drivers/spi/spi_bitbang_txrx.h

   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
  45 static inline u32
  46 bitbang_txrx_be_cpha0(struct spi_device *spi,
  47                 unsigned nsecs, unsigned cpol,
  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                 setmosi(spi, word & (1 << 31));
  57                 spidelay(nsecs);        /* T(setup) */
  58
  59                 setsck(spi, !cpol);
  60                 spidelay(nsecs);
  61
  62                 /* sample MSB (from slave) on leading edge */
  63                 word <<= 1;
  64                 word |= getmiso(spi);
  65                 setsck(spi, cpol);
  66         }
  67         return word;
  68 }
  69
  70 static inline u32
  71 bitbang_txrx_be_cpha1(struct spi_device *spi,
  72                 unsigned nsecs, unsigned cpol,
  73                 u32 word, u8 bits)
  74 {
  75         /* if (cpol == 0) this is SPI_MODE_1; else this is SPI_MODE_3 */
  76
  77         /* clock starts at inactive polarity */
  78         for (word <<= (32 - bits); likely(bits); bits--) {
  79
  80                 /* setup MSB (to slave) on leading edge */
  81                 setsck(spi, !cpol);
  82                 setmosi(spi, word & (1 << 31));
  83                 spidelay(nsecs); /* T(setup) */
  84
  85                 setsck(spi, cpol);
  86                 spidelay(nsecs);
  87
  88                 /* sample MSB (from slave) on trailing edge */
  89                 word <<= 1;
  90                 word |= getmiso(spi);
  91         }
  92         return word;
  93 }