linux-kernel - Re: [PATCH 3/4] irqchip, gicv3: Implement Cavium ThunderX erratum 23154

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Message-ID: <20150706104812.GD7557@n2100.arm.linux.org.uk>
Date:	Mon, 6 Jul 2015 11:48:12 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Robert Richter <rric@...nel.org>
Cc:	Marc Zygnier <marc.zyngier@....com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Jason Cooper <jason@...edaemon.net>,
	Robert Richter <rrichter@...ium.com>,
	Tirumalesh Chalamarla <tchalamarla@...ium.com>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 3/4] irqchip, gicv3: Implement Cavium ThunderX erratum
 23154

On Tue, Jun 30, 2015 at 04:14:02PM +0200, Robert Richter wrote:
> +static u64 gic_read_iar_cavium_thunderx(void)
>  {
>  	u64 irqstat;
>  
> +	asm volatile("nop;nop;nop;nop;");
> +	asm volatile("nop;nop;nop;nop;");
>  	asm volatile("mrs_s %0, " __stringify(ICC_IAR1_EL1) : "=r" (irqstat));
> +	asm volatile("nop;nop;nop;nop;");
> +	mb();

NAK.  Please read the GCC manual for proper use of asm().  Even with
"volatile" there, it doesn't stop GCC from inserting instructions
between these.

If you need an instruction sequence to be consecutive, then it _must_
be one single asm().

There should also be a comment explaining why that code is necessary
here.  Please think about the future when you've forgotten why those
nops are there.  The kernel is a long term project, and it's important
that people understand why things are coded the way they are so that
the kernel can be properly maintained into the future.

-- 
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