linux-kernel - [PATCH] regcache: Push async I/O request down into the rbtree cache

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Message-Id: <20230719-regcache-async-rbtree-v1-1-b03d30cf1daf@kernel.org>
Date:   Wed, 19 Jul 2023 00:30:40 +0100
From:   Mark Brown <broonie@...nel.org>
To:     Charles Keepax <ckeepax@...nsource.cirrus.com>
Cc:     linux-kernel@...r.kernel.org, Mark Brown <broonie@...nel.org>
Subject: [PATCH] regcache: Push async I/O request down into the rbtree
 cache

Currently the regcache core unconditionally enables async I/O for all cache
types, causing problems for the maple tree cache which dynamically allocates
the buffers used to write registers to the device since async requires the
buffers to be kept around until the I/O has been completed.

This use of async I/O is mainly for the rbtree cache which stores data in
a format directly usable for regmap_raw_write(), though there is a special
case for single register writes which would also have allowed it to be used
with the flat cache. It is a bit of a landmine for other caches since it
implicitly converts sync operations to async, and with modern hardware it
is not clear that async I/O is actually a performance win as shown by the
performance work David Jander did with SPI. In multi core systems the cost
of managing concurrency ends up swamping the performance benefit and almost
all modern systems are multi core.

Address this by pushing the enablement of async I/O down into the rbtree
cache where it is actively used, avoiding surprises for other cache
implementations.

Reported-by: Charles Keepax <ckeepax@...nsource.cirrus.com>
Fixes: bfa0b38c1483 ("regmap: maple: Implement block sync for the maple tree cache")
Signed-off-by: Mark Brown <broonie@...nel.org>
---
 drivers/base/regmap/regcache-rbtree.c | 4 ++++
 drivers/base/regmap/regcache.c        | 3 ---
 2 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/drivers/base/regmap/regcache-rbtree.c b/drivers/base/regmap/regcache-rbtree.c
index fabf87058d80..584bcc55f56e 100644
--- a/drivers/base/regmap/regcache-rbtree.c
+++ b/drivers/base/regmap/regcache-rbtree.c
@@ -471,6 +471,8 @@ static int regcache_rbtree_sync(struct regmap *map, unsigned int min,
 	unsigned int start, end;
 	int ret;

+	map->async = true;
+
 	rbtree_ctx = map->cache;
 	for (node = rb_first(&rbtree_ctx->root); node; node = rb_next(node)) {
 		rbnode = rb_entry(node, struct regcache_rbtree_node, node);
@@ -499,6 +501,8 @@ static int regcache_rbtree_sync(struct regmap *map, unsigned int min,
 			return ret;
 	}

+	map->async = false;
+
 	return regmap_async_complete(map);
 }

diff --git a/drivers/base/regmap/regcache.c b/drivers/base/regmap/regcache.c
index 28bc3ae9458a..7d3e47436056 100644
--- a/drivers/base/regmap/regcache.c
+++ b/drivers/base/regmap/regcache.c
@@ -368,8 +368,6 @@ int regcache_sync(struct regmap *map)
 	if (!map->cache_dirty)
 		goto out;

-	map->async = true;
-
 	/* Apply any patch first */
 	map->cache_bypass = true;
 	for (i = 0; i < map->patch_regs; i++) {
@@ -392,7 +390,6 @@ int regcache_sync(struct regmap *map)

 out:
 	/* Restore the bypass state */
-	map->async = false;
 	map->cache_bypass = bypass;
 	map->no_sync_defaults = false;
 	map->unlock(map->lock_arg);

---
base-commit: fdf0eaf11452d72945af31804e2a1048ee1b574c
change-id: 20230718-regcache-async-rbtree-dcce23303176

Best regards,
-- 
Mark Brown <broonie@...nel.org>