starting low (CPOL=0) and data stabilized for sampling during the
trailing clock edge (CPHA=1), that's SPI mode 1.
+Note that the clock mode is relevant as soon as the chipselect goes
+active. So the master must set the clock to inactive before selecting
+a slave, and the slave can tell the chosen polarity by sampling the
+clock level when its select line goes active. That's why many devices
+support for example both modes 0 and 3: they don't care about polarity,
+and alway clock data in/out on rising clock edges.
+
How do these driver programming interfaces work?
------------------------------------------------
So for example arch/.../mach-*/board-*.c files might have code like:
- #include <asm/arch/spi.h> /* for mysoc_spi_data */
+ #include <mach/spi.h> /* for mysoc_spi_data */
/* if your mach-* infrastructure doesn't support kernels that can
* run on multiple boards, pdata wouldn't benefit from "__init".
*/
- static struct mysoc_spi_data __init pdata = { ... };
+ static struct mysoc_spi_data __initdata pdata = { ... };
static __init board_init(void)
{
And SOC-specific utility code might look something like:
- #include <asm/arch/spi.h>
+ #include <mach/spi.h>
static struct platform_device spi2 = { ... };
.resume = CHIP_resume,
};
-The driver core will autmatically attempt to bind this driver to any SPI
+The driver core will automatically attempt to bind this driver to any SPI
device whose board_info gave a modalias of "CHIP". Your probe() code
might look like this unless you're creating a device which is managing
a bus (appearing under /sys/class/spi_master).
+ when bidirectional reads and writes start ... by how its
sequence of spi_transfer requests is arranged;
+ + which I/O buffers are used ... each spi_transfer wraps a
+ buffer for each transfer direction, supporting full duplex
+ (two pointers, maybe the same one in both cases) and half
+ duplex (one pointer is NULL) transfers;
+
+ optionally defining short delays after transfers ... using
- the spi_transfer.delay_usecs setting;
+ the spi_transfer.delay_usecs setting (this delay can be the
+ only protocol effect, if the buffer length is zero);
+ whether the chipselect becomes inactive after a transfer and
any delay ... by using the spi_transfer.cs_change flag;
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