block: rename and document BLK_DEF_MAX_SECTORS

Give BLK_DEF_MAX_SECTORS a _CAP postfix and document what it is used for.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20231227092305.279567-5-hch@lst.deSigned-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-settings.c

@@ -139,7 +139,7 @@ void blk_queue_max_hw_sectors(struct request_queue *q, unsigned int max_hw_secto
 	if (limits->max_user_sectors)
 		max_sectors = min(max_sectors, limits->max_user_sectors);
 	else
-		max_sectors = min(max_sectors, BLK_DEF_MAX_SECTORS);
+		max_sectors = min(max_sectors, BLK_DEF_MAX_SECTORS_CAP);
 
 	max_sectors = round_down(max_sectors,
 				 limits->logical_block_size >> SECTOR_SHIFT);

block/blk-sysfs.c

@@ -241,7 +241,7 @@ queue_max_sectors_store(struct request_queue *q, const char *page, size_t count)
 	if (max_sectors_kb == 0) {
 		q->limits.max_user_sectors = 0;
 		max_sectors_kb = min(max_hw_sectors_kb,
-				     BLK_DEF_MAX_SECTORS >> 1);
+				     BLK_DEF_MAX_SECTORS_CAP >> 1);
 	} else {
 		if (max_sectors_kb > max_hw_sectors_kb ||
 		    max_sectors_kb < page_kb)

drivers/scsi/sd.c

@@ -3500,7 +3500,7 @@ static int sd_revalidate_disk(struct gendisk *disk)
 	} else {
 		q->limits.io_opt = 0;
 		rw_max = min_not_zero(logical_to_sectors(sdp, dev_max),
-				      (sector_t)BLK_DEF_MAX_SECTORS);
+				      (sector_t)BLK_DEF_MAX_SECTORS_CAP);
 	}
 
 	/*

include/linux/blkdev.h

@@ -1056,7 +1056,14 @@ enum blk_default_limits {
 	BLK_SEG_BOUNDARY_MASK	= 0xFFFFFFFFUL,
 };
 
-#define BLK_DEF_MAX_SECTORS 2560u
+/*
+ * Default upper limit for the software max_sectors limit used for
+ * regular file system I/O.  This can be increased through sysfs.
+ *
+ * Not to be confused with the max_hw_sector limit that is entirely
+ * controlled by the driver, usually based on hardware limits.
+ */
+#define BLK_DEF_MAX_SECTORS_CAP	2560u
 
 static inline unsigned long queue_segment_boundary(const struct request_queue *q)
 {