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.

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