[PATCH printk v4 05/14] printk: ringbuffer: Clarify special lpos values

John Ogness
2024-02-07T13:41:19Z

For empty line records, no data blocks are created. Instead,
these valid records are identified by special logical position
values (in fields of @prb_desc.text_blk_lpos).

Currently the macro NO_LPOS is used for empty line records.
This name is confusing because it does not imply _why_ there is
no data block.

Rename NO_LPOS to EMPTY_LINE_LPOS so that it is clear why there
is no data block.

Also add comments explaining the use of EMPTY_LINE_LPOS as well
as clarification to the values used to represent data-less
blocks.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
---
kernel/printk/printk_ringbuffer.c | 20 ++++++++++++++++----
kernel/printk/printk_ringbuffer.h | 16 +++++++++++++++-
2 files changed, 31 insertions(+), 5 deletions(-)

diff --git a/kernel/printk/printk_ringbuffer.c b/kernel/printk/printk_ringbuffer.c
index 04c26cca546f..244d991ffd73 100644
--- a/kernel/printk/printk_ringbuffer.c
+++ b/kernel/printk/printk_ringbuffer.c
@@ -1034,9 +1034,13 @@ static char *data_alloc(struct printk_ringbuffer *rb, unsigned int size,
unsigned long next_lpos;

if (size == 0) {
- /* Specify a data-less block. */
- blk_lpos->begin = NO_LPOS;
- blk_lpos->next = NO_LPOS;
+ /*
+ * Data blocks are not created for empty lines. Instead, the
+ * reader will recognize these special lpos values and handle
+ * it appropriately.
+ */
+ blk_lpos->begin = EMPTY_LINE_LPOS;
+ blk_lpos->next = EMPTY_LINE_LPOS;
return NULL;
}

@@ -1214,10 +1218,18 @@ static const char *get_data(struct prb_data_ring *data_ring,

/* Data-less data block description. */
if (BLK_DATALESS(blk_lpos)) {
- if (blk_lpos->begin == NO_LPOS && blk_lpos->next == NO_LPOS) {
+ /*
+ * Records that are just empty lines are also valid, even
+ * though they do not have a data block. For such records
+ * explicitly return empty string data to signify success.
+ */
+ if (blk_lpos->begin == EMPTY_LINE_LPOS &&
+ blk_lpos->next == EMPTY_LINE_LPOS) {
*data_size = 0;
return "";
}
+
+ /* Data lost, invalid, or otherwise unavailable. */
return NULL;
}

diff --git a/kernel/printk/printk_ringbuffer.h b/kernel/printk/printk_ringbuffer.h
index 2d948cc82b5b..d49460f7578e 100644
--- a/kernel/printk/printk_ringbuffer.h
+++ b/kernel/printk/printk_ringbuffer.h
@@ -127,8 +127,22 @@ enum desc_state {
#define DESC_SV(id, state) (((unsigned long)state << DESC_FLAGS_SHIFT) | id)
#define DESC_ID_MASK (~DESC_FLAGS_MASK)
#define DESC_ID(sv) ((sv) & DESC_ID_MASK)
+
+/*
+ * Special data block logical position values (for fields of
+ * @prb_desc.text_blk_lpos).
+ *
+ * - Bit0 is used to identify if the record has no data block. (Implemented in
+ * the LPOS_DATALESS() macro.)
+ *
+ * - Bit1 specifies the reason for not having a data block.
+ *
+ * These special values could never be real lpos values because of the
+ * meta data and alignment padding of data blocks. (See to_blk_size() for
+ * details.)
+ */
#define FAILED_LPOS 0x1
-#define NO_LPOS 0x3
+#define EMPTY_LINE_LPOS 0x3

#define FAILED_BLK_LPOS \
{ \
--
2.39.2