[PATCH 03/12] util/fifo8: Introduce fifo8_peek_buf()

["Philippe Mathieu-Daudé" <philmd@linaro.org>]

To be able to poke at FIFO content without popping it,
introduce the fifo8_peek_buf() method by factoring
common content from fifo8_pop_buf().

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
---
 include/qemu/fifo8.h | 26 ++++++++++++++++++++++++++
 util/fifo8.c         | 22 ++++++++++++++++++----
 2 files changed, 44 insertions(+), 4 deletions(-)

diff --git a/include/qemu/fifo8.h b/include/qemu/fifo8.h
index d0d02bc73d..7acf6d1347 100644
--- a/include/qemu/fifo8.h
+++ b/include/qemu/fifo8.h
@@ -93,6 +93,32 @@ uint8_t fifo8_pop(Fifo8 *fifo);
  */
 const uint8_t *fifo8_pop_buf(Fifo8 *fifo, uint32_t max, uint32_t *numptr);
 
+/**
+ * fifo8_peek_buf:
+ * @fifo: FIFO to poke from
+ * @max: maximum number of bytes to pop
+ * @numptr: pointer filled with number of bytes returned (can be NULL)
+ *
+ * Pop a number of elements from the FIFO up to a maximum of max. The buffer
+ * containing the popped data is returned. This buffer points directly into
+ * the FIFO backing store and data is invalidated once any of the fifo8_* APIs
+ * are called on the FIFO.
+ *
+ * The function may return fewer bytes than requested when the data wraps
+ * around in the ring buffer; in this case only a contiguous part of the data
+ * is returned.
+ *
+ * The number of valid bytes returned is populated in *numptr; will always
+ * return at least 1 byte. max must not be 0 or greater than the number of
+ * bytes in the FIFO.
+ *
+ * Clients are responsible for checking the availability of requested data
+ * using fifo8_num_used().
+ *
+ * Returns: A pointer to peekable data.
+ */
+const uint8_t *fifo8_peek_buf(Fifo8 *fifo, uint32_t max, uint32_t *numptr);
+
 /**
  * fifo8_reset:
  * @fifo: FIFO to reset
diff --git a/util/fifo8.c b/util/fifo8.c
index 032e985440..e12477843e 100644
--- a/util/fifo8.c
+++ b/util/fifo8.c
@@ -66,7 +66,8 @@ uint8_t fifo8_pop(Fifo8 *fifo)
     return ret;
 }
 
-const uint8_t *fifo8_pop_buf(Fifo8 *fifo, uint32_t max, uint32_t *numptr)
+static const uint8_t *fifo8_peekpop_buf(Fifo8 *fifo, uint32_t max,
+                                        uint32_t *numptr, bool do_pop)
 {
     uint8_t *ret;
     uint32_t num;
@@ -74,15 +75,28 @@ const uint8_t *fifo8_pop_buf(Fifo8 *fifo, uint32_t max, uint32_t *numptr)
     assert(max > 0 && max <= fifo->num);
     num = MIN(fifo->capacity - fifo->head, max);
     ret = &fifo->data[fifo->head];
-    fifo->head += num;
-    fifo->head %= fifo->capacity;
-    fifo->num -= num;
+
+    if (do_pop) {
+        fifo->head += num;
+        fifo->head %= fifo->capacity;
+        fifo->num -= num;
+    }
     if (numptr) {
         *numptr = num;
     }
     return ret;
 }
 
+const uint8_t *fifo8_peek_buf(Fifo8 *fifo, uint32_t max, uint32_t *numptr)
+{
+    return fifo8_peekpop_buf(fifo, max, numptr, false);
+}
+
+const uint8_t *fifo8_pop_buf(Fifo8 *fifo, uint32_t max, uint32_t *numptr)
+{
+    return fifo8_peekpop_buf(fifo, max, numptr, true);
+}
+
 void fifo8_reset(Fifo8 *fifo)
 {
     fifo->num = 0;
-- 
2.38.1