Re: [PATCH] drivers: regmap: bugfix in regcache-rbtree.c

[David Jander <david.jander@protonic.nl>]

Hi Dimitris,

On Wed, 21 Aug 2013 15:02:35 +0200
David Jander <david@protonic.nl> wrote:

> The functionality of rbtree_ctx->cached_rbnode is broken. Remove it to
> avoid hitting the wrong rbnode when locating a register.
> rbnode register ranges can overlap, which is not a problem as long as
> every lookup for a register returns the same rbnode. Therefor we need
> to start searching from the top of the rb-tree _always_.

Some clarification to this patch. I want to be careful in "attacking" such a
core function that is used in quite a lot of places around the kernel, and it
strikes me that this has been gone unnoticed so far.

Here's the explanation:

1. If a driver initializes a regmap with a RB-tree cache, and starts writing to
registers in some arbitrary order, you might get overlapping rbnodes:
Suppose I have this rb-tree (a real case I happen to have):

# cat /sys/kernel/debug/regmap/1-000a/rbtree
2-19 (24)
4-1b (24)
20-37 (24)
22-39 (24)
3c-53 (24)
100-117 (24)
104-11c (25)
11e-135 (24)
8 nodes, 193 registers, average 24 registers, used 626 bytes

2. Suppose you just added node 20-37 by writing to previously uncached register
20. Now this node would be in rbtree_ctx->cached_rbnode, and would get search
first by the next access. Let's say that next access is register 30, then
rbtree_ctx->cached_rbnode would be a hit on the node 20-37.
Otherwise the hit would be in node 22-39 which is higher up the tree and
should be searched first. Two different possible hits for register 30 with
different contents! That cannot be right, right?

Therefor I dare to declare the rbtree_ctx->cached_rbnode functionality broken,
and removed it.

I stumbled upon this problem while debugging a non-functioning audio codec
(SGTL5000). This patch fixes the problem. Using a flat-cache in sgtl5000.c also
fixes it, so at least if I didn't fix this bug the right way, there sure is a
bug that needs to be fixed in regcache-rbtree.c.

Best regards,

-- 
David Jander
Protonic Holland.