spi_bitbang: add more lowlevel function documentation

This adds more documentation of the lowlevel API to avoid future bugs.
Signed-off-by: Michael Buesch <mb@bu3sch.de>
Acked-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/spi/spi_bitbang.h

@@ -83,6 +83,13 @@ extern int spi_bitbang_stop(struct spi_bitbang *spi);
  *  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).