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

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Message-ID: <5fnaskzlfybxiddwhm3fdm774ispvuxdmv4jb4wj5sts2btkf2@gmcfdzx3qvjy>
Date: Mon, 22 Jul 2024 09:51:04 +0200
From: Uwe Kleine-König <u.kleine-koenig@...libre.com>
To: Chris Packham <chris.packham@...iedtelesis.co.nz>
Cc: Guenter Roeck <linux@...ck-us.net>, jdelvare@...e.com, robh@...nel.org, 
	krzk+dt@...nel.org, conor+dt@...nel.org, 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 Mon, Jul 22, 2024 at 04:09:46PM +1200, 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.

My comment was only about the build bot finding a division where the gcc
stub was missing with is an indication that do_div should be used. 

Usually for PWMs perdiod and duty_cycle are u64, but here it's only
about values from the dtb, so they are u32 and a plain / should be fine.

> can probably get away with plain division, although 255 * args[3] / args[1]
> might overflow in theory but shouldn't in practice.

I don't like possible overflows, but I don't care enough for hwmon
drivers to object. Still a check for args[3] <= 0x1010101 would be easy
enough.

Best regards
Uwe

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