On Fri, Nov 21, 2014 at 05:04:28PM +0000, Arnd Bergmann wrote: > On Friday 21 November 2014 16:57:09 Catalin Marinas wrote: > > There is a scenario where smaller mask would work on arm64. For example > > Juno, you can have 2GB of RAM in the 32-bit phys range (starting at > > 0x80000000). A device with 31-bit mask and a dma_pfn_offset of > > 0x80000000 would still work (there isn't any but just as an example). So > > the check in dma_alloc_coherent() would be something like: > > > > phys_to_dma(top of ZONE_DMA) - dma_pfn_offset <= coherent_dma_mask > > > > (or assuming RAM starts at 0 and ignoring dma_pfn_offset for now) > > > > If the condition above fails, dma_alloc_coherent() would no longer fall > > back to swiotlb but issue a dev_warn() and return NULL. > > Ah, that looks like it should work on all architectures, very nice. > How about checking this condition, and then printing a small warning > (dev_warn, not WARN_ON) and setting the dma_mask pointer to NULL?
I would not add the above ZONE_DMA check to of_dma_configure(). For example on arm64, we may not support a small coherent_dma_mask but the same value for dma_mask could be fine via swiotlb bouncing (or IOMMU). However, that's an arch-specific decision. Maybe after the above setting of dev->coherent_dma_mask in of_dma_configure(), we could add: if (!dma_supported(dev, dev->coherent_dma_mask)) dev->dma_mask = NULL; The arch dma_supported() can check the swiotlb bouncing or ZONE_DMA limits. Strangely, we don't have a coherent_dma_supported() but we can defer such check to dma_alloc_coherent() and that's where we would check the top of ZONE_DMA. -- Catalin -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/