linux-kernel - Re: [PATCH] hwrng: Add TX4939 RNG driver

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Message-ID: <20090531161842.GB30640@linux-mips.org>
Date:	Sun, 31 May 2009 17:18:42 +0100
From:	Ralf Baechle <ralf@...ux-mips.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Atsushi Nemoto <anemo@....ocn.ne.jp>, linux-mips@...ux-mips.org,
	linux-kernel@...r.kernel.org,
	Herbert Xu <herbert@...dor.apana.org.au>
Subject: Re: [PATCH] hwrng: Add TX4939 RNG driver

On Fri, May 29, 2009 at 04:29:07PM -0700, Andrew Morton wrote:

> > +static u64 read_rng(void __iomem *base, unsigned int offset)
> > +{
> > +	/* Caller must disable interrupts */
> > +	return ____raw_readq(base + offset);
> > +}
> 
> What is the reasoning behind the local_irq_disable() requirement?
> 
> Because I'm wondering whether this is safe on SMP?

The problem are interrupts, not SMP.  readq is reading a 64-bit register
using a 64-bit load.  On a 32-bit kernel however interrupts or any
other processor exception would clobber the upper 32-bit of the processor
register so interrupts need to be disabled.  The "normal" readq
functions disable interrupts as necessary on a platform.  This code does
multiple read accesses so for efficiency sake it relies on the caller
handling interrupts explicitly.

Also this platform does not do SMP.

  Ralf
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