linux-kernel - Re: [PATCH 25/52] m68k: apollo: Replace set but not used variable by READ

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Date:   Fri, 8 Sep 2023 09:37:12 +1000 (AEST)
From:   Finn Thain <fthain@...ux-m68k.org>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
cc:     linux-m68k@...ts.linux-m68k.org, Arnd Bergmann <arnd@...db.de>,
        Michael Schmitz <schmitzmic@...il.com>,
        Philip Blundell <philb@....org>,
        Greg Ungerer <gerg@...ux-m68k.org>,
        Joshua Thompson <funaho@...ai.org>,
        Sam Creasey <sammy@...my.net>,
        Laurent Vivier <laurent@...ier.eu>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 25/52] m68k: apollo: Replace set but not used variable
 by READ_ONCE()

On Thu, 7 Sep 2023, Geert Uytterhoeven wrote:

> --- a/arch/m68k/apollo/config.c
> +++ b/arch/m68k/apollo/config.c
> @@ -146,13 +146,11 @@ void __init config_apollo(void)
>  
>  irqreturn_t dn_timer_int(int irq, void *dev_id)
>  {
> -	volatile unsigned char x;
> -
>  	legacy_timer_tick(1);
>  	timer_heartbeat();
>  
> -	x = *(volatile unsigned char *)(apollo_timer + 3);
> -	x = *(volatile unsigned char *)(apollo_timer + 5);
> +	READ_ONCE(*(volatile unsigned char *)(apollo_timer + 3));
> +	READ_ONCE(*(volatile unsigned char *)(apollo_timer + 5));
>  
>  	return IRQ_HANDLED;
>  }
> 

I believe the volatile cast is redundant here, as READ_ONCE does that.
Perhaps the remaining cast could be deduplicated like so:

-	volatile unsigned char x;
+	unsigned char *at = (unsigned char *)apollo_timer;

  	legacy_timer_tick(1);
  	timer_heartbeat();

-	x = *(volatile unsigned char *)(apollo_timer + 3);
-	x = *(volatile unsigned char *)(apollo_timer + 5);
+	READ_ONCE(*(at + 3));
+	READ_ONCE(*(at + 5));