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

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Date:	Tue, 7 Jul 2015 11:55:33 +0200
From:	Robert Richter <robert.richter@...iumnetworks.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
CC:	Robert Richter <rric@...nel.org>,
	Marc Zygnier <marc.zyngier@....com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Jason Cooper <jason@...edaemon.net>,
	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

Russell,

thanks for your comments.

On 06.07.15 11:48:12, Russell King - ARM Linux wrote:
> 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().

I will update this section.

> 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.

I will add a comment to the code with a brief description. Would the
current text from the patch description including the erratum number
be sufficient for that?

Thanks,

-Robert
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