linux-kernel - Re: [PATCH v6 3/3] hwmon: (adt7475) Add support for configuring initial PWM state

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Message-ID: <5d17aa29-8963-4951-9598-d1b9ca7a10d4@roeck-us.net>
Date: Sun, 21 Jul 2024 22:27:35 -0700
From: Guenter Roeck <linux@...ck-us.net>
To: Chris Packham <chris.packham@...iedtelesis.co.nz>, jdelvare@...e.com,
 robh@...nel.org, krzk+dt@...nel.org, conor+dt@...nel.org, ukleinek@...nel.org
Cc: linux-hwmon@...r.kernel.org, devicetree@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-pwm@...r.kernel.org
Subject: Re: [PATCH v6 3/3] hwmon: (adt7475) Add support for configuring
 initial PWM state

On 7/21/24 21:36, Chris Packham wrote:
> 
> On 22/07/24 16:27, Guenter Roeck wrote:
>> On 7/21/24 21:09, Chris Packham wrote:
>>>
>>> On 22/07/24 15:53, Guenter Roeck wrote:
>>>> On 7/21/24 17:58, Chris Packham wrote:
>>>>> By default the PWM duty cycle in hardware is 100%. On some systems this
>>>>> can cause unwanted fan noise. Add the ability to specify the fan
>>>>> connections and initial state of the PWMs via device properties.
>>>>>
>>>>> Signed-off-by: Chris Packham <chris.packham@...iedtelesis.co.nz>
>>>>> ---
>>>>>
>>>>> Notes:
>>>>>      Changes in v6:
>>>>>      - Use do_div() instead of plain /
>>>>>      - Use a helper function to avoid repetition between the of and non-of
>>>>>        code paths.
>>>>>      Changes in v5:
>>>>>      - Deal with PWM frequency and duty cycle being specified in nanoseconds
>>>>>      Changes in v4:
>>>>>      - Support DT and ACPI fwnodes
>>>>>      - Put PWM into manual mode
>>>>>      Changes in v3:
>>>>>      - Use the pwm provider/consumer bindings
>>>>>      Changes in v2:
>>>>>      - Use correct device property string for frequency
>>>>>      - Allow -EINVAL and only warn on error
>>>>>      - Use a frequency of 0 to indicate that the hardware should be left as-is
>>>>>
>>>>>   drivers/hwmon/adt7475.c | 130 ++++++++++++++++++++++++++++++++++++++++
>>>>>   1 file changed, 130 insertions(+)
>>>>>
>>>>> diff --git a/drivers/hwmon/adt7475.c b/drivers/hwmon/adt7475.c
>>>>> index 4224ffb30483..fc5605d34f36 100644
>>>>> --- a/drivers/hwmon/adt7475.c
>>>>> +++ b/drivers/hwmon/adt7475.c
>>>>> @@ -21,6 +21,8 @@
>>>>>   #include <linux/of.h>
>>>>>   #include <linux/util_macros.h>
>>>>>   +#include <dt-bindings/pwm/pwm.h>
>>>>> +
>>>>>   /* Indexes for the sysfs hooks */
>>>>>     #define INPUT        0
>>>>> @@ -1662,6 +1664,130 @@ static int adt7475_set_pwm_polarity(struct i2c_client *client)
>>>>>       return 0;
>>>>>   }
>>>>>   +struct adt7475_pwm_config {
>>>>> +    int index;
>>>>> +    int freq;
>>>>> +    int flags;
>>>>> +    int duty;
>>>>> +};
>>>>> +
>>>>> +static int _adt7475_pwm_properties_parse_args(u32 args[4], struct adt7475_pwm_config *cfg)
>>>>> +{
>>>>> +    unsigned long freq_hz;
>>>>> +    unsigned long duty;
>>>>> +
>>>>> +    if (args[1] == 0)
>>>>> +        return -EINVAL;
>>>>> +
>>>>> +    freq_hz = 1000000000UL;
>>>>> +    do_div(freq_hz, args[1]);
>>>>> +    duty = 255 * args[3];
>>>>> +    do_div(duty, args[1]);
>>>>> +
>>>>
>>>> Gues I am a bit at loss here, just as 0-day. Why use do_div ? It is only needed
>>>> for 64-bit divide operations.
>>>
>>> Mainly because of Uwe's comment on v5. I think I've avoided the original u64 issue now that I'm converting fwnode_reference_args::args to a u32 array. I can probably get away with plain division, although 255 * args[3] / args[1] might overflow in theory but shouldn't in practice.
>>>
>>> I'll let the earth turn and send out a v7 that uses plain division unless someone has a strong opinion that I should sprinkle some more u64s around.
>>>
>>
>> You lost me, sorry. Neither duty nor freq_hz are u64. What u64 variables
>> are you talking about ? Using so_div doesn't make those variables u64.
> 
> One way of fixing the 0-day complaint (I think) is to declare freq_hz and duty as u64 which would avoid all the theoretical overflow issues.
> 
> But plain division is probably easier to understand for everyone so I'll send out something like this in v7
> 
>    (unsigned?) int freq_hz;
>    (unsigned?) int duty;
>    ...
>    freq_hz = 1000000000UL / args[1];

This can not overflow.

>    duty = 255 * args[3] / args[1];

This will overflow if args[3] is larger than 16843009. What is its expected range ?
But even then you'd want something like
	duty = div_u64(255ULL * args[3], args[1]);

or
	if (args[3] >= args[1])
		duty = 255;
	else
		duty = div_u64(255ULL * args[3], args[1]);
to be able to drop the subsequent clamp_val() on duty.

Guenter