kfifo: introduce and use kfifo_skip_count()

kfifo_skip_count() is an extended version of kfifo_skip(), accepting
also count. This will be useful in the serial code later.

Now, it can be used to implement both kfifo_skip() and
kfifo_dma_out_finish(). In the latter, 'len' only needs to be divided by
'type' size (as it was until now).

And stop using statement expressions when the return value is cast to
'void'. Use classic 'do {} while (0)' instead.

Note: perhaps we should skip 'count' records for the 'recsize' case, but
the original (kfifo_dma_out_finish()) used to skip only one record. So
this is kept unchanged and 'count' is still ignored in the recsize case.
Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
Cc: Stefani Seibold <stefani@seibold.net>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: https://lore.kernel.org/r/20240405060826.2521-3-jirislaby@kernel.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/kfifo.h

@@ -304,19 +304,25 @@ __kfifo_uint_must_check_helper( \
 )
 
 /**
- * kfifo_skip - skip output data
+ * kfifo_skip_count - skip output data
  * @fifo: address of the fifo to be used
+ * @count: count of data to skip
  */
-#define	kfifo_skip(fifo) \
-(void)({ \
+#define	kfifo_skip_count(fifo, count) do { \
 	typeof((fifo) + 1) __tmp = (fifo); \
 	const size_t __recsize = sizeof(*__tmp->rectype); \
 	struct __kfifo *__kfifo = &__tmp->kfifo; \
 	if (__recsize) \
 		__kfifo_skip_r(__kfifo, __recsize); \
 	else \
-		__kfifo->out++; \
-})
+		__kfifo->out += (count); \
+} while(0)
+
+/**
+ * kfifo_skip - skip output data
+ * @fifo: address of the fifo to be used
+ */
+#define	kfifo_skip(fifo)	kfifo_skip_count(fifo, 1)
 
 /**
  * kfifo_peek_len - gets the size of the next fifo record
@@ -790,17 +796,10 @@ __kfifo_int_must_check_helper( \
  * Note that with only one concurrent reader and one concurrent
  * writer, you don't need extra locking to use these macros.
  */
-#define kfifo_dma_out_finish(fifo, len) \
-(void)({ \
-	typeof((fifo) + 1) __tmp = (fifo); \
-	unsigned int __len = (len); \
-	const size_t __recsize = sizeof(*__tmp->rectype); \
-	struct __kfifo *__kfifo = &__tmp->kfifo; \
-	if (__recsize) \
-		__kfifo_skip_r(__kfifo, __recsize); \
-	else \
-		__kfifo->out += __len / sizeof(*__tmp->type); \
-})
+#define kfifo_dma_out_finish(fifo, len) do { \
+	typeof((fifo) + 1) ___tmp = (fifo); \
+	kfifo_skip_count(___tmp, (len) / sizeof(*___tmp->type)); \
+} while (0)
 
 /**
  * kfifo_out_peek - gets some data from the fifo