linux-kernel - Re: [PATCH] clocksource: kona: avoid bogus warning

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Date:	Mon, 13 Jun 2016 15:34:38 -0700
From:	Ray Jui <ray.jui@...adcom.com>
To:	Arnd Bergmann <arnd@...db.de>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>
Cc:	Florian Fainelli <f.fainelli@...il.com>,
	Ray Jui <rjui@...adcom.com>,
	Scott Branden <sbranden@...adcom.com>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	bcm-kernel-feedback-list@...adcom.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] clocksource: kona: avoid bogus warning

Hi Arnd,

On 6/13/2016 1:40 PM, Arnd Bergmann wrote:
> I could not figure out why, but gcc cannot prove that the
> kona_timer_init function always initializes its two outputs,
> and we get a warning for the use of the 'lsw' variable later,
> which is obviously correct.
>
> drivers/clocksource/bcm_kona_timer.c: In function 'kona_timer_init':
> drivers/clocksource/bcm_kona_timer.c:119:13: error: 'lsw' may be used uninitialized in this function [-Werror=maybe-uninitialized]
>
> Slightly reordering the loop makes the warning disappear, after
> it becomes more obvious to the compiler that the loop is
> always entered on the first iteration.
>
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
>  drivers/clocksource/bcm_kona_timer.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/clocksource/bcm_kona_timer.c b/drivers/clocksource/bcm_kona_timer.c
> index 70d9c1e482dd..bbbfb03b46dd 100644
> --- a/drivers/clocksource/bcm_kona_timer.c
> +++ b/drivers/clocksource/bcm_kona_timer.c
> @@ -69,7 +69,7 @@ static void kona_timer_disable_and_clear(void __iomem *base)
>  static void
>  kona_timer_get_counter(void __iomem *timer_base, uint32_t *msw, uint32_t *lsw)
>  {
> -	int loop_limit = 4;
> +	int loop_limit = 3;
>
>  	/*
>  	 * Read 64-bit free running counter
> @@ -83,12 +83,12 @@ kona_timer_get_counter(void __iomem *timer_base, uint32_t *msw, uint32_t *lsw)
>  	 *      if new hi-word is equal to previously read hi-word then stop.
>  	 */
>
> -	while (--loop_limit) {
> +	do {
>  		*msw = readl(timer_base + KONA_GPTIMER_STCHI_OFFSET);
>  		*lsw = readl(timer_base + KONA_GPTIMER_STCLO_OFFSET);
>  		if (*msw == readl(timer_base + KONA_GPTIMER_STCHI_OFFSET))
>  			break;
> -	}
> +	} while (--loop_limit);
>  	if (!loop_limit) {
>  		pr_err("bcm_kona_timer: getting counter failed.\n");
>  		pr_err(" Timer will be impacted\n");
>

This fix of compiler warning looks fine to me.

Btw, this also exposes another issue in the code:

Does it make more sense to have "kona_timer_get_counter" return 
-ETIMEDOUT in the case when "loop_limit" reaches zero, and in 
"kona_timer_set_net_event", it should bail out immediately when error is 
returned from "kona_timer_get_counter":

ret = kona_timer_get_counter(timers.tmr_regs, &msw, &lsw);
if (ret)
     return ret;

Thanks,

Ray