linux-kernel - Re: [PATCH] ALSA: opti9xx: shut up gcc-10 range warning

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Date:   Thu, 30 Apr 2020 10:24:36 +0200
From:   Takashi Iwai <tiwai@...e.de>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     'Arnd Bergmann' <arnd@...db.de>, Jaroslav Kysela <perex@...ex.cz>,
        "Takashi Iwai" <tiwai@...e.com>,
        "alsa-devel@...a-project.org" <alsa-devel@...a-project.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ALSA: opti9xx: shut up gcc-10 range warning

On Thu, 30 Apr 2020 10:15:02 +0200,
David Laight wrote:
> 
> From: Arnd Bergmann
> > Sent: 29 April 2020 20:02
> > gcc-10 points out a few instances of suspicious integer arithmetic
> > leading to value truncation:
> > 
> > sound/isa/opti9xx/opti92x-ad1848.c: In function 'snd_opti9xx_configure':
> > sound/isa/opti9xx/opti92x-ad1848.c:322:43: error: overflow in conversion from 'int' to 'unsigned char'
> > changes value from '(int)snd_opti9xx_read(chip, 3) & -256 | 240' to '240' [-Werror=overflow]
> >   322 |   (snd_opti9xx_read(chip, reg) & ~(mask)) | ((value) & (mask)))
> >       |   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~
> > sound/isa/opti9xx/opti92x-ad1848.c:351:3: note: in expansion of macro 'snd_opti9xx_write_mask'
> >   351 |   snd_opti9xx_write_mask(chip, OPTi9XX_MC_REG(3), 0xf0, 0xff);
> >       |   ^~~~~~~~~~~~~~~~~~~~~~
> > sound/isa/opti9xx/miro.c: In function 'snd_miro_configure':
> > sound/isa/opti9xx/miro.c:873:40: error: overflow in conversion from 'int' to 'unsigned char' changes
> > value from '(int)snd_miro_read(chip, 3) & -256 | 240' to '240' [-Werror=overflow]
> >   873 |   (snd_miro_read(chip, reg) & ~(mask)) | ((value) & (mask)))
> >       |   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~
> > sound/isa/opti9xx/miro.c:1010:3: note: in expansion of macro 'snd_miro_write_mask'
> >  1010 |   snd_miro_write_mask(chip, OPTi9XX_MC_REG(3), 0xf0, 0xff);
> >       |   ^~~~~~~~~~~~~~~~~~~
> > 
> > These are all harmless here as only the low 8 bit are passed down
> > anyway. Change the macros to inline functions to make the code
> > more readable and also avoid the warning.
> > 
> > Strictly speaking those functions also need locking to make the
> > read/write pair atomic, but it seems unlikely that anyone would
> > still run into that issue.
> > 
> > Fixes: 1841f613fd2e ("[ALSA] Add snd-miro driver")
> > Signed-off-by: Arnd Bergmann <arnd@...db.de>
> > ---
> >  sound/isa/opti9xx/miro.c           | 9 ++++++---
> >  sound/isa/opti9xx/opti92x-ad1848.c | 9 ++++++---
> >  2 files changed, 12 insertions(+), 6 deletions(-)
> > 
> > diff --git a/sound/isa/opti9xx/miro.c b/sound/isa/opti9xx/miro.c
> > index e764816a8f7a..b039429e6871 100644
> > --- a/sound/isa/opti9xx/miro.c
> > +++ b/sound/isa/opti9xx/miro.c
> > @@ -867,10 +867,13 @@ static void snd_miro_write(struct snd_miro *chip, unsigned char reg,
> >  	spin_unlock_irqrestore(&chip->lock, flags);
> >  }
> > 
> > +static inline void snd_miro_write_mask(struct snd_miro *chip,
> > +		unsigned char reg, unsigned char value, unsigned char mask)
> > +{
> > +	unsigned char oldval = snd_miro_read(chip, reg);
> > 
> > -#define snd_miro_write_mask(chip, reg, value, mask)	\
> > -	snd_miro_write(chip, reg,			\
> > -		(snd_miro_read(chip, reg) & ~(mask)) | ((value) & (mask)))
> > +	snd_miro_write(chip, reg, (oldval & ~mask) | (value & mask));
> > +}
> 
> Isn't that likely to add additional masking with 0xff at the call sites?
> You will probably get better code if the arguments are 'unsigned int'.

I don't think such a micro optimization is needed.  
All registers, values and masks in the driver are 8bit, so keeping all
unsigned char is rather an improvement of readability.


thanks,

Takashi