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octoi: Fix frame_rifo_depth() function

We use the 'correct' math in frame_rifo_depth now so the
count is accurate down to zero.

We need to fixup the logic that drags the 'write pointer'
(last_in_fn) along when reading from an empty FIFO.

We also need proper init (which was missing alltogether
beforehand).

Change-Id: I088f181e74358eb2c96a7aab7a7c875b9276d980
Signed-off-by: Sylvain Munaut <tnt@246tNt.com>
---
M src/octoi/frame_rifo.c
M src/octoi/frame_rifo.h
2 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/src/octoi/frame_rifo.c b/src/octoi/frame_rifo.c
index 94758e9..876f023 100644
--- a/src/octoi/frame_rifo.c
+++ b/src/octoi/frame_rifo.c
@@ -97,6 +97,7 @@
 	memset(rifo->buf, 0xff, sizeof(rifo->buf));
 	rifo->next_out = rifo->buf;
 	rifo->next_out_fn = 0;
+	rifo->last_in_fn = -1;
 	memset(rifo->bitvec, 0, sizeof(rifo->bitvec));
 }
 
@@ -151,10 +152,11 @@
 	if (rifo->next_out >= RIFO_BUF_END(rifo))
 		rifo->next_out -= sizeof(rifo->buf);
 
+	/* if we're empty we 'drag' last_in along to avoid overflows */
+	if (frame_rifo_depth(rifo) == 0)
+		rifo->last_in_fn += 1;
+
 	rifo->next_out_fn += 1;
-	/* make sure that frame_rifo_depth() doing last_in - next_out won't overflow */
-	if (rifo->next_out_fn == rifo->last_in_fn + 1)
-		rifo->last_in_fn = rifo->next_out_fn;
 
 	return rc;
 }
diff --git a/src/octoi/frame_rifo.h b/src/octoi/frame_rifo.h
index 401e873..ac97506 100644
--- a/src/octoi/frame_rifo.h
+++ b/src/octoi/frame_rifo.h
@@ -24,7 +24,7 @@
 /* current depth of RIFO */
 static inline unsigned int frame_rifo_depth(struct frame_rifo *rifo)
 {
-	return rifo->last_in_fn - rifo->next_out_fn;
+	return rifo->last_in_fn - rifo->next_out_fn + 1;
 }
 
 void frame_rifo_init(struct frame_rifo *rifo);

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