printk: ringbuffer: Cleanup reader terminology

With the lockless ringbuffer, it is allowed that multiple
CPUs/contexts write simultaneously into the buffer. This creates
an ambiguity as some writers will finalize sooner.

The documentation for the prb_read functions is not clear as it
refers to "not yet written" and "no data available". Clarify the
return values and language to be in terms of the reader: records
available for reading.
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20240207134103.1357162-9-john.ogness@linutronix.deSigned-off-by: Petr Mladek <pmladek@suse.com>

kernel/printk/printk_ringbuffer.c

@@ -1987,11 +1987,13 @@ u64 prb_first_seq(struct printk_ringbuffer *rb)
 }
 
 /*
- * Non-blocking read of a record. Updates @seq to the last finalized record
- * (which may have no data available).
+ * Non-blocking read of a record.
  *
- * See the description of prb_read_valid() and prb_read_valid_info()
- * for details.
+ * On success @seq is updated to the record that was read and (if provided)
+ * @r and @line_count will contain the read/calculated data.
+ *
+ * On failure @seq is updated to a record that is not yet available to the
+ * reader, but it will be the next record available to the reader.
  */
 static bool _prb_read_valid(struct printk_ringbuffer *rb, u64 *seq,
 			    struct printk_record *r, unsigned int *line_count)
@@ -2010,7 +2012,7 @@ static bool _prb_read_valid(struct printk_ringbuffer *rb, u64 *seq,
 			*seq = tail_seq;
 
 		} else if (err == -ENOENT) {
-			/* Record exists, but no data available. Skip. */
+			/* Record exists, but the data was lost. Skip. */
 			(*seq)++;
 
 		} else {
@@ -2043,7 +2045,7 @@ static bool _prb_read_valid(struct printk_ringbuffer *rb, u64 *seq,
  * On success, the reader must check r->info.seq to see which record was
  * actually read. This allows the reader to detect dropped records.
  *
- * Failure means @seq refers to a not yet written record.
+ * Failure means @seq refers to a record not yet available to the reader.
  */
 bool prb_read_valid(struct printk_ringbuffer *rb, u64 seq,
 		    struct printk_record *r)
@@ -2073,7 +2075,7 @@ bool prb_read_valid(struct printk_ringbuffer *rb, u64 seq,
  * On success, the reader must check info->seq to see which record meta data
  * was actually read. This allows the reader to detect dropped records.
  *
- * Failure means @seq refers to a not yet written record.
+ * Failure means @seq refers to a record not yet available to the reader.
  */
 bool prb_read_valid_info(struct printk_ringbuffer *rb, u64 seq,
 			 struct printk_info *info, unsigned int *line_count)