Re: [PATCH net-next v3 1/3] net: phy: mxl-86110: add basic support for led_brightness_set op

[Daniel Golle]

On Wed, Aug 20, 2025 at 02:34:48PM +0200, Andrew Lunn wrote:
> > +# define MXL86110_COM_EXT_LED_GEN_CFG_LFM(x)		({ typeof(x) _x = (x); \
> > +							  GENMASK(1 + (3 * (_x)), \
> > +								 3 * (_x)); })
> 
> > +static int mxl86110_led_brightness_set(struct phy_device *phydev,
> > +				       u8 index, enum led_brightness value)
> > +{
> > +	u16 mask, set;
> > +	int ret;
> > +
> > +	if (index >= MXL86110_MAX_LEDS)
> > +		return -EINVAL;
> > +
> > +	/* force manual control */
> > +	set = MXL86110_COM_EXT_LED_GEN_CFG_LFE(index);
> > +	/* clear previous force mode */
> > +	mask = MXL86110_COM_EXT_LED_GEN_CFG_LFM(index);
> > +
> > +	/* force LED to be permanently on */
> > +	if (value != LED_OFF)
> > +		set |= MXL86110_COM_EXT_LED_GEN_CFG_LFME(index);
> 
> That is particularly complex. We know index is a u8, so why not
> GENMASK_U8(1 + 3 * index, 3 * index)? But set is a u16, so
> GENMASK_U16() would also be valid.

I chose this construct to avoid reusing the macro parameter as gcc would
rightously complain about that potentially having unexpected side-effects.

Eg.

#define FOO(a) ((a)+(a))

Now with var=10, when calling FOO(var++) the result will be 21 and
var will be equal to 12, which isn't intuitive without seeing the
macro definition.

Also using GENMASK_TYPE would not avoid the problem of macro
parameter reuse.

However, I agree that the macro itself is also weirdly complex and
confusing (but at the same time also very common, a quick grep reveals
hundreds of occurances of that pattern in Linux sources), so maybe we
should introduce some generic helpers for this (quite common) use-case?
I can do that, but I certainly can't take care of migrating all the
existing uses of this pattern to switch to the new helper.