linux-kernel - Re: [PATCH] hwmon: (max6639) : Allow setting target RPM

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Message-ID: <be099cf4-338b-45c8-b0d3-24b2cefe386e@roeck-us.net>
Date: Wed, 26 Mar 2025 09:49:04 -0700
From: Guenter Roeck <linux@...ck-us.net>
To: Naresh Solanki <naresh.solanki@...ements.com>
Cc: Jean Delvare <jdelvare@...e.com>, linux-hwmon@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] hwmon: (max6639) : Allow setting target RPM

On 3/26/25 09:36, Naresh Solanki wrote:
> Hi Guenter,
> 
> On Tue, 25 Mar 2025 at 05:00, Guenter Roeck <linux@...ck-us.net> wrote:
>>
>> On 3/24/25 11:57, Your Name wrote:
>>> From: Naresh Solanki <naresh.solanki@...ements.com>
>>>
>>> Currently, during startup, the fan is set to its maximum RPM by default,
>>> which may not be suitable for all use cases.
>>> This patch introduces support for specifying a target RPM via the Device
>>> Tree property "target-rpm".
>>>
>>> Changes:
>>> - Added `target_rpm` field to `max6639_data` structure to store the
>>>     target RPM for each fan channel.
>>> - Modified `max6639_probe_child_from_dt()` to read the `"target-rpm"`
>>>     property from the Device Tree and set `target_rpm` accordingly.
>>> - Updated `max6639_init_client()` to use `target_rpm` to compute the
>>>     initial PWM duty cycle instead of defaulting to full speed (120/120).
>>>
>>> Behavior:
>>> - If `"target-rpm"` is specified, the fan speed is set accordingly.
>>> - If `"target-rpm"` is not specified, the previous behavior (full speed
>>>     at startup) is retained.
>>>
>>
>> Unless I am missing something, that is not really correct. See below.
>>
>>> This allows better control over fan speed during system initialization.
>>>
>>> Signed-off-by: Naresh Solanki <naresh.solanki@...ements.com>
>>> ---
>>>    drivers/hwmon/max6639.c | 15 ++++++++++++---
>>>    1 file changed, 12 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/drivers/hwmon/max6639.c b/drivers/hwmon/max6639.c
>>> index 32b4d54b2076..ca8a8f58d133 100644
>>> --- a/drivers/hwmon/max6639.c
>>> +++ b/drivers/hwmon/max6639.c
>>> @@ -80,6 +80,7 @@ struct max6639_data {
>>>        /* Register values initialized only once */
>>>        u8 ppr[MAX6639_NUM_CHANNELS];   /* Pulses per rotation 0..3 for 1..4 ppr */
>>>        u8 rpm_range[MAX6639_NUM_CHANNELS]; /* Index in above rpm_ranges table */
>>> +     u32 target_rpm[MAX6639_NUM_CHANNELS];
>>>
>>>        /* Optional regulator for FAN supply */
>>>        struct regulator *reg;
>>> @@ -560,8 +561,14 @@ static int max6639_probe_child_from_dt(struct i2c_client *client,
>>>        }
>>>
>>
>> target_rpm[] is 0 here.
>>
>>>        err = of_property_read_u32(child, "max-rpm", &val);
>>> -     if (!err)
>>> +     if (!err) {
>>>                data->rpm_range[i] = rpm_range_to_reg(val);
>>> +             data->target_rpm[i] = val;
>>> +     }
>>
>> If there is no max-rpm property, or if there is no devicetree support,
>> target_rpm[i] is still 0.
>>
>>> +
>>> +     err = of_property_read_u32(child, "target-rpm", &val);
>>> +     if (!err)
>>> +             data->target_rpm[i] = val;
>>
>> If there is neither max-rpm nor target-rpm, target_rpm[i] is still 0.
>>
>>>
>>>        return 0;
>>>    }
>>> @@ -573,6 +580,7 @@ static int max6639_init_client(struct i2c_client *client,
>>>        const struct device_node *np = dev->of_node;
>>>        struct device_node *child;
>>>        int i, err;
>>> +     u8 target_duty;
>>>
>>>        /* Reset chip to default values, see below for GCONFIG setup */
>>>        err = regmap_write(data->regmap, MAX6639_REG_GCONFIG, MAX6639_GCONFIG_POR);
>>> @@ -639,8 +647,9 @@ static int max6639_init_client(struct i2c_client *client,
>>>                if (err)
>>>                        return err;
>>>
>>> -             /* PWM 120/120 (i.e. 100%) */
>>> -             err = regmap_write(data->regmap, MAX6639_REG_TARGTDUTY(i), 120);
>>> +             /* Set PWM based on target RPM if specified */
>>> +             target_duty = 120 * data->target_rpm[i] / rpm_ranges[data->rpm_range[i]];
>>
>> If there is no devicetree support, or if neither max-rpm nor target-rpm are
>> provided, target_duty will be 0, and the fans will stop.
>>
>> Maybe my interpretation is wrong, but I think both target_rpm[] and rpm_range[]
>> will need to be initialized. Also, it seems to me that there will need to be an
>> upper bound for target_rpm[]; without it, it is possible that target_duty > 120,
>> which would probably not be a good idea.
> Yes you're right. I missed it in my analysis.
> 
> Here is the logic that would address:
>                  target_rpm = 120;
>                  /* Set PWM based on target RPM if specified */
>                  if (data->target_rpm[i] != 0 &&
>                      data->target_rpm[i]  <= rpm_ranges[data->rpm_range[i]]) {
> 
>                          target_duty = 120 * data->target_rpm[i] /
> rpm_ranges[data->rpm_range[i]];
>                  }
> 
> Please let me know your thoughts & suggestions.
> 

I would prefer if target_rpm[] and rpm_range[] were pre-initialized with default
values in the probe function. That would avoid runtime checks.

Thanks,
Guenter