util/fifo8: Introduce fifo8_peek_buf()

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

Reviewed-by: Francisco Iglesias <frasse.iglesias@gmail.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Tested-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20231109192814.95977-3-philmd@linaro.org>
Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>

include/qemu/fifo8.h

@@ -93,6 +93,33 @@ uint8_t fifo8_pop(Fifo8 *fifo);
  */
 const uint8_t *fifo8_pop_buf(Fifo8 *fifo, uint32_t max, uint32_t *numptr);
 
+/**
+ * fifo8_peek_buf: read upto max bytes from the fifo
+ * @fifo: FIFO to read from
+ * @max: maximum number of bytes to peek
+ * @numptr: pointer filled with number of bytes returned (can be NULL)
+ *
+ * Peek into a number of elements from the FIFO up to a maximum of max.
+ * The buffer containing the data peeked into is returned. This buffer points
+ * directly into the FIFO backing store. Since data is invalidated once any
+ * of the fifo8_* APIs are called on the FIFO, it is the caller responsibility
+ * to access it before doing further API calls.
+ *
+ * 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

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;