linux-kernel - Re: [PATCH 2/2] staging: et131x: Fix 64bit tx dma address handling

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Date:	Thu, 18 Oct 2012 10:59:56 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Jeffrey Ladouceur <jladouceur@...il.com>
Cc:	Mark Einon <mark.einon@...il.com>, devel@...verdev.osuosl.org,
	gregkh@...uxfoundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] staging: et131x: Fix 64bit tx dma address handling

> > +                                                         skb_headlen(skb),
> > +                                                         DMA_TO_DEVICE);
> > +                               desc[frag].addr_lo = dma_addr & 0xFFFFFFFF;
> > +                               desc[frag].addr_hi = dma_addr >> 32;
> 
> Maybe use macros defined in kernel.h instead:
> 
> desc[frag].addr_lo = lower_32_bits(dma_addr);
> desc[frag].addr_hi = upper_32_bits(dma_addr);
> 
> A few more instances below.

This is actually important because >> 32 of a 32bit value is undefined in
C. The compiler is free to do what it likes with this. While the results
are usually sane some architectures do generate the equivalent of

		x >> (n % wordsize);


Alan
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