How does kernel code knows which spi bus is using?

Question

I modified device tree file and enable spi using 4 GPIO pins, which support pinmux and switch from gpio to spi function. But in Linux kernel code, how does the code know which spi bus/pins is used? For example, I find a Linux kernel driver: max1111.c, which drives a spi ADC chip. But I checked its code, and don't find where the spi bus/pins is specified.

I paste max1111.c below.

/*
 * max1111.c - +2.7V, Low-Power, Multichannel, Serial 8-bit ADCs
 *
 * Based on arch/arm/mach-pxa/corgi_ssp.c
 *
 * Copyright (C) 2004-2005 Richard Purdie
 *
 * Copyright (C) 2008 Marvell International Ltd.
 *  Eric Miao <[email protected]>
 *
 *  This program is free software; you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License version 2 as
 *  publishhed by the Free Software Foundation.
 */

#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/err.h>
#include <linux/hwmon.h>
#include <linux/hwmon-sysfs.h>
#include <linux/spi/spi.h>
#include <linux/slab.h>

enum chips { max1110, max1111, max1112, max1113 };

#define MAX1111_TX_BUF_SIZE 1
#define MAX1111_RX_BUF_SIZE 2

/* MAX1111 Commands */
#define MAX1111_CTRL_PD0      (1u << 0)
#define MAX1111_CTRL_PD1      (1u << 1)
#define MAX1111_CTRL_SGL      (1u << 2)
#define MAX1111_CTRL_UNI      (1u << 3)
#define MAX1110_CTRL_SEL_SH   (4)
#define MAX1111_CTRL_SEL_SH   (5)   /* NOTE: bit 4 is ignored */
#define MAX1111_CTRL_STR      (1u << 7)

struct max1111_data {
    struct spi_device   *spi;
    struct device       *hwmon_dev;
    struct spi_message  msg;
    struct spi_transfer xfer[2];
    uint8_t tx_buf[MAX1111_TX_BUF_SIZE];
    uint8_t rx_buf[MAX1111_RX_BUF_SIZE];
    struct mutex        drvdata_lock;
    /* protect msg, xfer and buffers from multiple access */
    int         sel_sh;
    int         lsb;
};

static int max1111_read(struct device *dev, int channel)
{
    struct max1111_data *data = dev_get_drvdata(dev);
    uint8_t v1, v2;
    int err;

    /* writing to drvdata struct is not thread safe, wait on mutex */
    mutex_lock(&data->drvdata_lock);

    data->tx_buf[0] = (channel << data->sel_sh) |
        MAX1111_CTRL_PD0 | MAX1111_CTRL_PD1 |
        MAX1111_CTRL_SGL | MAX1111_CTRL_UNI | MAX1111_CTRL_STR;

    err = spi_sync(data->spi, &data->msg);
    if (err < 0) {
        dev_err(dev, "spi_sync failed with %d\n", err);
        mutex_unlock(&data->drvdata_lock);
        return err;
    }

    v1 = data->rx_buf[0];
    v2 = data->rx_buf[1];

    mutex_unlock(&data->drvdata_lock);

    if ((v1 & 0xc0) || (v2 & 0x3f))
        return -EINVAL;

    return (v1 << 2) | (v2 >> 6);
}

#ifdef CONFIG_SHARPSL_PM
static struct max1111_data *the_max1111;

int max1111_read_channel(int channel)
{
    return max1111_read(&the_max1111->spi->dev, channel);
}
EXPORT_SYMBOL(max1111_read_channel);
#endif

/*
 * NOTE: SPI devices do not have a default 'name' attribute, which is
 * likely to be used by hwmon applications to distinguish between
 * different devices, explicitly add a name attribute here.
 */
static ssize_t show_name(struct device *dev,
             struct device_attribute *attr, char *buf)
{
    return sprintf(buf, "%s\n", to_spi_device(dev)->modalias);
}

static ssize_t show_adc(struct device *dev,
            struct device_attribute *attr, char *buf)
{
    struct max1111_data *data = dev_get_drvdata(dev);
    int channel = to_sensor_dev_attr(attr)->index;
    int ret;

    ret = max1111_read(dev, channel);
    if (ret < 0)
        return ret;

    /*
     * Assume the reference voltage to be 2.048V or 4.096V, with an 8-bit
     * sample. The LSB weight is 8mV or 16mV depending on the chip type.
     */
    return sprintf(buf, "%d\n", ret * data->lsb);
}

#define MAX1111_ADC_ATTR(_id)       \
    SENSOR_DEVICE_ATTR(in##_id##_input, S_IRUGO, show_adc, NULL, _id)

static DEVICE_ATTR(name, S_IRUGO, show_name, NULL);
static MAX1111_ADC_ATTR(0);
static MAX1111_ADC_ATTR(1);
static MAX1111_ADC_ATTR(2);
static MAX1111_ADC_ATTR(3);
static MAX1111_ADC_ATTR(4);
static MAX1111_ADC_ATTR(5);
static MAX1111_ADC_ATTR(6);
static MAX1111_ADC_ATTR(7);

static struct attribute *max1111_attributes[] = {
    &dev_attr_name.attr,
    &sensor_dev_attr_in0_input.dev_attr.attr,
    &sensor_dev_attr_in1_input.dev_attr.attr,
    &sensor_dev_attr_in2_input.dev_attr.attr,
    &sensor_dev_attr_in3_input.dev_attr.attr,
    NULL,
};

static const struct attribute_group max1111_attr_group = {
    .attrs  = max1111_attributes,
};

static struct attribute *max1110_attributes[] = {
    &sensor_dev_attr_in4_input.dev_attr.attr,
    &sensor_dev_attr_in5_input.dev_attr.attr,
    &sensor_dev_attr_in6_input.dev_attr.attr,
    &sensor_dev_attr_in7_input.dev_attr.attr,
    NULL,
};

static const struct attribute_group max1110_attr_group = {
    .attrs  = max1110_attributes,
};

static int setup_transfer(struct max1111_data *data)
{
    struct spi_message *m;
    struct spi_transfer *x;

    m = &data->msg;
    x = &data->xfer[0];

    spi_message_init(m);

    x->tx_buf = &data->tx_buf[0];
    x->len = MAX1111_TX_BUF_SIZE;
    spi_message_add_tail(x, m);

    x++;
    x->rx_buf = &data->rx_buf[0];
    x->len = MAX1111_RX_BUF_SIZE;
    spi_message_add_tail(x, m);

    return 0;
}

static int max1111_probe(struct spi_device *spi)
{
    enum chips chip = spi_get_device_id(spi)->driver_data;
    struct max1111_data *data;
    int err;

    spi->bits_per_word = 8;
    spi->mode = SPI_MODE_0;
    err = spi_setup(spi);
    if (err < 0)
        return err;

    data = devm_kzalloc(&spi->dev, sizeof(struct max1111_data), GFP_KERNEL);
    if (data == NULL) {
        dev_err(&spi->dev, "failed to allocate memory\n");
        return -ENOMEM;
    }

    switch (chip) {
    case max1110:
        data->lsb = 8;
        data->sel_sh = MAX1110_CTRL_SEL_SH;
        break;
    case max1111:
        data->lsb = 8;
        data->sel_sh = MAX1111_CTRL_SEL_SH;
        break;
    case max1112:
        data->lsb = 16;
        data->sel_sh = MAX1110_CTRL_SEL_SH;
        break;
    case max1113:
        data->lsb = 16;
        data->sel_sh = MAX1111_CTRL_SEL_SH;
        break;
    }
    err = setup_transfer(data);
    if (err)
        return err;

    mutex_init(&data->drvdata_lock);

    data->spi = spi;
    spi_set_drvdata(spi, data);

    err = sysfs_create_group(&spi->dev.kobj, &max1111_attr_group);
    if (err) {
        dev_err(&spi->dev, "failed to create attribute group\n");
        return err;
    }
    if (chip == max1110 || chip == max1112) {
        err = sysfs_create_group(&spi->dev.kobj, &max1110_attr_group);
        if (err) {
            dev_err(&spi->dev,
                "failed to create extended attribute group\n");
            goto err_remove;
        }
    }

    data->hwmon_dev = hwmon_device_register(&spi->dev);
    if (IS_ERR(data->hwmon_dev)) {
        dev_err(&spi->dev, "failed to create hwmon device\n");
        err = PTR_ERR(data->hwmon_dev);
        goto err_remove;
    }

#ifdef CONFIG_SHARPSL_PM
    the_max1111 = data;
#endif
    return 0;

err_remove:
    sysfs_remove_group(&spi->dev.kobj, &max1110_attr_group);
    sysfs_remove_group(&spi->dev.kobj, &max1111_attr_group);
    return err;
}

static int max1111_remove(struct spi_device *spi)
{
    struct max1111_data *data = spi_get_drvdata(spi);

    hwmon_device_unregister(data->hwmon_dev);
    sysfs_remove_group(&spi->dev.kobj, &max1110_attr_group);
    sysfs_remove_group(&spi->dev.kobj, &max1111_attr_group);
    mutex_destroy(&data->drvdata_lock);
    return 0;
}

static const struct spi_device_id max1111_ids[] = {
    { "max1110", max1110 },
    { "max1111", max1111 },
    { "max1112", max1112 },
    { "max1113", max1113 },
    { },
};
MODULE_DEVICE_TABLE(spi, max1111_ids);

static struct spi_driver max1111_driver = {
    .driver     = {
        .name   = "max1111",
        .owner  = THIS_MODULE,
    },
    .id_table   = max1111_ids,
    .probe      = max1111_probe,
    .remove     = max1111_remove,
};

module_spi_driver(max1111_driver);

MODULE_AUTHOR("Eric Miao <[email protected]>");
MODULE_DESCRIPTION("MAX1110/MAX1111/MAX1112/MAX1113 ADC Driver");
MODULE_LICENSE("GPL");

Answer 1

SPI device driver (max1111 in your case) get pointer to underlying SPI-controller (struct spi_device *spi) during probe stage (max1111_probe). Driver should use it send requests to controller (using spi_sync, for example). Driver doesn't know about what PINS SPI-controller use.

What SPI-controller is passed to SPI device driver? SPI device, should be declared in the DTS-file inside SPI-controller node. The controller initialized from SPI-controller node is passed to device probe.

SPI-controller can be hardware (specific to SoC), or SPI-GPIO. In case of hardware SPI, pins usually dedicated and specified in SoC TRM. In case of SPI-GPIO, GPIO names are specified inside DTS-properties of SPI-GPIO. The properties names are: gpio-sck, gpio-miso, gpio-mosi, num-chipselects and cs-gpios (list).

Answer 2

The device tree file where the SPI device(max1111) is listed as child will have the corresponding base address of SPI module as shown in below example, this base address is related to the SPI bus not device.

spix@ABCDXYZ {
         compatible = "busdriver,variant";  

    /*This naming convention depends on the device tree*/    
    mosi = <gpioA> /*replace gpioA/B/C/D with your gpios*/
    miso = <gpioB>
    gpio-clk  = <gpioC>
    gpio-cs0  = <gpioD>
    spi-max-frequency = <freq1>;
                ;
                ;
                ;

max1111@0 {
        compatible = "max1111";
        reg = <0>;
        spi-max-frequency = <freq2>;
               ;
               ;
               ;
    };
        ;
        ;
        ;
 }

In the above device tree file you can see the device node "max1111" listed as child under the bus node "spix" whose register address range starts at address @ABCDXYZ. You need to refer userguide of MCU you are using to know this base address. The driver file max1111.c, is the device driver file for one of the SPI devices sitting on the spix bus. The driver max1111.c is agnostic to the SPI lines that it is sitting on & just enjoys the APIs (Ex: data transfer & chip select handling) provided by the SPI bus driver associated with "busdriver,variant". The SPI bus driver will take care of configuring/multiplexing the MCU pins into SPI mode (handled explicitly in machine/board specific initialisation code).

So all you have to do is to,

1. Identify which SPI bus max1111 device is using.
2. Then go to the device tree file corresponding to you board.
3. Add your device driver details (+ SPI parameters) as part of child to that bus node
4. include your max1111.c into the kernel biuld directory (mostly drivers/spi folder)

that is it.

Note: here the assumption is your machine is already having that particular SPI bus driver enabled.