linux-kernel - Re: [PATCH RFC 1/8] arm: make SWIOTLB available

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Message-ID: <alpine.DEB.2.02.1308021302340.4893@kaball.uk.xensource.com>
Date:	Fri, 2 Aug 2013 13:09:51 +0100
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
CC:	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	<xen-devel@...ts.xensource.com>, <linux-kernel@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>, <konrad.wilk@...cle.com>,
	<Ian.Campbell@...rix.com>, <will.deacon@....com>
Subject: Re: [PATCH RFC 1/8] arm: make SWIOTLB available

On Thu, 1 Aug 2013, Russell King - ARM Linux wrote:
> On Wed, Jul 31, 2013 at 06:45:25PM +0100, Stefano Stabellini wrote:
> > +static inline dma_addr_t phys_to_dma(struct device *dev, phys_addr_t paddr)
> > +{
> > +	unsigned int offset = paddr & ~PAGE_MASK;
> > +	return pfn_to_dma(dev, paddr >> PAGE_SHIFT) + offset;
> > +}
> > +
> > +static inline phys_addr_t dma_to_phys(struct device *dev, dma_addr_t dev_addr)
> > +{
> > +	unsigned int offset = dev_addr & ~PAGE_MASK;
> > +	return (dma_to_pfn(dev, dev_addr) << PAGE_SHIFT) + offset;
> > +}
> 
> These two helpers look fine on the face of it.
> 
> > +static inline bool dma_capable(struct device *dev, dma_addr_t addr, size_t size)
> > +{
> > +	if (!dev->dma_mask)
> > +		return 0;
> > +
> > +	return addr + size - 1 <= *dev->dma_mask;
> > +}
> 
> You may wish to have a closer look at the DMA bounce code, because this
> assumes that DMA masks are a set of zeros followed by a set of ones.
> That may not always be the case (and we have the odd platform where that
> isn't the case.)
> 
> It has always bugged me that we call this thing a dma _mask_ and then
> much kernel code treats it as a limit - it should've been called "dma
> limit" if that's how it was to be interpreted.  If it really is a _mask_
> then the right way to test whether a DMA address/size is possible is:
> 
> 	u64 limit, mask = *dev->dma_mask;
> 
> 	limit = (mask + 1) & ~mask;
> 	if (limit && size > limit)
> 		return 0;
> 
> 	if ((addr | (addr + size - 1)) & ~mask)
> 		return 0;
> 
> 	return 1;
> 
> The first checks whether 'size' fits within the least significant
> contiguous set of '1' bits in the DMA mask, and the second checks
> whether the region itself contains any address bits which may not
> meet the DMA mask.
 
I'll make the change, thanks for the explanation.


> I guess if we aren't going to encounter any of these cases anymore,
> your test is entirely sufficient.
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