spi: documentation: emphasise spi_master.setup() semantics

This is a doc-only patch which I hope will reduce the number of
spi_master controller driver patches starting out with a common
implementation bug.

(As in: almost every spi_master driver I see starts out with its
version of this bug.  Sigh.)

It just re-emphasizes that the setup() method may be called for one
device while a transfer is active on another ...  which means that most
driver implementations shouldn't touch any registers.

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>

2 files changed, 12 insertions, 1 deletions

diff --git a/Documentation/spi/spi-summary b/Documentation/spi/spi-summary
index 0f5122eb282b..4a02d2508bc8 100644
--- a/Documentation/spi/spi-summary
+++ b/Documentation/spi/spi-summary
@@ -511,10 +511,16 @@ SPI MASTER METHODS
         This sets up the device clock rate, SPI mode, and word sizes.
         Drivers may change the defaults provided by board_info, and then
         call spi_setup(spi) to invoke this routine.  It may sleep.
+
         Unless each SPI slave has its own configuration registers, don't
         change them right away ... otherwise drivers could corrupt I/O
         that's in progress for other SPI devices.
 
+                ** BUG ALERT:  for some reason the first version of
+                ** many spi_master drivers seems to get this wrong.
+                ** When you code setup(), ASSUME that the controller
+                ** is actively processing transfers for another device.
+
     master->transfer(struct spi_device *spi, struct spi_message *message)
         This must not sleep.  Its responsibility is arrange that the
         transfer happens and its complete() callback is issued.  The two
diff --git a/include/linux/spi/spi.h b/include/linux/spi/spi.h
index 2cc43fa380cb..a0faa18f7b1b 100644
--- a/include/linux/spi/spi.h
+++ b/include/linux/spi/spi.h
@@ -245,7 +245,12 @@ struct spi_master {
          */
         u16                     dma_alignment;
 
-        /* setup mode and clock, etc (spi driver may call many times) */
+        /* Setup mode and clock, etc (spi driver may call many times).
+         *
+         * IMPORTANT:  this may be called when transfers to another
+         * device are active.  DO NOT UPDATE SHARED REGISTERS in ways
+         * which could break those transfers.
+         */
         int                     (*setup)(struct spi_device *spi);
 
         /* bidirectional bulk transfers