UsbMidiDeviceJNI: fix race condition
Try multiple times to open the MIDI control device.
This fixes a race condition that caused Android to
sometimes not see a USB MIDI keyboard when it was plugged in.
Bug: 25328161
Change-Id: Ic72c5859364fc56bf7a40c1b1c9791c42827ea63
Signed-off-by: Phil Burk <philburk@google.com>
diff --git a/services/core/jni/com_android_server_UsbMidiDevice.cpp b/services/core/jni/com_android_server_UsbMidiDevice.cpp
index 06b9bc3..e12a016 100644
--- a/services/core/jni/com_android_server_UsbMidiDevice.cpp
+++ b/services/core/jni/com_android_server_UsbMidiDevice.cpp
@@ -43,12 +43,26 @@
jint card, jint device)
{
char path[100];
+ int fd;
+ const int kMaxRetries = 10;
+ const int kSleepMicroseconds = 2000;
snprintf(path, sizeof(path), "/dev/snd/controlC%d", card);
- int fd = open(path, O_RDWR);
- if (fd < 0) {
- ALOGE("could not open %s", path);
- return 0;
+ // This control device may not have been created yet. So we should
+ // try to open it several times to prevent intermittent failure
+ // from a race condition.
+ int retryCounter = 0;
+ while ((fd = open(path, O_RDWR)) < 0) {
+ if (++retryCounter > kMaxRetries) {
+ ALOGE("timed out after %d tries, could not open %s", retryCounter, path);
+ return 0;
+ } else {
+ ALOGW("attempt #%d, could not open %s", retryCounter, path);
+ // Increase the sleep interval each time.
+ // 10 retries will total 2 * sum(1..10) = 110 milliseconds.
+ // Typically the device should be ready in 5-10 milliseconds.
+ usleep(kSleepMicroseconds * retryCounter);
+ }
}
struct snd_rawmidi_info info;