alsa: Avoid busy loop with dmix.

When using alsa's direct mixing plugin, the alsa poll fd might be ready
for IO even though nothing can be written to the alsa handle. This
leads to a busy loop in para_audiod and para_write and makes these
applications eat up considerable amounts of CPU time during playback.

This patch fixes the problem by reading from the alsa fd after a
failed write. It also adjusts the test for when to return early from
alsa_post_select(). The old code was not wrong, however, since we
should never reach the test if poll_fd is negative.

diff --git a/alsa_write.c b/alsa_write.c
index 047e88f2200a45f2518c7f1508874108e26a6bce..b89c8c31c8007ec6267ecac96acec6ca0d1ad1ce 100644 (file)
--- a/alsa_write.c
+++ b/alsa_write.c
@@ -248,12 +248,23 @@ again:
                wn->min_iqs = pad->bytes_per_frame;
                goto again;
        }
-       if (pad->poll_fd >= 0 && !FD_ISSET(pad->poll_fd, &s->rfds))
+       if (pad->poll_fd < 0 || !FD_ISSET(pad->poll_fd, &s->rfds))
                return;
        frames = bytes / pad->bytes_per_frame;
        frames = snd_pcm_writei(pad->handle, data, frames);
-       if (frames == 0 || frames == -EAGAIN)
+       if (frames == 0 || frames == -EAGAIN) {
+               /*
+                * The alsa poll fd was ready for IO but we failed to write to
+                * the alsa handle. This means another event is pending. We
+                * don't care about that but we have to dispatch the event in
+                * order to avoid a busy loop. So we simply read from the poll
+                * fd.
+                */
+               char buf[100];
+               if (read(pad->poll_fd, buf, 100))
+                       do_nothing;
                return;
+       }
        if (frames > 0) {
                btr_consume(btrn, frames * pad->bytes_per_frame);
                goto again;