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====== Platform Data ======

For compile time configuration, it’s common Linux practice to keep board- and application-specific configuration out of the main driver file, instead putting it into the board support file.

For devices on custom boards, as typical of embedded and SoC-(system-on-chip) based hardware, Linux uses platform_data to point to board-specific structures describing devices and how they are connected to the SoC. This can include available ports, chip variants, preferred modes, default initialization, additional pin roles, and so on. This shrinks the board-support packages (BSPs) and minimizes board and application specific #ifdefs in drivers.

====== Platform Data Example ======

The following example shows a set of these configuration options. These variables are fully documented in adxl34x header file (include/linux/input/adxl34x.h).
<code c>
#include <linux/input/adxl34x.h>
static const struct adxl34x_platform_data adxl34x_info = {
	.x_axis_offset = 0,
	.y_axis_offset = 0,
	.z_axis_offset = 0,
	.tap_threshold = 0x31,
	.tap_duration = 0x10,
	.tap_latency = 0x60,
	.tap_window = 0xF0,
	.tap_axis_control = ADXL_TAP_X_EN | ADXL_TAP_Y_EN | ADXL_TAP_Z_EN,
	.act_axis_control = 0xFF,
	.activity_threshold = 5,
	.inactivity_threshold = 3,
	.inactivity_time = 4,
	.free_fall_threshold = 0x7,
	.free_fall_time = 0x20,
	.data_rate = 0x8,
	.data_range = ADXL_FULL_RES,

	.ev_type = EV_ABS,
	.ev_code_x = ABS_X,		/* EV_REL */
	.ev_code_y = ABS_Y,		/* EV_REL */
	.ev_code_z = ABS_Z,		/* EV_REL */

	.ev_code_tap = {BTN_TOUCH, BTN_TOUCH, BTN_TOUCH}, /* EV_KEY x,y,z */ 

.ev_code_ff = KEY_F,		/* EV_KEY */
.ev_code_act_inactivity = KEY_A,	/* EV_KEY */
	.power_mode = ADXL_AUTO_SLEEP | ADXL_LINK,
	.fifo_mode = ADXL_FIFO_STREAM,
};
</code>

====== Platform and Bus Model ======

To attach devices to drivers, the platform and bus model eliminates the need for device drivers to contain hard coded physical addresses or bus IDs of the devices they control. The platform and bus model also prevents resource conflicts, greatly improves portability, and cleanly interfaces with the kernel's power-management features.

With the platform and bus model, device drivers know how to control a device once informed of its physical location and interrupt lines. This information is provided as a data structure passed to the driver during probing.


====== Declaring I2C devices ======


Unlike PCI or USB devices, I2C devices are not enumerated at the hardware level. Instead, the software must know which devices are connected on each I2C bus segment, and what address these devices are using. For this reason, the kernel code must instantiate I2C devices explicitly. There are different ways to achieve this, depending on the context and requirements. However the most common method is to declare the I2C devices by bus number.

This method is appropriate when the I2C bus is a system bus, as in many embedded systems, wherein each I2C bus has a number which is known in advance. It is thus possible to pre-declare the I2C devices that inhabit this bus. This is done with an array of struct i2c_board_info, which is registered by calling i2c_register_board_info().

So, to enable such a driver one need only edit the board support file by adding an appropriate entry to i2c_board_info.

For more information see: [[git.linux.org>Documentation/i2c/instantiating-devices.rst]] 
====== Example Declaring I2C Devices ======

<code c>
static struct i2c_board_info __initdata bfin_i2c_board_info[] = {
#if defined(CONFIG_TOUCHSCREEN_AD7879_I2C) || defined(CONFIG_TOUCHSCREEN_AD7879_I2C_MODULE)
	{
		I2C_BOARD_INFO("ad7879", 0x2F),
		.irq = IRQ_PG5,
		.platform_data = (void *)&bfin_ad7879_ts_info,
	},
#endif
#if defined(CONFIG_KEYBOARD_ADP5588) || defined(CONFIG_KEYBOARD_ADP5588_MODULE)
	{
		I2C_BOARD_INFO("adp5588-keys", 0x34),
		.irq = IRQ_PG0,
		.platform_data = (void *)&adp5588_kpad_data,
	},
#endif
#if defined(CONFIG_PMIC_ADP5520) || defined(CONFIG_PMIC_ADP5520_MODULE)
	{
		I2C_BOARD_INFO("pmic-adp5520", 0x32),
		.irq = IRQ_PG0,
		.platform_data = (void *)&adp5520_pdev_data,
	},
#endif
#if defined(CONFIG_INPUT_ADXL34X_I2C) || defined(CONFIG_INPUT_ADXL34X_I2C_MODULE)
	{
		I2C_BOARD_INFO("adxl34x", 0x53),
		.irq = IRQ_PG0,
		.platform_data = (void *)&adxl34x_info,
	},
#endif
};
</code>

<code c>
static void __init blackfin_init(void)
{
	(...)
	i2c_register_board_info(0, bfin_i2c_board_info, ARRAY_SIZE(bfin_i2c_board_info));
	(...)
}
</code>


====== Declaring SPI slave devices ======

Unlike PCI or USB devices, SPI devices are not enumerated at the hardware level. Instead, the software must know which devices are connected on each SPI bus segment, and what slave selects these devices are using. For this reason, the kernel code must instantiate SPI devices explicitly. The most common method is to declare the SPI devices by bus number.

This method is appropriate when the SPI  bus is a system bus, as in many embedded systems, wherein each SPI bus has a number which is known in advance. It is thus possible to pre-declare the SPI devices that inhabit this bus. This is done with an array of struct spi_board_info, which is registered by calling spi_register_board_info().

For more information see: [[git.linux.org>Documentation/spi/spi-summary.rst]] 
====== Example Declaring SPI Slave Devices ======

So, to enable such a driver one need only edit the board support file by adding an appropriate entry to spi_board_info.

<code c>
static struct spi_board_info bfin_spi_board_info[] __initdata = {
#if defined(CONFIG_TOUCHSCREEN_AD7877) || defined(CONFIG_TOUCHSCREEN_AD7877_MODULE)
	{
		.modalias		= "ad7877",
		.platform_data		= &bfin_ad7877_ts_info,
		.irq			= IRQ_PF6,
		.max_speed_hz	= 12500000,     /* max spi clock (SCK) speed in HZ */
		.bus_num	= 0,
		.chip_select  = 1,
		.controller_data = &spi_ad7877_chip_info,
	},
#endif
#if defined(CONFIG_TOUCHSCREEN_AD7879_SPI) || defined(CONFIG_TOUCHSCREEN_AD7879_SPI_MODULE)
	{
		.modalias = "ad7879",
		.platform_data = &bfin_ad7879_ts_info,
		.irq = IRQ_PF7,
		.max_speed_hz = 5000000,     /* max spi clock (SCK) speed in HZ */
		.bus_num = 0,
		.chip_select = 1,
		.controller_data = &spi_ad7879_chip_info,
		.mode = SPI_CPHA | SPI_CPOL,
	},
#endif
#if defined(CONFIG_INPUT_ADXL34X_SPI) || defined(CONFIG_INPUT_ADXL34X_SPI_MODULE)
	{
		.modalias	= "adxl34x",
		.platform_data	= &adxl34x_info,
		.irq		= IRQ_PF6,
		.max_speed_hz	= 5000000,    /* max spi clock (SCK) speed in HZ */
		.bus_num	= 0,
		.chip_select	= 2,
		.controller_data = &spi_adxl34x_chip_info,
		.mode = SPI_MODE_3,
	},
#endif
};
</code>

<code c>
static void __init blackfin_init(void)
{
	(...)
	spi_register_board_info(bfin_spi_board_info, ARRAY_SIZE(bfin_spi_board_info));
	(...)
}
</code>
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