linux-kernel - Re: [PATCH v4 1/1] mfd: Add I2C based System Configuaration (SYSCON) access

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Date:   Wed, 01 Jul 2020 00:32:13 +0200
From:   Michael Walle <michael@...le.cc>
To:     Lee Jones <lee.jones@...aro.org>
Cc:     robh+dt@...nel.org, broonie@...nel.org, gregkh@...uxfoundation.org,
        andriy.shevchenko@...ux.intel.com, devicetree@...r.kernel.org,
        linus.walleij@...aro.org, bgolaszewski@...libre.com, arnd@...db.de,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 1/1] mfd: Add I2C based System Configuaration (SYSCON)
 access

Hi Lee,

Am 2020-06-30 11:16, schrieb Michael Walle:
> I'm just trying to use this for my sl28 driver. Some remarks, see 
> below.
> 
> Am 2020-06-22 09:51, schrieb Lee Jones:
>> The existing SYSCON implementation only supports MMIO (memory mapped)
>> accesses, facilitated by Regmap.  This extends support for registers
>> held behind I2C busses.
>> 
>> Signed-off-by: Lee Jones <lee.jones@...aro.org>
>> ---
>> Changelog:
>> 
>> v3 => v4
>>   - Add ability to provide a non-default Regmap configuration
>> 
>> v2 => v3
>>   - Change 'is CONFIG' present check to include loadable modules
>>     - s/#ifdef CONFIG_MFD_SYSCON_I2C/#if 
>> IS_ENABLED(CONFIG_MFD_SYSCON_I2C)/
>> 
>> v1 => v2
>>   - Remove legacy references to OF
>>   - Allow building as a module (fixes h8300 0-day issue)
>> 
>> drivers/mfd/Kconfig            |   7 +++
>>  drivers/mfd/Makefile           |   1 +
>>  drivers/mfd/syscon-i2c.c       | 104 
>> +++++++++++++++++++++++++++++++++
>>  include/linux/mfd/syscon-i2c.h |  36 ++++++++++++
>>  4 files changed, 148 insertions(+)
>>  create mode 100644 drivers/mfd/syscon-i2c.c
>>  create mode 100644 include/linux/mfd/syscon-i2c.h
>> 
> 
> [..]
> 
>> +static struct regmap *syscon_i2c_get_regmap(struct i2c_client 
>> *client,
>> +					    struct regmap_config *regmap_config)
>> +{
>> +	struct device *dev = &client->dev;
>> +	struct syscon *entry, *syscon = NULL;
>> +
>> +	spin_lock(&syscon_i2c_list_slock);
>> +
>> +	list_for_each_entry(entry, &syscon_i2c_list, list)
>> +		if (entry->dev == dev) {
>> +			syscon = entry;
>> +			break;
>> +		}
>> +
>> +	spin_unlock(&syscon_i2c_list_slock);
>> +
>> +	if (!syscon)
>> +		syscon = syscon_i2c_register(client, regmap_config);
>> +
>> +	if (IS_ERR(syscon))
>> +		return ERR_CAST(syscon);
>> +
>> +	return syscon->regmap;
>> +}
>> +
>> +struct regmap *syscon_i2c_to_regmap_config(struct i2c_client *client,
>> +					   struct regmap_config *regmap_config)
>> +{
>> +	return syscon_i2c_get_regmap(client, regmap_config);
>> +}
>> +EXPORT_SYMBOL_GPL(syscon_i2c_to_regmap_config);
>> +
>> +struct regmap *syscon_i2c_to_regmap(struct i2c_client *client)
>> +{
>> +	return syscon_i2c_get_regmap(client, &syscon_i2c_regmap_config);
>> +}
>> +EXPORT_SYMBOL_GPL(syscon_i2c_to_regmap);
> 
> What do you think about
> 
> struct regmap *syscon_i2c_to_regmap(struct device *dev)
> {
> 	struct i2c_client *client = i2c_verify_client(dev);
> 
> 	if (!client)
> 		return ERR_PTR(-EINVAL);
> 
> 	return syscon_i2c_get_regmap(client, &syscon_i2c_regmap_config);
> }
> 
> Or even move it to syscon_i2c_get_regmap().
> 
> This way, (a) a driver doesn't have to use "#include <linux/i2c.h>" 
> just
> to call to_i2c_client() (or i2c_verify_client()) and (b) you won't do 
> it
> all over again in all sub drivers.
> 
> So you could just do a
>   regmap = syscon_i2c_to_regmap(pdev->dev.parent);
> 
> I've also noticed that the mmio syscon uses device_node as parameter. 
> What
> was the reason to divert from that? Just curious.

How is this supposed to be used?

I had something like the following in mind:

&i2c {
   cpld@4a {
     compatible = "simple-mfd";
     reg = <0x4a>;

     gpio@4 {
       compatible = "vendor,gpio";
       reg = <0x4>;
     };
   };
};

But I think the childen are not enumerated if its an I2C device. And
the actual i2c driver is also missing.

-michael