline u32
  46bitbang_txrx_be_cpha0(struct spi_device *spi,
  47                unsigned nsecs, unsigned cpol, unsigned flags,
  48                u32 word, u8 bits)
  49{
  50        /* if (cpol == 0) this is SPI_MODE_0; else this is SPI_MODE_2 */
  51
  52        /* clock starts at inactive polarity */
  53        for (word <<= (32 - bits); likely(bits); bits--) {
  54
  55                /* setup MSB (to slave) on trailing edge */
  56                if ((flags & SPI_MASTER_NO_TX) == 0)
  57                        setmosi(spi, word & (1 << 31));
  58                spidelay(nsecs);        /* T(setup) */
  59
  60                setsck(spi, !cpol);
  61                spidelay(nsecs);
  62
  63                /* sample MSB (from slave) on leading edge */
  64                word <<= 1;
  65                if ((flags & SPI_MASTER_NO_RX) == 0)
  66                        word |= getmiso(spi);
  67                setsck(spi, cpol);
  68        }
  69        return word;
  70}
  71
  72static inline u32
  73bitbang_txrx_be_cpha1(struct spi_device *spi,
  74                unsigned nsecs, unsigned cpol, unsigned flags,
  75                u32 word, u8 bits)
  76{
  77        /* if (cpol == 0) this is SPI_MODE_1; else this is SPI_MODE_3 */
  78
  79        /* clock starts at inactive polarity */
  80        for (word <<= (32 - bits); likely(bits); bits--) {
  81
  82                /* setup MSB (to slave) on leading edge */
  83                setsck(spi, !cpol);
  84                if ((flags & SPI_MASTER_NO_TX) == 0)
  85                        setmosi(spi, word & (1 << 31));
  86                spidelay(nsecs); /* T(setup) */
  87
  88                setsck(spi, cpol);
  89                spidelay(nsecs);
  90
  91                /* sample MSB (from slave) on trailing edge */
  92                word <<= 1;
  93                if ((flags & SPI_MASTER_NO_RX) == 0)
  94                        word |= getmiso(spi);
  95        }
  96        return word;
  97}
  98
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