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Nils Werner 19cd24f54f Add udev rules to use GPIO ports as non-root user (#18 )

2019-02-14 19:06:53 -07:00

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Audio

alsa-utils should supply the needed programs to use onboard sound. Default volume can be adjusted using alsamixer.

A key change with Linux kernel version 4.4.x for ARM related to ALSA and to the needed sound module: in order to use tools such as alsamixer with the current kernel, users must modify /boot/config.txt to contain the following line:

dtparam=audio=on

Caveats for Audio

To force audio over HDMI, add this to /boot/config.txt:

hdmi_drive=2

If you experience distortion using the 3.5mm analogue output:

audio_pwm_mode=2

Bluetooth

firmware

To be able to talk to the bluetooth chip, you must install the services, firmware, and UDEV rules. The pi-bluetooth package from the AUR is available to do this.

wifi-coexistence

The BCM43* series chips are notorious for problems when both wifi and bluetooth are used at the same time. This coexistence issue comes in varying degrees of severity but for most users makes using the Pi in bluetooth A2DP mode while also using wifi impossible. Bluetooth buffer underruns are caused by sharing the UART device, resulting in skipping, popping, hissing, and generally unusable audio. For some users, this also effects peripherals such as mice and keyboards. A firmware fix was found for both the Pi3 and Pi ZeroW, discussion of this fix can be found in this github issue.

To deploy the fix to the current firmware in Arch add the following lines:

btc_mode=1
btc_params8=0x4e20
btc_params1=0x7530

to the end of these firmware config files:

/usr/lib/firmware/updates/brcm/brcmfmac43430-sdio.txt

/usr/lib/firmware/updates/brcm/brcmfmac43455-sdio.txt

This fix can be found in this git commit. The fix is experimental and is not yet deployed to the upstream package

Video

The X.org driver for Raspberry Pi can be installed with the xf86-video-fbdev or xf86-video-fbturbo-git package.

CPU/GPU RAM split

Memory split between the CPU and GPU can be set in boot/config.txt by adjusting the parameter gpu_mem which stands for the amount of RAM in MB that is available to the GPU (minimum 16, default 64) and the rest is available to the ARM CPU.

HDMI/Analogue TV-Out

With the default configuration, the Raspberry Pi uses HDMI video if a HDMI monitor is connected. Otherwise, it uses analog TV-Out (also known as composite output or RCA) To turn the HDMI or analog TV-Out on or off, have a look at /opt/vc/bin/tvservice

Use the -s parameter to check the status; the -o parameter to turn the display off and -p parameter to power on HDMI with preferred settings.

Adjustments are likely required to correct proper overscan/underscan and are easily achieved in /boot/config.txt in which many tweaks are set. To fix, simply uncomment the corresponding lines and setup per the commented instructions:

# uncomment the following to adjust overscan. Use positive numbers if console
# goes off screen, and negative if there is too much border
#overscan_left=16
overscan_right=8
overscan_top=-16
overscan_bottom=-16

Or simply disable overscan if the TV/monitor has a "fit to screen" option.

disable_overscan=1

Users wishing to use the analog video out should consult this config file which contains options for non-NTSC outputs.

Camera

The commands for the camera module are included as part of the raspberrypi-firmware package:

$ /opt/vc/bin/raspistill
$ /opt/vc/bin/raspivid

Append to /boot/config.txt:

gpu_mem=128
start_file=start_x.elf
fixup_file=fixup_x.dat

Optionally

disable_camera_led=1

The following is a common error:

mmal: mmal_vc_component_enable: failed to enable component: ENOSPC
mmal: camera component couldn't be enabled
mmal: main: Failed to create camera component
mmal: Failed to run camera app. Please check for firmware updates

which can be corrected by setting these values in /boot/config.txt:

cma_lwm=
cma_hwm=
cma_offline_start=

Another common error:

mmal: mmal_vc_component_create: failed to create component 'vc.ril.camera' (1:ENOMEM)
mmal: mmal_component_create_core: could not create component 'vc.ril.camera' (1)
mmal: Failed to create camera component
mmal: main: Failed to create camera component
mmal: Only 64M of gpu_mem is configured. Try running "sudo raspi-config" and ensure that "memory_split" has a value of 128 or greater

can be corrected by adding the following line to /etc/modprobe.d/blacklist.conf:

blacklist i2c_bcm2708

In order to use standard applications (those that look for /dev/video0) the V4L2 driver must be loaded. This can be done automatically at boot by creating an autoload file, /etc/modules-load.d/rpi-camera.conf:

bcm2835-v4l2

The V4L2 driver by default only allows video recording up to 1280x720, else it glues together consecutive still screens resulting in videos of 4 fps or lower. Adding the following options removes this limitation, /etc/modprobe.d/rpi-camera.conf:

options bcm2835-v4l2 max_video_width=3240 max_video_height=2464

Onboard Hardware Sensors

Temperature

Temperatures sensors can be queried with utils in the raspberrypi-firmware package.

$ /opt/vc/bin/vcgencmd measure_temp
temp=49.8'C

Voltage

Four different voltages can be monitored via /opt/vc/bin/vcgencmd:

$ /opt/vc/bin/vcgencmd measure_volts <id>

Where <id> is:

core for core voltage
sdram_c for sdram Core voltage
sdram_i for sdram I/O voltage
sdram_p for sdram PHY voltage

Watchdog

BCM2708 has a hardware watchdog which can be utilized by enabling the bcm2708_wdog kernel module.

For proper operation the watchdog daemon also has to be installed, configured (by uncommenting the "watchdog-device" line in /etc/watchdog.conf) and enabled.

This should also apply for Raspberry Pi 2 by using the bcm2709_wdog module and Raspberry Pi 3 by using the bcm2835_wdt module.

Hardware Random Number Generator

Arch Linux ARM for the Raspberry Pi had the bcm2708-rng module set to load at boot; starting with kernel 4.4.7 the bcm2835_rng module replaces the former on Raspberry Pi 2 and Raspberry Pi 3 units.

Install rng-tools and tell the Hardware RNG Entropy Gatherer Daemon (rngd) where to find the hardware random number generator. This can be done by editing /etc/conf.d/rngd:

RNGD_OPTS="-o /dev/random -r /dev/hwrng"

and enabling and starting the rngd service.

If haveged is running, it should be stopped and disabled, as it might compete with rngd and is only preferred when there is no hardware random number generator available.

Once completed, this change ensures that data from the hardware random number generator is fed into the kernel's entropy pool at /dev/random. To check the available entropy, run:

# cat /proc/sys/kernel/random/entropy_avail

The number it reports should be around 3000, whereas before setting up rngd it would have been closer to 1000.

I/O Pins

GPIO

To be able to use the GPIO pins from Python, use the RPi.GPIO library. Install the python-raspberry-gpio package from the AUR.

SPI

To enable the /dev/spidev* devices, uncomment the following line in /boot/config.txt:

device_tree_param=spi=on

I2C

Install i2c-tools and lm_sensors packages.

Configure the bootloader to enable the i2c hardware by appending to /boot/config.txt:

dtparam=i2c_arm=on

Configure the i2c-dev and i2c-bcm2708 (if not blacklisted for the camera) modules to be loaded at boot in /etc/modules-load.d/raspberrypi.conf:

i2c-dev i2c-bcm2708

Reboot the Raspberry Pi and issue the following command to get the hardware address:

i2cdetect -y 0

Note: When using the I2C1 port instead of I2C0, one will need to run i2cdetect -y 1 instead and replace i2c-0 with i2c-1 in the following steps.

Now instantiate the device. Change the hardware address to the address found in the previous step with '0x' as prefix (e.g. 0x48) and choose a device name:

echo <devicename> <hardware address> >/sys/class/i2c-adapter/i2c-0/new_device

Check dmesg for a new entry:

i2c-0: new_device: Instantiated device ds1621 at 0x48

Finally, read the sensor output: sensors

1-WIRE

To enable the 1-wire interface add this line to /boot/config.txt and reboot. dtoverlay=w1-gpio

Non-Root GPIO

To use the GPIO/SPI pins as a regular non-root user (in group tty), add the following lines to a new file /usr/lib/udev/rules.d/99-spi-permissions.rules

KERNEL=="spidev*", GROUP="tty", MODE="0660"
SUBSYSTEM=="gpio*", PROGRAM="/bin/sh -c 'chown -R root:tty /sys/class/gpio && chmod -R 775 /sys/class/gpio; chown -R root:tty /sys/devices/virtual/gpio && chmod -R 775 /sys/devices/virtual/gpio; chown -R root:tty /sys/devices/platform/soc/*.gpio/gpio && chmod -R 775 /sys/devices/platform/soc/*.gpio/gpio'"
SUBSYSTEM=="gpio", KERNEL=="gpiochip*", ACTION=="add", PROGRAM="/bin/sh -c 'chown root:tty /sys/class/gpio/export /sys/class/gpio/unexport ; chmod 220 /sys/class/gpio/export /sys/class/gpio/unexport'"
SUBSYSTEM=="gpio", KERNEL=="gpio*", ACTION=="add", PROGRAM="/bin/sh -c 'chown root:tty /sys%p/active_low /sys%p/direction /sys%p/edge /sys%p/value ; chmod 660 /sys%p/active_low /sys%p/direction /sys%p/edge /sys%p/value'"

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