block: add ability to flag write back caching on a device

Add an internal helper and flag for setting whether a queue has
write back caching, or write through (or none). Add a sysfs file
to show this as well, and make it changeable from user space.

This will replace the (awkward) blk_queue_flush() interface that
drivers currently use to inform the block layer of write cache state
and capabilities.

Signed-off-by: Jens Axboe <axboe@fb.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>

1 files changed, 39 insertions, 0 deletions

diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
index 995b58d46ed1..99205965f559 100644
--- a/block/blk-sysfs.c
+++ b/block/blk-sysfs.c
@@ -347,6 +347,38 @@ static ssize_t queue_poll_store(struct request_queue *q, const char *page,
         return ret;
 }
 
+static ssize_t queue_wc_show(struct request_queue *q, char *page)
+{
+        if (test_bit(QUEUE_FLAG_WC, &q->queue_flags))
+                return sprintf(page, "write back\n");
+
+        return sprintf(page, "write through\n");
+}
+
+static ssize_t queue_wc_store(struct request_queue *q, const char *page,
+                              size_t count)
+{
+        int set = -1;
+
+        if (!strncmp(page, "write back", 10))
+                set = 1;
+        else if (!strncmp(page, "write through", 13) ||
+                 !strncmp(page, "none", 4))
+                set = 0;
+
+        if (set == -1)
+                return -EINVAL;
+
+        spin_lock_irq(q->queue_lock);
+        if (set)
+                queue_flag_set(QUEUE_FLAG_WC, q);
+        else
+                queue_flag_clear(QUEUE_FLAG_WC, q);
+        spin_unlock_irq(q->queue_lock);
+
+        return count;
+}
+
 static struct queue_sysfs_entry queue_requests_entry = {
         .attr = {.name = "nr_requests", .mode = S_IRUGO | S_IWUSR },
         .show = queue_requests_show,
@@ -478,6 +510,12 @@ static struct queue_sysfs_entry queue_poll_entry = {
         .store = queue_poll_store,
 };
 
+static struct queue_sysfs_entry queue_wc_entry = {
+        .attr = {.name = "write_cache", .mode = S_IRUGO | S_IWUSR },
+        .show = queue_wc_show,
+        .store = queue_wc_store,
+};
+
 static struct attribute *default_attrs[] = {
         &queue_requests_entry.attr,
         &queue_ra_entry.attr,
@@ -503,6 +541,7 @@ static struct attribute *default_attrs[] = {
         &queue_iostats_entry.attr,
         &queue_random_entry.attr,
         &queue_poll_entry.attr,
+        &queue_wc_entry.attr,
         NULL,
 };