linux-kernel - Re: [PATCH v2 02/11] hwmon: (amc6821) Make reading and writing fan speed limits consistent

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Message-ID: <b556e39d-93f4-42e6-a702-715f5498d3d9@roeck-us.net>
Date: Thu, 4 Jul 2024 10:50:01 -0700
From: Guenter Roeck <linux@...ck-us.net>
To: Quentin Schulz <quentin.schulz@...rry.de>, linux-hwmon@...r.kernel.org
Cc: linux-kernel@...r.kernel.org, Farouk Bouabid <farouk.bouabid@...rry.de>
Subject: Re: [PATCH v2 02/11] hwmon: (amc6821) Make reading and writing fan
 speed limits consistent

On 7/4/24 00:52, Quentin Schulz wrote:
> Hi Guenter,
> 
> On 7/3/24 11:48 PM, Guenter Roeck wrote:
>> On 7/3/24 07:35, Quentin Schulz wrote:
>>> Hi Guenter,
>>>
>>> On 7/1/24 11:23 PM, Guenter Roeck wrote:
>>>> The default value of the maximum fan speed limit register is 0,
>>>> essentially translating to an unlimited fan speed. When reading
>>>> the limit, a value of 0 is reported in this case. However, writing
>>>> a value of 0 results in writing a value of 0xffff into the register,
>>>> which is inconsistent.
>>>>
>>>> To solve the problem, permit writing a limit of 0 for the maximim fan
>>>> speed, effectively translating to "no limit". Write 0 into the register
>>>> if a limit value of 0 is written. Otherwise limit the range to
>>>> <1..6000000> and write 1..0xffff into the register. This ensures that
>>>> reading and writing from and to a limit register return the same value
>>>> while at the same time not changing reported values when reading the
>>>> speed or limits.
>>>>
>>>> While at it, restrict fan limit writes to non-negative numbers; writing
>>>> a negative limit does not make sense and should be reported instead of
>>>> being corrected.
>>>>
>>>> Signed-off-by: Guenter Roeck <linux@...ck-us.net>
>>>> ---
>>>> v2: Do not accept negative fan speed values
>>>>      Display fan speed and speed limit as 0 if register value is 0
>>>>      (instead of 6000000), as in original code.
>>>>      Only permit writing 0 (unlimited) for the maximum fan speed.
>>>>
>>>>   drivers/hwmon/amc6821.c | 13 +++++++++----
>>>>   1 file changed, 9 insertions(+), 4 deletions(-)
>>>>
>>>> diff --git a/drivers/hwmon/amc6821.c b/drivers/hwmon/amc6821.c
>>>> index eb2d5592a41a..9c19d4d278ec 100644
>>>> --- a/drivers/hwmon/amc6821.c
>>>> +++ b/drivers/hwmon/amc6821.c
>>>> @@ -617,15 +617,20 @@ static ssize_t fan_store(struct device *dev, struct device_attribute *attr,
>>>>   {
>>>>       struct amc6821_data *data = dev_get_drvdata(dev);
>>>>       struct i2c_client *client = data->client;
>>>> -    long val;
>>>> +    unsigned long val;
>>>>       int ix = to_sensor_dev_attr(attr)->index;
>>>> -    int ret = kstrtol(buf, 10, &val);
>>>> +    int ret = kstrtoul(buf, 10, &val);
>>>>       if (ret)
>>>>           return ret;
>>>> -    val = 1 > val ? 0xFFFF : 6000000/val;
>>>> +
>>>> +    /* The minimum fan speed must not be unlimited (0) */
>>>> +    if (ix == IDX_FAN1_MIN && !val)
>>>> +        return -EINVAL;
>>>> +
>>>> +    val = val > 0 ? 6000000 / clamp_val(val, 1, 6000000) : 0;
>>>
>>> I'm wondering if we shouldn't check !val for min after this line instead? Otherwise we allow 6000001+RPM speeds... which is technically unlimited.
>>>
>>
>> If ix == IDX_FAN1_MIN, val must be positive because of the check above.
>> The expression "6000000 / clamp_val(val, 1, 6000000)" is therefore always
>> positive as well because val is clamped. Its minimum result would be
>> 6000000/6000000 = 1. The alternate case of the ternary expression would
>> never hit because it is guaranteed that val > 0. Am I missing something ?
>>
> 
> No, I misread the code and I didn't see the clamp_val, which means we cannot have the denominator be > 6000000, meaning val cannot be 0 after that line (well, except if it is 0 **before** already).
> 
> So no, just brain fart.
> 
> Also, we probably could swap clamp_val(val, 1, 6000000) for min(val, 6000000) as val > 0 because of the ternary operator condition. But that's nothing important nor interesting.
> 

Good point. I may do that if I have to send v4 (if I remember), but for now the
existing code should be good enough.

Thanks,
Guenter