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

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Date:   Wed, 13 Dec 2023 13:40:54 +0200
From:   Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
To:     Arnd Bergmann <arnd@...nel.org>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        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>
Cc:     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 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 =).

  Tomi