[alsa-devel] [PATCH TINYCOMPRESS 10/14 v2] compress: Block if unable to write all data in single write()

[Richard Fitzgerald]

Previously compress_write() would only block on poll() if the
available space < fragment_size. If the device has a small fragment
size this could lead to it never blocking and instead looping around
doing many small writes. This is bad for power saving.

If we were unable to write all the remaining data in a single write
we want to block until the device reaches a buffer high water mark,
to allow the CPU to sleep.

This change will always attempt to issue the first write providing
avail >= fragment_size. All subsequent loops will block on poll()
before attempting another write() unless there is enough buffer space
to write all remaining data.

---
 compress.c |   25 ++++++++++++-------------
 1 files changed, 12 insertions(+), 13 deletions(-)

diff --git a/compress.c b/compress.c
index 7174041..45b34d0 100644
--- a/compress.c
+++ b/compress.c
@@ -329,8 +329,10 @@ int compress_write(struct compress *compress, const void *buf, unsigned int size
 {
 	struct snd_compr_avail avail;
 	struct pollfd fds;
-	int to_write, written, total = 0, ret;
+	int to_write = 0;	/* zero indicates we haven't written yet */
+	int written, total = 0, ret;
 	const char* cbuf = buf;
+	const unsigned int frag_size = compress->config->fragment_size;
 
 	if (!(compress->flags & COMPRESS_IN))
 		return oops(compress, -EINVAL, "Invalid flag set");
@@ -344,24 +346,21 @@ int compress_write(struct compress *compress, const void *buf, unsigned int size
 		if (ioctl(compress->fd, SNDRV_COMPRESS_AVAIL, &avail))
 			return oops(compress, errno, "cannot get avail");
 
-		/* we will write only when avail > fragment size */
-		if (avail.avail < compress->config.fragment_size) {
-			/* nothing to write so wait */
-			ret = poll(&fds, 1, compress->max_poll_wait_ms);
-			/* A pause will cause -EBADFD or zero return from driver
-			 * This is not an error, just stop writing
+		if ( (avail.avail < frag_size)
+			|| ((to_write != 0) && (avail.avail < size)) ) {
+			/* not enough space for one fragment, or we have done
+			 * a short write and there isn't enough space for all
+			 * the remaining data
 			 */
+			ret = poll(&fds, 1, compress->max_poll_wait_ms);
+			/* A pause will cause -EBADFD or zero.
+			 * This is not an error, just stop writing */
 			if ((ret == 0) || (ret == -EBADFD))
 				break;
 			if (ret < 0)
 				return oops(compress, errno, "poll error");
 			if (fds.revents & POLLOUT) {
-				if (ioctl(compress->fd, SNDRV_COMPRESS_AVAIL, &avail))
-					return oops(compress, errno, "cannot get avail");
-				if (avail.avail == 0) {
-					oops(compress, -EIO, "woken up even when avail is 0!!!");
-					continue;
-				}
+				continue;
 			}
 			if (fds.revents & POLLERR) {
 				return oops(compress, -EIO, "poll returned error!");
-- 
1.7.2.5