QtMultimedia: FATAL: cannot locate cpu MHz in /proc/cpuinfo

This became a problem for my Raspberry Pi (armv6h) running Archlinux (with a relatively newer kernel). Some users of my Grooveshark server reported some bufferring problems where it stopped streaming songs and reported “FATAL: cannot locate cpu MHz in /proc/cpuinfo”.

Since the Grooveshark server is based on Qt5 I thought the new Qt5.3 version had some multimedia library changes that caused the issue, but turned out the issue is /proc/cpuinfo not producing CPU frequency for the ARM based CPUs. But what program is using a proc cpuinfo to determine CPU frequency without getting the value from sys filesystem exposed by the kernel? or using a monotonic clock as per newer kernels, which would provide timing capabilities without depending on system level changes, turned out there are quite a few programs around that depends on the value specified in “cpuinfo” file in proc filesystem. Including “jack” audio server which is a dependancy for QtMultimedia framework and number of other applications.

After hours of searching I came up with the commit that broke this, http://lists.infradead.org/pipermail/linux-arm-kernel/2013-June/178645.html, after reading the mailing list which explains why they have removed BogoMips (bogus mips rating of the CPU) from the /proc/cpuinfo, which gets set in the kernel bootup process for the purpose of setting its internal busy-loop.

How to fix QtMultimedia on ARM

Since qtmultimedia libraries depend on jack audio server for sinking audio-out and jack server being broken because of the removal of bogus mips value, I found out that there is jack2 version which seems to be working fine on ARM based computers, so here is set of steps to get JACK2 (a drop-in replacement for JACK as far as I understand) working on your Raspberry Pi,

Login in as root (if you are running Archlinux)

cd ~
git clone git://github.com/jackaudio/jack2.git
cd jack2
./waf configure --alsa
./waf build # this takes a while
./waf install
cd ..
rm -r jack2


That’s it, you should not get “FATAL: cannot locate cpu MHz in /proc/cpuinfo” error when using QtMultimedia framwork under ARM (Raspberry Pi, etc) boards.

Interested in kernel level timers and clocks, go on to

Multiple/Second SPI Buses on BeagleBone

How to: Get SPI to work on Beaglebone

If you want just one SPI port running, then most up to date images for beaglebone has enabled them by default.
Location to get pre-compiled images with SPI1 enabled,  http://rcn-ee.net/deb/precise-armhf/v3.2.25-psp20/

Enabling SPI0

Step 1

  • In order to enable userland SPI we need to be able to rebuild the kernel, so grab the Linux source:
    git clone git://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git
  • Install the required tools and libs by running:
    sudo apt-get install gcc-4.4-arm-linux-gnueabi git ccache libncurses5-dev u-boot-tools lzma

I tend to compile the kernel using hardfloat, so arm-linux-gnueabihf compiler would be the way to go.

  • As per Robert C. Nelson’s repo, get the patches required to enable spidev
    git clone git://github.com/RobertCNelson/linux-dev.git
    cd linux-dev
  • switch to the am33x-v3.2 branch with:
    git checkout origin/am33x-v3.2 -b am33x-v3.2
  • Now copy system.sh.sample to system.sh and update system.sh with the settings for your setup:
    cp system.sh.sample system.sh
  • For my setup I had to make the following changes to system.sh:
    uncomment line 14, change from:
    at line 60 update the path to your linux source, change from:
    uncomment line 70 to set the kernel entry point, change from:
    uncomment line 80 to set the BUILD_UIMAGE flag, change from:
    and finally at line 89 uncomment and set the path to the SD card, change from:
  • Build kernel command

Step 2

Note: This step will discuss how to enable SPI0 (SPI1 is enabled by default with latest kernels, or the one you download from above URL which contains precompiled images).

  • Goto “/KERNEL/arch/arm/mach-omap2” directory in kernel source which is in the clone you got from “ git://github.com/RobertCNelson/linux-dev.git” in above steps.
  • Make the following modification after you get to Kernel configuration menu.
    • Note: if you make the change before you get to config menu, the git update process will overwrite it again with the files from repository.
      *** /home/purinda/Software/Linux/linux-dev/KERNEL/arch/arm/mach-omap2/board-am335xevm.c 2012-08-10 00:12:53.937992451 +1000
      --- board-am335xevm.c 2012-08-09 23:36:24.774040301 +1000
      *** 2275,2280 ****
      --- 2275,2289 ----
      static struct spi_board_info bone_spidev2_info[] = {
      + /* [PG] I added the following struct call in order to get SPI0 working
      + */
      + {
      + .modalias = "spidev",
      + .irq = -1,
      + .max_speed_hz = 12000000,
      + .bus_num = 1,
      + .chip_select = 0,
      + },
       .modalias = "spidev",
       .irq = -1,
      *** 3189,3195 ****
       if(beaglebone_spi1_free == 1) {
       beaglebone_spi1_free = 0;
       pr_info("BeagleBone cape: exporting SPI pins as spidev\n");
      ! setup_pin_mux(spi1_pin_mux);
       spi_register_board_info(bone_spidev2_info, ARRAY_SIZE(bone_spidev2_info));
       if(beaglebone_w1gpio_free == 1) {
      --- 3198,3207 ----
       if(beaglebone_spi1_free == 1) {
       beaglebone_spi1_free = 0;
       pr_info("BeagleBone cape: exporting SPI pins as spidev\n");
      ! /* [PG] I made the following pin_mux call in order to get SPI0 working
      ! */
      ! setup_pin_mux(spi0_pin_mux);
      ! setup_pin_mux(spi1_pin_mux);
       spi_register_board_info(bone_spidev2_info, ARRAY_SIZE(bone_spidev2_info));
       if(beaglebone_w1gpio_free == 1) {

Step 3

  • Call ./build_kernel.sh (inside linux-dev directory).

Step 4

  • Call ./tools/install_image.sh
    • you may need to modify system.sh file according to the changes displayed on above.
    • if you have set the correct memory card device in system.sh it will load the compiled driver correctly into the boot partition. In my case the mmc dev was /dev/sdb

How to test

  • Once you boot the new Kernel you should see two devices in /dev/ directory as follows when you run the following command
    ls /dev/spidev* -lah
    Output should be
    crw------- 1 root root 153, 0 Aug 9 23:58 /dev/spidev1.0
    crw------- 1 root root 153, 1 Aug 9 23:58 /dev/spidev2.0
  • spidev1.0 is spi0 in beaglebone board and spidev2.0 is the spi1(the default spi master active on precompiled kernel images) in beaglebone.
  • once you can see these two devices you are ready to test.
  • connect pin 18 and 21 in header P9 with a cable, compile and run spidev_test.c in documentation/spi folder in Kernel source (refer to 1st site in references section) to test “spidev1.0”.
  • connect pin 28 and 30 in header P9 with a cable, compile and run spidev_test.c in documentation/spi folder in Kernel source (refer to 1st site in references section) to test “spidev2.0”.
  • When you do so make sure you change make sure you change the following line (in spidev_test.c) to spidev1.0 or spidev2.0, depending on what you need to test.
    static const char *device = "/dev/spidev1.1";

Example SPI driver with modifications

Modified am335_omap_spi.c file from kernel source

Beaglebone RF24(nRF24L01) Library


I started working on a project which require communication with multiple radios. I started looking at nRF24L01 chip and found it to be very useful, cheap little device which has many features built-in. Initially I got two AVRs to talk to each other through the simple library found here (http://www.tinkerer.eu/AVRLib/nRF24L01). But I found multiple issues with the library when the number of nodes in the network increases,

  • Didn’t support retries
  • Dynamic packet sizes
  • Channel scanning, etc

It was a simple library to start with and understand how the radio actually works in different configurations. I didn’t want to spend lot of time implementing all features I wanted so started looking around for complete libraries for nRF24L01, and I found RF24.

RF24 is one of the most complete libraries available for the nRF24L01[+]. So I implemented my project using RF24, It was an excellent library for AVRs (Arduino), but there was no port for devices running linux, as one of the nodes in my network was running linux I had to hack the code in order to get it to work on the beaglebone. So I had to spend a weekend hacking the RF24 library to come up with a very thin layer below the library to support all its features on embedded linux boards. I used a beaglebone for testing.

This is a Beaglebone port of the popular RF24(nrf24l01) library posted on https://github.com/maniacbug/RF24/.
If you find the library is out of sync with the Arduino library please drop an email to purinda@gmail.com

Contains portions of the GPIO library posted here,

Complete wiki can be found here


  • Refer to RF24 documentation.


How to connect the pins

  • I used one of these boards to test the library. http://iteadstudio.com/store/images/produce/Wireless/RF2401/RF24013.jpg
  • Connect PIN 1 (GND) of NRF24L01 board to Expansion header P9 PIN 1(GND) of Beagleboard
  • Connect PIN 2 (VCC) of NRF24L01 board to Expansion header P9 PIN 3(3v3) of Beagleboard
  • Connect PIN 3 (CE) of NRF24L01 board to Expansion header P9 PIN 27(GPIO3_19) of Beagleboard
  • Connect PIN 4 (CSN) of NRF24L01 board to Expansion header P9 PIN 25(GPIO3_21) of Beagleboard
  • Connect PIN 5 (SCK) of NRF24L01 board to Expansion header P9 PIN 31(SPI1_SCLK) of Beagleboard
  • Connect PIN 6 (MOSI) of NRF24L01 board to Expansion header P9 PIN 30(SPI1_D1) of Beagleboard
  • Connect PIN 7 (MISO) of NRF24L01 board to Expansion header P9 PIN 29(SPI1_D0) of Beagleboard


  • Note that the library uses a GPIO pin in BeagleBone as CS pin, which is an workaround/issue exist in the code. Planning to fix it whenever I have time.
  • Project is written using NetBeans and I have commited some files used by netbeans IDE.
  • main.c contains a program which prints out the information of the connected NRF24L01 device and ping demo.
  • you may need to upload the Arduino sketch in ‘pingpair_dyn_arduino_pongback’ directory to a Arduino board in order for both Beaglebone and Arduino to run interactively.

Download the source here