How to use BMP280 SPI Kernel Module

Question

I got a adafruit BME280 sensor board and I wired it to the raspberry pi zero via spi. According to the datasheet I can read sensor data from the device via the userspace spi interface. Everything works fine from userspace.

I discovered, that there is a kernel module for this sensor available utilizing the iio subsystem. And it implements the algorithms from the datasheet. A naive modprobe bcmp280_spi loaded the module:

# lsmod
Module                  Size  Used by
bmp280_spi              3451  0
bmp280                 11976  1 bmp280_spi
industrialio           50711  1 bmp280
spidev                  7034  0
...

But there is nothing to interface with:

# tree /sys/bus/iio/
/sys/bus/iio/
|-- devices
|-- drivers
|-- drivers_autoprobe
|-- drivers_probe
`-- uevent

2 directories, 3 files

Could someone help me with setting up the BMP280 SPI Kernel Module for the raspberry pi zero? I guess I need a device tree overlay, but I can't find any documentation how to interface this sensor module correctly. The raspberry pi includes a configuration for i2c but I want to use spi.

Answer 1

After searching in google for several painful hours days, I found the solution. Here is my device tree overlay:

/dts-v1/;
/plugin/;

/ {
    compatible = "brcm,bcm2708";

    fragment@0 {
        target = <&spidev0>;

        __overlay__ {
            status = "disabled";
        };
    };

    fragment@1 {
        target = <&spi0>;

        __overlay__ {
            status = "okay";
            #address-cells = <1>;
            #size-cells = <0>;

            bme280@0 {
                compatible = "bosch,bme280";
                reg = <0x0>;
                spi-max-frequency = <500000>;
                default-oversampling = <1>;
                status = "okay";
            };
        };
    };
};

It is important to disable the spidev node, since otherwise it conflicts with the chipselect. If you see something like this in dmesg, then you have disabled the wrong node:

[   85.280101] spi-bcm2835 20204000.spi: chipselect 0 already in use
[   85.280123] spi_master spi0: spi_device register error /soc/spi@7e204000/bme280@0
[   85.280138] of_spi_notify: failed to create for '/soc/spi@7e204000/bme280@0'
[   85.280144] OF: changeset notifier error @/soc/spi@7e204000/bme280@0

You can use @spidev0 for chipselect 0 and @spidev1 for chipselect 1. It also also important to disable the spidev node (=fragment@0) before enabling the sensor driver (=fragment@1).

An out of tree device tree compiler is needed to compile the overlay. I used this script to get one. The device tree overlay can be compiled and applied as follows:

# dtc -@ -I dts -O dtb -o /boot/overlays/bmp280-spi.dtbo bmp280-spi.dts
# dtoverlay /boot/overlays/bmp280-spi.dtbo

After the device tree overlay has been applied, sensor is there:

# cat /sys/bus/iio/devices/iio:device0/in_temp_input 
26980

To apply the overlay at bootup, add this line to /boot/config.txt

dtoverlay=bmp280-spi