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-rw-r--r--include/linux/spi/spi.h18
1 files changed, 17 insertions, 1 deletions
diff --git a/include/linux/spi/spi.h b/include/linux/spi/spi.h
index 9d8d63109a8..4f0f8c2e58a 100644
--- a/include/linux/spi/spi.h
+++ b/include/linux/spi/spi.h
@@ -163,7 +163,8 @@ static inline void spi_unregister_driver(struct spi_driver *sdrv)
* each slave has a chipselect signal, but it's common that not
* every chipselect is connected to a slave.
* @setup: updates the device mode and clocking records used by a
- * device's SPI controller; protocol code may call this.
+ * device's SPI controller; protocol code may call this. This
+ * must fail if an unrecognized or unsupported mode is requested.
* @transfer: adds a message to the controller's transfer queue.
* @cleanup: frees controller-specific state
*
@@ -305,6 +306,16 @@ extern struct spi_master *spi_busnum_to_master(u16 busnum);
* shifting out three bytes with word size of sixteen or twenty bits;
* the former uses two bytes per word, the latter uses four bytes.)
*
+ * In-memory data values are always in native CPU byte order, translated
+ * from the wire byte order (big-endian except with SPI_LSB_FIRST). So
+ * for example when bits_per_word is sixteen, buffers are 2N bytes long
+ * and hold N sixteen bit words in CPU byte order.
+ *
+ * When the word size of the SPI transfer is not a power-of-two multiple
+ * of eight bits, those in-memory words include extra bits. In-memory
+ * words are always seen by protocol drivers as right-justified, so the
+ * undefined (rx) or unused (tx) bits are always the most significant bits.
+ *
* All SPI transfers start with the relevant chipselect active. Normally
* it stays selected until after the last transfer in a message. Drivers
* can affect the chipselect signal using cs_change:
@@ -462,6 +473,11 @@ static inline void spi_message_free(struct spi_message *m)
* 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.
+ *
+ * Note that this call wil fail if the protocol driver specifies an option
+ * that the underlying controller or its driver does not support. For
+ * example, not all hardware supports wire transfers using nine bit words,
+ * LSB-first wire encoding, or active-high chipselects.
*/
static inline int
spi_setup(struct spi_device *spi)