linux/Documentation/blackfin/bfin-spi-notes.txt
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   1SPI Chip Select behavior:
   2
   3With the Blackfin on-chip SPI peripheral, there is some logic tied to the CPHA
   4bit whether the Slave Select Line is controlled by hardware (CPHA=0) or
   5controlled by software (CPHA=1). However, the Linux SPI bus driver assumes that
   6the Slave Select is always under software control and being asserted during
   7the entire SPI transfer. - And not just bits_per_word duration.
   8
   9In most cases you can utilize SPI MODE_3 instead of MODE_0 to work-around this
  10behavior. If your SPI slave device in question requires SPI MODE_0 or MODE_2
  11timing, you can utilize the GPIO controlled SPI Slave Select option instead.
  12In this case, you should use GPIO based CS for all of your slaves and not just
  13the ones using mode 0 or 2 in order to guarantee correct CS toggling behavior.
  14
  15You can even use the same pin whose peripheral role is a SSEL,
  16but use it as a GPIO instead.
  17
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