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

From: Christoph Hellwig
Date: Thu Sep 27 2018 - 09:49:57 EST


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.