Re: [PATCH v2] pinctrl: mcp23s08: delete regmap reg_defaults to avoid cache sync issues

From: Mike Looijmans

Date: Wed Oct 22 2025 - 10:45:59 EST

On 10/20/25 21:40, Sander Vanheule wrote:
Hi,

On Thu, 2025-10-09 at 16:26 +0300, bigunclemax@xxxxxxxxx wrote:
From: Maksim Kiselev <bigunclemax@xxxxxxxxx>

The probe function does not guarantee that chip registers are in their
default state. Thus using reg_defaults for regmap is incorrect.

---

@@ -82,25 +71,12 @@ const struct regmap_config mcp23x08_regmap = {
  .reg_stride = 1,
  .volatile_table = &mcp23x08_volatile_table,
  .precious_table = &mcp23x08_precious_table,
- .reg_defaults = mcp23x08_defaults,
- .num_reg_defaults = ARRAY_SIZE(mcp23x08_defaults),
  .cache_type = REGCACHE_FLAT,
  .max_register = MCP_OLAT,
  .disable_locking = true, /* mcp->lock protects the regmap */
As Andy mentioned, the problem you will now have to deal with is that your cache
is not initialized at all. Unlike the other cache types, REGCACHE_FLAT will
zero-initialize its cache, perhaps making your cache sync issues worse.

Ouch...

I have access to hardware this week (boards with 2 and 3 of the I2C chips), I'll be able to do some hands-on testing, and report back.

You have two options to initialize the cache properly:
* Provide .num_reg_defaults_raw (= MCP_OLAT + 1). This will give you a warning
on probe about the cache defaults being initialized from hardware.
* Switch to another cache type (REGCACHE_MAPLE), which is aware of (in)valid
cache entries. regmap will then init the cache on the first access to a
register.

Using REGCACHE_MAPLE sounds like the obvious solution to me. That's what most other drivers use.


You could also combine the two, like the Cypress driver Andy referred to
(pinctrl-cy8c95x0.c). In that case you get cache loading at init, instead of at
first use, but without the risk of missing something.


--
Mike Looijmans