spi doc updates

Update two points in the SPI interface documentation:

- Update description of the "chip stays selected after message ends"
  mode.  In some cases it's required for correctness; it isn't just a
  performance tweak.  (Yes: to use this mode on mult-device busses, another
  programming interface will be needed.  One draft has been circulated
  already.)

- Clarify spi_setup(), highlighting that callers must ensure that no
  requests are queued (can't change configuration except between I/Os), and
  that the device must be deselected when this returns (which is a key part
  of why it's called during device init).

Signed-off-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.h

@@ -341,9 +341,14 @@ extern struct spi_master *spi_busnum_to_master(u16 busnum);
  * chip transactions together.
  *
  * (ii) When the transfer is the last one in the message, the chip may
- * stay selected until the next transfer.  This is purely a performance
- * hint; the controller driver may need to select a different device
- * for the next message.
+ * stay selected until the next transfer.  On multi-device SPI busses
+ * with nothing blocking messages going to other devices, this is just
+ * a performance hint; starting a message to another device deselects
+ * this one.  But in other cases, this can be used to ensure correctness.
+ * Some devices need protocol transactions to be built from a series of
+ * spi_message submissions, where the content of one message is determined
+ * by the results of previous messages and where the whole transaction
+ * ends when the chipselect goes intactive.
  *
  * The code that submits an spi_message (and its spi_transfers)
  * to the lower layers is responsible for managing its memory.
@@ -480,14 +485,15 @@ static inline void spi_message_free(struct spi_message *m)
 /**
  * spi_setup - setup SPI mode and clock rate
  * @spi: the device whose settings are being modified
- * Context: can sleep
+ * Context: can sleep, and no requests are queued to the device
  *
  * SPI protocol drivers may need to update the transfer mode if the
- * device doesn't work with the mode 0 default.  They may likewise need
+ * device doesn't work with its default.  They may likewise need
  * to update clock rates or word sizes from initial values.  This function
  * changes those settings, and must be called from a context that can sleep.
- * The changes take effect the next time the device is selected and data
- * is transferred to or from it.
+ * Except for SPI_CS_HIGH, which takes effect immediately, the changes take
+ * effect the next time the device is selected and data is transferred to
+ * or from it.  When this function returns, the spi device is deselected.
  *
  * Note that this call will fail if the protocol driver specifies an option
  * that the underlying controller or its driver does not support.  For