[android-porting] Using Android Nougat's shared memory for media decoding

[François de Cugnaux]

Hi everyone,


I am porting my Android AOSP-based distribution from Android K to Android
N. It includes a modified version of the Media Player that decodes DVD
subtitles. The architecture of the Media Player evolved a lot between those
2 versions. In particular, it is now split into 3 processes (see
https://source.android.com/devices/media/framework-hardening).

I am thus trying to use Shared Memory to make the *MediaCodecService* send
decoded bitmap subtitles to the *MediaServer*. I modified the contents of
the structure that was previously created by *MediaCodecService* and added
a *subtitle_fd* attribute, file descriptor to the decoded bitmap subtitle.
When a message is received by the *MediaServer*'s *Nuplayer* for rendering,
the code tries to map the aforementioned file descriptor.


Unfortunately, the result of the call to *::mmap* is always *MAP_FAILED*.


Do you have an idea of what I missed ?


-

*Code of the MediaCodecService part*

AVSubtitleRect *rect = sub->rects[0];size_t len = sizeof(*rect);int fd = ashmem_create_region("subtitle rect", len);
ashmem_set_prot_region(fd, PROT_READ | PROT_WRITE);void* ptr = ::mmap(NULL, len, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);if (ptr == MAP_FAILED) {
ALOGI("%s[%d] dvb ptr == MAP_FAILED", __FUNCTION__, __LINE__);} else {
ALOGI("Success creating FD with value %d", fd);}
memcpy(ptr, rect, len);sub->subtitle_fd = fd;sub->subtitle_size = len;

-

*Code of the MediaServer part*

int fd = mSubtitle->subtitle_fd;size_t len = mSubtitle->subtitle_size;
ALOGI("Trying to map shared memory with FD = %d", fd);void* ptr = ::mmap(NULL, len, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);if (ptr == MAP_FAILED) {
ALOGI("Subtitle mmap ptr==MAP_FAILED %s", strerror(errno));} else {
ALOGI("Subtitle get ptr %p", ptr);}AVSubtitleRect *rect = (AVSubtitleRect *)ptr;


Thank you very much for your help !