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

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Date:   Wed, 13 Dec 2023 13:56:48 +0200
From:   Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
To:     Laurent Pinchart <laurent.pinchart@...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 13/12/2023 13:48, Laurent Pinchart wrote:
> 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.

I think HAVE_CLK would be more correct.

  Tomi