linux-kernel - Re: [PATCH v2] pinctrl: mcp23s08: delete regmap reg

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Message-ID: <aaa1208a-f11a-4537-94a8-afed77c7fe84@topic.nl>
Date: Fri, 24 Oct 2025 08:54:02 +0200
From: Mike Looijmans <mike.looijmans@...ic.nl>
To: bigunclemax@...il.com
CC: Linus Walleij <linus.walleij@...aro.org>, linux-gpio@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] pinctrl: mcp23s08: delete regmap reg_defaults to avoid
 cache sync issues

On 10/9/25 15:26, bigunclemax@...il.com wrote:
> From: Maksim Kiselev <bigunclemax@...il.com>
>
> The probe function does not guarantee that chip registers are in their
> default state. Thus using reg_defaults for regmap is incorrect.

Did some testing on actual hardware with this patch "as is". As Sander 
mentioned, the REGCACHE_FLAT caching mode assumes all registers are "0" 
initially, which causes similar issues as what we're trying to solve.

I've verified that by setting a bit to "1" in the OLAT register (using 
i2cset) for an unused GPIO line. After a reboot, the kernel resets it to 
"0" because of the caching issue.

I then changed the cache_type to REGCACHE_MAPLE as indicated below, and 
then the kernel doesn't touch that bit any longer (as it should).

So once you've amended the patch with REGCACHE_MAPLE, you have my:

Tested-by: Mike Looijmans <mike.looijmans@...ic.nl>



> ...
>   static const struct regmap_range mcp23x08_volatile_range = {
>   	.range_min = MCP_INTF,
>   	.range_max = MCP_GPIO,
> @@ -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,

Must be REGCACHE_MAPLE


>   	.max_register = MCP_OLAT,
>   	.disable_locking = true, /* mcp->lock protects the regmap */
>   };
>   EXPORT_SYMBOL_GPL(mcp23x08_regmap);
>   
> -static const struct reg_default mcp23x17_defaults[] = {
> -	{.reg = MCP_IODIR << 1,		.def = 0xffff},
> -	{.reg = MCP_IPOL << 1,		.def = 0x0000},
> -	{.reg = MCP_GPINTEN << 1,	.def = 0x0000},
> -	{.reg = MCP_DEFVAL << 1,	.def = 0x0000},
> -	{.reg = MCP_INTCON << 1,	.def = 0x0000},
> -	{.reg = MCP_IOCON << 1,		.def = 0x0000},
> -	{.reg = MCP_GPPU << 1,		.def = 0x0000},
> -	{.reg = MCP_OLAT << 1,		.def = 0x0000},
> -};
> -
>   static const struct regmap_range mcp23x17_volatile_range = {
>   	.range_min = MCP_INTF << 1,
>   	.range_max = MCP_GPIO << 1,
> @@ -129,8 +105,6 @@ const struct regmap_config mcp23x17_regmap = {
>   	.max_register = MCP_OLAT << 1,
>   	.volatile_table = &mcp23x17_volatile_table,
>   	.precious_table = &mcp23x17_precious_table,
> -	.reg_defaults = mcp23x17_defaults,
> -	.num_reg_defaults = ARRAY_SIZE(mcp23x17_defaults),
>   	.cache_type = REGCACHE_FLAT,

Must be REGCACHE_MAPLE


>   	.val_format_endian = REGMAP_ENDIAN_LITTLE,
>   	.disable_locking = true, /* mcp->lock protects the regmap */
> @@ -614,14 +588,6 @@ int mcp23s08_probe_one(struct mcp23s08 *mcp, struct device *dev,
>   ...


-- 
Mike Looijmans
System Expert

TOPIC Embedded Products B.V.
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The Netherlands

T: +31 (0) 499 33 69 69
E: mike.looijmans@...ic.nl
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