From: John Ogness Date: Mon, 23 Oct 2023 11:11:05 +0000 Subject: [PATCH 05/50] printk: ringbuffer: Clarify special lpos values Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/6.7/older/patches-6.7-rt6.tar.xz 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 Signed-off-by: Sebastian Andrzej Siewior --- kernel/printk/printk_ringbuffer.c | 20 ++++++++++++++++---- kernel/printk/printk_ringbuffer.h | 16 +++++++++++++++- 2 files changed, 31 insertions(+), 5 deletions(-) --- a/kernel/printk/printk_ringbuffer.c +++ b/kernel/printk/printk_ringbuffer.c @@ -1034,9 +1034,13 @@ static char *data_alloc(struct printk_ri 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_d /* 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; } --- 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 \ { \