kfifo: add kfifo_out_linear{,_ptr}()

These are helpers which are going to be used in the serial layer. We
need a wrapper around kfifo which provides us with a tail (sometimes
"tail" offset, sometimes a pointer) to the kfifo data. And which returns
count of available data -- but not larger than to the end of the buffer
(hence _linear in the names). I.e. something like CIRC_CNT_TO_END() in
the legacy circ_buf.

This patch adds such two helpers.

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/20240405060826.2521-4-jirislaby@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/kfifo.h

@@ -827,6 +827,63 @@ __kfifo_uint_must_check_helper( \
 }) \
 )
 
+/**
+ * kfifo_out_linear - gets a tail of/offset to available data
+ * @fifo: address of the fifo to be used
+ * @tail: pointer to an unsigned int to store the value of tail
+ * @n: max. number of elements to point at
+ *
+ * This macro obtains the offset (tail) to the available data in the fifo
+ * buffer and returns the
+ * numbers of elements available. It returns the available count till the end
+ * of data or till the end of the buffer. So that it can be used for linear
+ * data processing (like memcpy() of (@fifo->data + @tail) with count
+ * returned).
+ *
+ * Note that with only one concurrent reader and one concurrent
+ * writer, you don't need extra locking to use these macro.
+ */
+#define kfifo_out_linear(fifo, tail, n) \
+__kfifo_uint_must_check_helper( \
+({ \
+	typeof((fifo) + 1) __tmp = (fifo); \
+	unsigned int *__tail = (tail); \
+	unsigned long __n = (n); \
+	const size_t __recsize = sizeof(*__tmp->rectype); \
+	struct __kfifo *__kfifo = &__tmp->kfifo; \
+	(__recsize) ? \
+	__kfifo_out_linear_r(__kfifo, __tail, __n, __recsize) : \
+	__kfifo_out_linear(__kfifo, __tail, __n); \
+}) \
+)
+
+/**
+ * kfifo_out_linear_ptr - gets a pointer to the available data
+ * @fifo: address of the fifo to be used
+ * @ptr: pointer to data to store the pointer to tail
+ * @n: max. number of elements to point at
+ *
+ * Similarly to kfifo_out_linear(), this macro obtains the pointer to the
+ * available data in the fifo buffer and returns the numbers of elements
+ * available. It returns the available count till the end of available data or
+ * till the end of the buffer. So that it can be used for linear data
+ * processing (like memcpy() of @ptr with count returned).
+ *
+ * Note that with only one concurrent reader and one concurrent
+ * writer, you don't need extra locking to use these macro.
+ */
+#define kfifo_out_linear_ptr(fifo, ptr, n) \
+__kfifo_uint_must_check_helper( \
+({ \
+	typeof((fifo) + 1) ___tmp = (fifo); \
+	unsigned int ___tail; \
+	unsigned int ___n = kfifo_out_linear(___tmp, &___tail, (n)); \
+	*(ptr) = ___tmp->kfifo.data + ___tail * kfifo_esize(___tmp); \
+	___n; \
+}) \
+)
+
+
 extern int __kfifo_alloc(struct __kfifo *fifo, unsigned int size,
 	size_t esize, gfp_t gfp_mask);
 
@@ -856,6 +913,9 @@ extern unsigned int __kfifo_dma_out_prepare(struct __kfifo *fifo,
 extern unsigned int __kfifo_out_peek(struct __kfifo *fifo,
 	void *buf, unsigned int len);
 
+extern unsigned int __kfifo_out_linear(struct __kfifo *fifo,
+	unsigned int *tail, unsigned int n);
+
 extern unsigned int __kfifo_in_r(struct __kfifo *fifo,
 	const void *buf, unsigned int len, size_t recsize);
 
@@ -885,6 +945,9 @@ extern void __kfifo_skip_r(struct __kfifo *fifo, size_t recsize);
 extern unsigned int __kfifo_out_peek_r(struct __kfifo *fifo,
 	void *buf, unsigned int len, size_t recsize);
 
+extern unsigned int __kfifo_out_linear_r(struct __kfifo *fifo,
+	unsigned int *tail, unsigned int n, size_t recsize);
+
 extern unsigned int __kfifo_max_r(unsigned int len, size_t recsize);
 
 #endif

lib/kfifo.c

@@ -163,6 +163,19 @@ unsigned int __kfifo_out_peek(struct __kfifo *fifo,
 }
 EXPORT_SYMBOL(__kfifo_out_peek);
 
+unsigned int __kfifo_out_linear(struct __kfifo *fifo,
+		unsigned int *tail, unsigned int n)
+{
+	unsigned int size = fifo->mask + 1;
+	unsigned int off = fifo->out & fifo->mask;
+
+	if (tail)
+		*tail = off;
+
+	return min3(n, fifo->in - fifo->out, size - off);
+}
+EXPORT_SYMBOL(__kfifo_out_linear);
+
 unsigned int __kfifo_out(struct __kfifo *fifo,
 		void *buf, unsigned int len)
 {
@@ -473,6 +486,19 @@ unsigned int __kfifo_out_peek_r(struct __kfifo *fifo, void *buf,
 }
 EXPORT_SYMBOL(__kfifo_out_peek_r);
 
+unsigned int __kfifo_out_linear_r(struct __kfifo *fifo,
+		unsigned int *tail, unsigned int n, size_t recsize)
+{
+	if (fifo->in == fifo->out)
+		return 0;
+
+	if (tail)
+		*tail = fifo->out + recsize;
+
+	return min(n, __kfifo_peek_n(fifo, recsize));
+}
+EXPORT_SYMBOL(__kfifo_out_linear_r);
+
 unsigned int __kfifo_out_r(struct __kfifo *fifo, void *buf,
 		unsigned int len, size_t recsize)
 {