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

[Sander Vanheule <sander@svanheule.net>]

Hi,

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

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.

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.

Best,
Sander