Re: [PATCH v8 2/8] mfd: simple-mfd-i2c: specify max_register

Alex Elder
2025-07-23T12:42:49Z

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@riscstar.com>
>> Suggested-by: Lee Jones <lee@kernel.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
>>
>