linux-kernel - Re: [PATCH] [v2] media: i2c: mt9m114: use fsleep() in place of udelay()

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Message-ID: <20231213114812.GB769@pendragon.ideasonboard.com>
Date:   Wed, 13 Dec 2023 13:48:12 +0200
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
Cc:     Arnd Bergmann <arnd@...nel.org>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Jacopo Mondi <jacopo.mondi@...asonboard.com>,
        Hans Verkuil <hverkuil-cisco@...all.nl>,
        Arnd Bergmann <arnd@...db.de>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Bill Wendling <morbo@...gle.com>,
        Justin Stitt <justinstitt@...gle.com>,
        Shuah Khan <skhan@...uxfoundation.org>,
        linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev
Subject: Re: [PATCH] [v2] media: i2c: mt9m114: use fsleep() in place of
 udelay()

On Wed, Dec 13, 2023 at 01:40:54PM +0200, Tomi Valkeinen wrote:
> On 13/12/2023 13:23, Arnd Bergmann wrote:
> > From: Arnd Bergmann <arnd@...db.de>
> > 
> > With clang-16, building without COMMON_CLK triggers a range check on
> > udelay() because of a constant division-by-zero calculation:
> > 
> > ld.lld: error: undefined symbol: __bad_udelay
> >>>> referenced by mt9m114.c
> >>>>                drivers/media/i2c/mt9m114.o:(mt9m114_power_on) in archive vmlinux.a
> > 
> > In this configuration, the driver already fails to probe, before
> > this function gets called, so it's enough to suppress the assertion.
> > 
> > Do this by using fsleep(), which turns long delays into sleep() calls
> > in place of the link failure.
> > 
> > This is probably a good idea regardless to avoid overly long dynamic
> > udelay() calls on a slow clock.
> > 
> > Cc: Sakari Ailus <sakari.ailus@...ux.intel.com>
> > Fixes: 24d756e914fc ("media: i2c: Add driver for onsemi MT9M114 camera sensor")
> > Signed-off-by: Arnd Bergmann <arnd@...db.de>
> > ---
> >   drivers/media/i2c/mt9m114.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/drivers/media/i2c/mt9m114.c b/drivers/media/i2c/mt9m114.c
> > index 0a22f328981d..68adaecaf481 100644
> > --- a/drivers/media/i2c/mt9m114.c
> > +++ b/drivers/media/i2c/mt9m114.c
> > @@ -2116,7 +2116,7 @@ static int mt9m114_power_on(struct mt9m114 *sensor)
> >   		duration = DIV_ROUND_UP(2 * 50 * 1000000, freq);
> >   
> >   		gpiod_set_value(sensor->reset, 1);
> > -		udelay(duration);
> > +		fsleep(duration);
> >   		gpiod_set_value(sensor->reset, 0);
> >   	} else {
> >   		/*
> 
> I think this is fine, so:
> 
> Reviewed-by: Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
> 
> But: If we don't have COMMON_CLK (or rather, I think, HAVE_CLK), the 
> freq will be zero at compile time. So won't the compiler give a warning 
> for the DIV_ROUND_UP() call?
> 
> Interestingly, for me, this doesn't give a div-by-zero warning:
> 
> 	int x;
> 	int y = 0;
> 	x = DIV_ROUND_UP(10, y);
> 
> but this does:
> 
> 	int x;
> 	const int y = 0;
> 	x = DIV_ROUND_UP(10, y);
> 
> And looks like this gives the warning too:
> 
> 	int x;
> 	const int y = 0;
> 	if (y)
> 		x = DIV_ROUND_UP(10, y);
> 
> So, I think, the code in the driver could fail to compile at some later 
> point, if the compiler warnings are improved (?), or if someone adds a 
> 'const' in front of 'long freq = clk_get_rate(sensor->clk);' line.
> 
> Maybe worry about that if it actually happens =).

Maybe :-) I would be tempted to make VIDEO_CAMERA_SENSOR depend on
COMMON_CLK.

-- 
Regards,

Laurent Pinchart