linux-kernel - Re: [PATCH 1/3] watchdog: kempld: fix gcc-4.3 build

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Date:   Wed, 1 Mar 2017 06:13:03 -0800
From:   Guenter Roeck <linux@...ck-us.net>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Wim Van Sebroeck <wim@...ana.be>, linux-watchdog@...r.kernel.org,
        Lee Jones <lee.jones@...aro.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] watchdog: kempld: fix gcc-4.3 build

On 03/01/2017 01:15 AM, Arnd Bergmann wrote:
> gcc-4.3 can't decide whether the constant value in
> kempld_prescaler[PRESCALER_21] is built-time constant or
> not, and gets confused by the logic in do_div():
>
> drivers/watchdog/kempld_wdt.o: In function `kempld_wdt_set_stage_timeout':
> kempld_wdt.c:(.text.kempld_wdt_set_stage_timeout+0x130): undefined reference to `__aeabi_uldivmod'
>
> This adds a call to ACCESS_ONCE() to force it to not consider
> it to be constant, and leaves the more efficient normal case
> in place for modern compilers, using an #ifdef to annotate
> why we do this hack.
>
> Signed-off-by: Arnd Bergmann <arnd@...db.de>

Reviewed-by: Guenter Roeck <linux@...ck-us.net>

> ---
>  drivers/watchdog/kempld_wdt.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/watchdog/kempld_wdt.c b/drivers/watchdog/kempld_wdt.c
> index 73c46b3a09ab..2f3b049ea301 100644
> --- a/drivers/watchdog/kempld_wdt.c
> +++ b/drivers/watchdog/kempld_wdt.c
> @@ -140,12 +140,19 @@ static int kempld_wdt_set_stage_timeout(struct kempld_wdt_data *wdt_data,
>  					unsigned int timeout)
>  {
>  	struct kempld_device_data *pld = wdt_data->pld;
> -	u32 prescaler = kempld_prescaler[PRESCALER_21];
> +	u32 prescaler;
>  	u64 stage_timeout64;
>  	u32 stage_timeout;
>  	u32 remainder;
>  	u8 stage_cfg;
>
> +#if GCC_VERSION < 40400
> +	/* work around a bug compiling do_div() */
> +	prescaler = READ_ONCE(kempld_prescaler[PRESCALER_21]);
> +#else
> +	prescaler = kempld_prescaler[PRESCALER_21];
> +#endif
> +
>  	if (!stage)
>  		return -EINVAL;
>
>