Message-ID: <efa85637-7a6b-475c-b7ca-3a3c8fc3429b@roeck-us.net>
Date: Wed, 26 Mar 2025 10:42:03 -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 09:59, Naresh Solanki wrote:
> Hi Guenter,
> 
> On Wed, 26 Mar 2025 at 22:19, Guenter Roeck <linux@...ck-us.net> wrote:
>>
>> 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.
> rpm_range is pre-initialized to 4000 RPM [1]
> I can also init target_rpm[] to 4000 RPM as default along with above init.
> 
Yes.

> But still there might be a case wherein DT doesn't provide max-rpm but
> target-rpm is set to greater than 4000 RPM
> Thus there will be a need to check to cover this kind of scenario.
> 
> Please let me know your thoughts & will implement that.
> 

You'll need to validate target_rpm against max-rpm either way,
to make sure that target_rpm <= max_rpm.

Thanks,
Guenter

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