A bug has been discovered in the vb2 and m2m implementations of poll where EPOLLIN and EPOLLOUT events are never signaled to epoll_wait() unless at least one of them is included in the initial epoll_ctl() call. The presence of this bug can be detected by setting an initial empty set of events with EPOLL_CTL_ADD, then setting the events we are interested in using EPOLL_CTL_MOD. epoll_wait() will then invariably timeout even if there is a queue-related event pending. Signed-off-by: Alexandre Courbot <gnurou@xxxxxxxxx> --- This is the follow-up patch for the discussion on https://www.spinics.net/lists/linux-media/msg179618.html. I have tested it with vivid and vicodec, and got the expected result with both (epoll streaming tests fails without the kernel fix, passes with it). utils/v4l2-compliance/v4l2-test-buffers.cpp | 17 ++++++++++++++++- 1 file changed, 16 insertions(+), 1 deletion(-) diff --git a/utils/v4l2-compliance/v4l2-test-buffers.cpp b/utils/v4l2-compliance/v4l2-test-buffers.cpp index 8000db23..291019c7 100644 --- a/utils/v4l2-compliance/v4l2-test-buffers.cpp +++ b/utils/v4l2-compliance/v4l2-test-buffers.cpp @@ -903,6 +903,17 @@ static int captureBufs(struct node *node, struct node *node_m2m_cap, const cv4l_ epollfd = epoll_create1(0); fail_on_test(epollfd < 0); + + /* + * Many older versions of the vb2 and m2m have a bug where + * EPOLLIN and EPOLLOUT events are never signaled unless they + * are part of the initial EPOLL_CTL_ADD. We set an initial + * empty set of events, which we then modify with EPOLL_CTL_MOD, + * in order to detect that condition. + */ + ev.events = 0; + fail_on_test(epoll_ctl(epollfd, EPOLL_CTL_ADD, node->g_fd(), &ev)); + if (node->is_m2m) ev.events = EPOLLIN | EPOLLOUT | EPOLLPRI; else if (v4l_type_is_output(q.g_type())) @@ -910,7 +921,7 @@ static int captureBufs(struct node *node, struct node *node_m2m_cap, const cv4l_ else ev.events = EPOLLIN; ev.data.fd = node->g_fd(); - fail_on_test(epoll_ctl(epollfd, EPOLL_CTL_ADD, node->g_fd(), &ev)); + fail_on_test(epoll_ctl(epollfd, EPOLL_CTL_MOD, node->g_fd(), &ev)); } if (pollmode) @@ -944,6 +955,10 @@ static int captureBufs(struct node *node, struct node *node_m2m_cap, const cv4l_ can_read = FD_ISSET(node->g_fd(), &rfds); have_event = FD_ISSET(node->g_fd(), &efds); } else if (pollmode == POLL_MODE_EPOLL) { + /* + * This can fail with a timeout on older kernels for + * drivers using vb2_core_poll() v4l2_m2m_poll(). + */ ret = epoll_wait(epollfd, &ev, 1, 2000); fail_on_test(ret == 0); fail_on_test(ret < 0); -- 2.29.2
