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

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Date:	Thu, 18 Oct 2012 11:17:52 +0100
From:	Mark Einon <mark.einon@...il.com>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Jeffrey Ladouceur <jladouceur@...il.com>
Cc:	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

On Thu, Oct 18, 2012 at 10:59:56AM +0100, Alan Cox wrote:
> > > +                                                         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

Thanks for the comments both. I'll send an updated patch this evening.

The intention is to eventually store this value as a dma_addr_t anyway,
but some more work needs to be done to copy the address correctly to the
desc ring.

Cheers,

Mark
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