[PATCH] SPI: devices can require LSB-first encodings

Add spi_device hook for LSB-first word encoding, and update all the
(in-tree) controller drivers to reject such devices.  Eventually,
some controller drivers will be updated to support lsb-first encodings
on the wire; no current drivers need this.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

2 files changed, 15 insertions, 3 deletions

diff --git a/drivers/spi/spi_bitbang.c b/drivers/spi/spi_bitbang.c
index 6c3da64d609a..0f7f5c64391c 100644
--- a/drivers/spi/spi_bitbang.c
+++ b/drivers/spi/spi_bitbang.c
@@ -187,13 +187,22 @@ int spi_bitbang_setup(struct spi_device *spi)
         if (!spi->max_speed_hz)
                 return -EINVAL;
 
+        bitbang = spi_master_get_devdata(spi->master);
+
+        /* REVISIT: some systems will want to support devices using lsb-first
+         * bit encodings on the wire.  In pure software that would be trivial,
+         * just bitbang_txrx_le_cphaX() routines shifting the other way, and
+         * some hardware controllers also have this support.
+         */
+        if ((spi->mode & SPI_LSB_FIRST) != 0)
+                return -EINVAL;
+
         if (!cs) {
                 cs = kzalloc(sizeof *cs, SLAB_KERNEL);
                 if (!cs)
                         return -ENOMEM;
                 spi->controller_state = cs;
         }
-        bitbang = spi_master_get_devdata(spi->master);
 
         if (!spi->bits_per_word)
                 spi->bits_per_word = 8;
diff --git a/include/linux/spi/spi.h b/include/linux/spi/spi.h
index 082006714b85..77add901691d 100644
--- a/include/linux/spi/spi.h
+++ b/include/linux/spi/spi.h
@@ -35,10 +35,13 @@ extern struct bus_type spi_bus_type;
  * @chip-select: Chipselect, distinguishing chips handled by "master".
  * @mode: The spi mode defines how data is clocked out and in.
  *      This may be changed by the device's driver.
+ *      The "active low" default for chipselect mode can be overridden,
+ *      as can the "MSB first" default for each word in a transfer.
  * @bits_per_word: Data transfers involve one or more words; word sizes
  *      like eight or 12 bits are common.  In-memory wordsizes are
  *      powers of two bytes (e.g. 20 bit samples use 32 bits).
- *      This may be changed by the device's driver.
+ *      This may be changed by the device's driver, or left at the
+ *      default (0) indicating protocol words are eight bit bytes.
  *      The spi_transfer.bits_per_word can override this for each transfer.
  * @irq: Negative, or the number passed to request_irq() to receive
  *      interrupts from this device.
@@ -67,6 +70,7 @@ struct spi_device {
 #define SPI_MODE_2      (SPI_CPOL|0)
 #define SPI_MODE_3      (SPI_CPOL|SPI_CPHA)
 #define SPI_CS_HIGH     0x04                    /* chipselect active high? */
+#define SPI_LSB_FIRST   0x08                    /* per-word bits-on-wire */
         u8                      bits_per_word;
         int                     irq;
         void                    *controller_state;
@@ -75,7 +79,6 @@ struct spi_device {
 
         // likely need more hooks for more protocol options affecting how
         // the controller talks to each chip, like:
-        //  - bit order (default is wordwise msb-first)
         //  - memory packing (12 bit samples into low bits, others zeroed)
         //  - priority
         //  - drop chipselect after each word