attempt to open /dev/radio0 on Xperia E C1504 returns EBUSY

Question

I have a rooted Xperia E C1504 on which I would like to access the FM receiver chipset for a custom FM radio app I'm working on. Trouble is that my fcntl open() call to /dev/radio0 keeps returning EBUSY (errno 16: device or resource busy). Here is the code I'm using to try to open the device (executed in a root shell):

#define DEFAULT_RADIO_DEVICE "/dev/radio0"
...
radio_fd = open(DEFAULT_RADIO_DEVICE, O_RDWR);

I also tried open(DEFAULT_RADIO_DEVICE,O_RDONLY), though I'm pretty sure I need write access to the device, and still received EBUSY.

I verified that the radio device is functional with the stock FM Radio app, which was able to tune to a frequency and receive PCM successfully. I turned this app off via its power button icon (this should release /dev/radio0, correct?) and explicitly force-stopped the stock FM Radio app from the Settings->Apps menu, and even deleting Radio.apk (the stock FM Radio app package) from /system/apps with Root Browser and then rebooting the phone, but my program continues to return EBUSY when it executes the above instruction.

What is the best way to investigate what process might be holding a lock on /dev/radio0 and kill it? I tried [adb shell "su -c 'lsof /dev/radio0'"] but the returned list didn't have any entries exactly matching /dev/radio0. There were quite a few cases of '/dev/log/radio' and almost 300 cases of just the word 'radio', but I was expecting to see something listed as using exactly /dev/radio0. I also tried [adb shell ps | grep radio] which returned

root      79    2     0      0     ffffffff 00000000 S kfmradio
radio     155   140   20416  3232  ffffffff 00000000 S /system/bin/rild
radio     178   140   7820   2472  ffffffff 00000000 S /system/bin/cnd
radio     215   140   6152   500   ffffffff 00000000 S /system/bin/qmuxd
radio     231   140   7288   752   ffffffff 00000000 S /system/bin/netmgrd
radio     610   157   311448 35704 ffffffff 00000000 S com.android.phone

the kfmradio process looked suspicious so I tried killing it, which didn't return any errors, but re-running the filtered ps list above showed that kfmradio was still in the process list (I suppose the OS restarted it?) Any advice regarding troubleshooting EBUSY returns from fcntl open() calls would be very helpful.

Device Model: Sony Xperia E C1504

Linux kernel: 3.4.0

Android OS: 4.1.1

Firmware version: Stock Kernel Xperia E C1505_11.3.A.0.47 (supposed to work for the C1504 as well)

Rooted with: SRSRoot and the 'Gandalf' exploit.

Answer 1

turns out my own program was to blame; my main() looks something like:

bindFmRx(); //wrapped the open() fcntl function
testV4L2Functionality(); //tested a number of V4L2 functions, and also included an errant 
                         //open() call on /dev/radio0 that I thought I'd removed X|
releaseFmRx();//close()ed the /dev/radio0 device

so the EBUSY I was seeing was coming from the second, erroneous, open() call in testV4L2Functionality(). I fixed the problem by removing fcntl function calls from the testV4L2Functionality() routine.