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authorDavid Brownell <david-b@pacbell.net>2007-06-16 10:16:08 -0700
committerLinus Torvalds <torvalds@woody.linux-foundation.org>2007-06-16 13:16:16 -0700
commitf5a9c77df45b113d21b64cbc2bf6c72a0da48998 (patch)
treec68315e2d5e3e3b8075bfbe01fab054efc1d9bf2
parented45666271f4fafa95b9d8ad44050e9a9bd2376e (diff)
downloadlinux-f5a9c77df45b113d21b64cbc2bf6c72a0da48998.tar.gz
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>
-rw-r--r--include/linux/spi/spi.h20
1 files changed, 13 insertions, 7 deletions
diff --git a/include/linux/spi/spi.h b/include/linux/spi/spi.h
index b6bedc3ee95..1be5ea05947 100644
--- a/include/linux/spi/spi.h
+++ b/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