r5c_state have 4 states:
* no-cache;
* write-through (write journal only);
* write-back (w/ write cache);
* cache-broken (journal missing or Faulty)

When there is functional write cache, r5c_state is a knob to
switch between write-back and write-through.

When the journal device is broken, the raid array is forced
in readonly mode. In this case, r5c_state can be used to
remove "journal feature", and thus make the array read-write
without journal. By writing into r5c_cache_mode, the array
can transit from cache-broken to no-cache, which removes
journal feature for the array.

To remove the journal feature:
- When journal fails, the raid array is forced readonly mode
  (enforced by kernel)
- User uses the new interface to remove journal (writing 0
  to r5c_state, I will add a mdadm option for that later)
- User forces array read-write;
- Kernel updates superblock and array can run read/write.

Signed-off-by: Song Liu <redacted>
---
 drivers/md/raid5-cache.c | 58 ++++++++++++++++++++++++++++++++++++++++++++++++
 drivers/md/raid5.c       |  1 +
 drivers/md/raid5.h       |  1 +
 3 files changed, 60 insertions(+)

diff --git a/drivers/md/raid5-cache.c b/drivers/md/raid5-cache.c
index 2774f93..b19024c 100644
--- a/drivers/md/raid5-cache.c
+++ b/drivers/md/raid5-cache.c

@@ -57,6 +57,8 @@ enum r5c_state {
 	R5C_STATE_CACHE_BROKEN = 3,
 };
 
+static char *r5c_state_str[] = {"no-cache", "write-through",
+				"write-back", "cache-broken"};
 /*
  * raid5 cache state machine
  *

@@ -1519,6 +1521,62 @@ int r5c_flush_cache(struct r5conf *conf, int num)
 	return count;
 }
 
+ssize_t r5c_state_show(struct mddev *mddev, char *page)
+{
+	struct r5conf *conf = mddev->private;
+	int val = 0;
+	int ret = 0;
+
+	if (conf->log)
+		val = conf->log->r5c_state;
+	else if (test_bit(MD_HAS_JOURNAL, &mddev->flags))
+		val = R5C_STATE_CACHE_BROKEN;
+	ret += snprintf(page, PAGE_SIZE - ret, "%d: %s\n",
+			val, r5c_state_str[val]);