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spi: documentation: emphasise spi_master.setup() semantics

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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>
This commit is contained in:
David Brownell 2009-04-21 12:24:49 -07:00 committed by Linus Torvalds
parent 14fadca793
commit 6e538aaf50
2 changed files with 12 additions and 1 deletions
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6
Documentation/spi/spi-summary
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@ -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

7
include/linux/spi/spi.h
View file

@ -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