On Wednesday 08 May 2013, Peter Chen wrote: > > > > This probably could be initialized from some DT property. However, > > there's no such property defined right now, and considering that DT is > > supposed to be an ABI, we'd always need the code in this patch as a > > fallback for DTs that were created before any such property was defined. > > > > Equally, since the data is SoC-specific rather than board-specific, and > > is even fairly unlikely to vary between SoC versions since these values > > are all 0xffffffff anyway, I don't really see much point in putting it > > into DT, rather than just putting the static data into the driver. > > I mean there is already dev->dev.coherent_dma_mask = DMA_BIT_MASK(32); > at function of_platform_device_create, why can't add > dev->dev.dma_mask = &dev->dev.coherent_dma_mask after that? > > If DT core can do above things, can we delete dma_mask assignment > at every driver? It probably should. The main thing is that the dma_mask setting in of_platform (and elsewhere) is a mess and that nobody so far had the guts to try to get it right for good. Setting a 32 bit DMA mask is /probably/ the right default on all ARM systems, but there are caveats: - Once you get to systems with larger than 32 bit addressing (powerpc64, arm64, arm32 with LPAE), it's not so obvious: you may have some devices that need a 32 bit mask and some that need a 64 bit mask. - Some (very rare these days, thankfully) devices require a mask that is less than 32 bits. Since that knowledge is device specific, not platform specific, it should probably stay in the driver. - There are cases (I know them only on powerpc, but they probably exist on ARM and other places too) where the mapping from bus addresses to physical addresses is not linear. There is a device-tree binding for a "dma-ranges" property that can accurately describe the specific mapping. Actually using this on architecture independent code requires not only setting the dma_mask but also supporting the remapping in the dma_map_ops. - Things get more interesting in combination with an IOMMU. If we have an IOMMU, I think we should set the dma_mask pointer to the mask of the IOMMU and set the map_ops accordingly. - Whether we actually need coherent_dma_mask these days is another hard question to answer. I suspect that the only thing really needing it was some version of the Itanium based Altix machine for its PCI devices and we'd be better off finding a simpler solution for platform devices. For all practical purposes I think coherent_dma_mask must be the same as dma_mask. Arnd -- To unsubscribe from this list: send the line "unsubscribe linux-tegra" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
