linux-kernel - Re: [PATCH v8 2/8] mfd: simple-mfd-i2c: specify max

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Message-ID: <877dcf99-107e-4d96-8790-6608976d13ca@riscstar.com>
Date: Wed, 23 Jul 2025 07:42:45 -0500
From: Alex Elder <elder@...cstar.com>
To: Lee Jones <lee@...nel.org>
Cc: lgirdwood@...il.com, broonie@...nel.org, alexandre.belloni@...tlin.com,
 robh@...nel.org, krzk+dt@...nel.org, conor+dt@...nel.org, mat.jonczyk@...pl,
 dlan@...too.org, paul.walmsley@...ive.com, palmer@...belt.com,
 aou@...s.berkeley.edu, alex@...ti.fr, troymitchell988@...il.com,
 guodong@...cstar.com, linux-rtc@...r.kernel.org, devicetree@...r.kernel.org,
 linux-riscv@...ts.infradead.org, spacemit@...ts.linux.dev,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v8 2/8] mfd: simple-mfd-i2c: specify max_register

On 7/23/25 4:51 AM, Lee Jones wrote:
> On Thu, 10 Jul 2025, Alex Elder wrote:
> 
>> All devices supported by simple MFD use the same 8-bit register 8-bit
>> value regmap configuration.  There is an option available for a device
>> to specify a custom configuration, but no existing device uses it.
>>
>> Rather than specify a "full" regmap configuration to change only
>> the max_register value, Lee Jones suggested allowing max_register
>> to be specified in the simple_mfd_data structure.  If regmap_config
>> and max_register are both supplied, the max_register field is ignored.
>>
>> Signed-off-by: Alex Elder <elder@...cstar.com>
>> Suggested-by: Lee Jones <lee@...nel.org>
>> ---
>> v8: - Use regmap_config_8r_8v, modifying it if max_register supplied
>>
>>   drivers/mfd/simple-mfd-i2c.c | 8 ++++++--
>>   drivers/mfd/simple-mfd-i2c.h | 3 ++-
>>   2 files changed, 8 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/mfd/simple-mfd-i2c.c b/drivers/mfd/simple-mfd-i2c.c
>> index 22159913bea03..5138aa72140b5 100644
>> --- a/drivers/mfd/simple-mfd-i2c.c
>> +++ b/drivers/mfd/simple-mfd-i2c.c
>> @@ -24,15 +24,16 @@
>>   
>>   #include "simple-mfd-i2c.h"
>>   
>> -static const struct regmap_config regmap_config_8r_8v = {
>> +static struct regmap_config regmap_config_8r_8v = {
>>   	.reg_bits = 8,
>>   	.val_bits = 8,
>> +	/* .max_register can be specified in simple_mfd_data */
> 
> Drop this comment please.
> 
>>   };
>>   
>>   static int simple_mfd_i2c_probe(struct i2c_client *i2c)
>>   {
>>   	const struct simple_mfd_data *simple_mfd_data;
>> -	const struct regmap_config *regmap_config;
>> +	struct regmap_config *regmap_config;
>>   	struct regmap *regmap;
>>   	int ret;
>>   
>> @@ -43,8 +44,11 @@ static int simple_mfd_i2c_probe(struct i2c_client *i2c)
>>   		regmap_config = &regmap_config_8r_8v;
>>   	else
>>   		regmap_config = simple_mfd_data->regmap_config;
>> +	if (simple_mfd_data && !simple_mfd_data->regmap_config)
>> +		regmap_config->max_register = simple_mfd_data->max_register;
> 
> If max_register is set in simple_mfd_data, it should take precedence.

I don't really agree with that.  If simple_mfd_data->regmap_config
is provided, why not use the max_register field already available
there?

This is why I said above that I think this feature doesn't add
much value.  It provides a second way to specify something, but
in the end it complicates the code more than it's worth.

The only time this new simple_mfd_data->max_register field seems
to make sense is if it were the only thing provided (without
simple_mfd_data->regmap_config being supplied).  In that case,
I see the benefit--a null simple_mfd_data->regmap_config means
use regmap_config_8r_8v, and overlay it with the max_register
value.  The new max_register field avoids defining another huge
but mostly empty regmap_config structure.

Anyway, back to your original point:  I said in v7 "If both
are specified, the max_register value is ignored" and I think
that's the simplest.  Specify one or the other--if you want
to define things in regmap_config, then that's where you add
your max_register.  If you like regmap_config_8r_8v but want
to define a max_register value, just provide max_register.

If you insist, I'll do what you say but before I sent another
version I wanted to explain my reasoning.


> if (simple_mfd_data && simple_mfd_data->max_register)
> 	regmap_config->max_register = simple_mfd_data->max_register;
> 
>>   	regmap = devm_regmap_init_i2c(i2c, regmap_config);
>> +	regmap_config->max_register = 0;
> 
> Does max_register definitely have persistence over subsequent calls?

It is a global variable.  Isn't that how they work?  When
it was read-only there was no concern about that, nor about
any possible concurrent access (though I don't think multiple
probes can be using this code at once).

We could allocate a new one each time instead.

I think what I offered in v5 was acceptable.  If you're
willing to accept that I will be happy to keep discussing
(and implementing) the max_register feature.

					-Alex

> 
>>   	if (IS_ERR(regmap))
>>   		return PTR_ERR(regmap);
>>   
>> diff --git a/drivers/mfd/simple-mfd-i2c.h b/drivers/mfd/simple-mfd-i2c.h
>> index 7cb2bdd347d97..ea2a96af8bce4 100644
>> --- a/drivers/mfd/simple-mfd-i2c.h
>> +++ b/drivers/mfd/simple-mfd-i2c.h
>> @@ -24,7 +24,8 @@
>>   #include <linux/regmap.h>
>>   
>>   struct simple_mfd_data {
>> -	const struct regmap_config *regmap_config;
>> +	struct regmap_config *regmap_config;
>> +	unsigned int max_register;	/* Ignored if regmap_config supplied */
>>   	const struct mfd_cell *mfd_cell;
>>   	size_t mfd_cell_size;
>>   };
>> -- 
>> 2.45.2
>>
>