linux-kernel - Re: [PATCH 5/5] dma-direct: always allow dma mask <= physiscal memory size

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Message-ID: <20180927134954.GB8281@lst.de>
Date:   Thu, 27 Sep 2018 15:49:54 +0200
From:   Christoph Hellwig <hch@....de>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:     Christoph Hellwig <hch@....de>, iommu@...ts.linux-foundation.org,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH 5/5] dma-direct: always allow dma mask <= physiscal
 memory size

On Thu, Sep 27, 2018 at 11:50:14AM +1000, Benjamin Herrenschmidt wrote:
> > -	 * to be able to satisfy them - either by not supporting more physical
> > -	 * memory, or by providing a ZONE_DMA32.  If neither is the case, the
> > -	 * architecture needs to use an IOMMU instead of the direct mapping.
> > -	 */
> > -	if (mask < phys_to_dma(dev, DMA_BIT_MASK(32)))
> > +	u64 min_mask;
> > +
> > +	if (IS_ENABLED(CONFIG_ZONE_DMA))
> > +		min_mask = DMA_BIT_MASK(ARCH_ZONE_DMA_BITS);
> > +	else
> > +		min_mask = min_t(u64, DMA_BIT_MASK(32),
> > +				 (max_pfn - 1) << PAGE_SHIFT);
> > +
> > +	if (mask >= phys_to_dma(dev, min_mask))
> >  		return 0;
> 
> nitpick ... to be completely "correct", I would have written
> 
> 	if (IS_ENABLED(CONFIG_ZONE_DMA))
> 		min_mask = DMA_BIT_MASK(ARCH_ZONE_DMA_BITS);
> 	else
> 		min_mask = DMA_BIT_MASK(32);
> 
> 	min_mask = min_t(u64, min_mask,	(max_pfn - 1) << PAGE_SHIFT);
> 
> In "theory" it's also ok to have a mask < ZONE_DMA_BITS as long as it's
> big enough to fit all memory :-)

Yeah, we could do that.