linux-kernel - Re: [PATCH net-next v3 1/3] net: phy: mxl-86110: add basic support for led_brightness

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Message-ID: <20250821173620.367ce05c@kernel.org>
Date: Thu, 21 Aug 2025 17:36:20 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Daniel Golle <daniel@...rotopia.org>
Cc: Andrew Lunn <andrew@...n.ch>, Xu Liang <lxu@...linear.com>, Heiner
 Kallweit <hkallweit1@...il.com>, Russell King <linux@...linux.org.uk>,
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet
 <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>,
 netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next v3 1/3] net: phy: mxl-86110: add basic support
 for led_brightness_set op

On Wed, 20 Aug 2025 14:34:07 +0100 Daniel Golle wrote:
> > > +	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.

IMHO this is one of the cases where the code would be far clearer
without he macros..

#define MXL86110_COM_EXT_LED_GEN_CFG			0xA00B
# define MXL86110_COM_EXT_LED_GEN_CFG_LFME(x)		(1 << (3 * (x)))
# define MXL86110_COM_EXT_LED_GEN_CFG_LFE(x)		(2 << (3 * (x)))
# define MXL86110_COM_EXT_LED_GEN_CFG_MASK(x)		(3 << (3 * (x)))

Right?