On Tuesday 06 May 2014 05:40 AM, Arnd Bergmann wrote: > On Monday 05 May 2014 17:47:32 Santosh Shilimkar wrote: > >> + dev->coherent_dma_mask = DMA_BIT_MASK(32); >> + if (!dev->dma_mask) >> + dev->dma_mask = &dev->coherent_dma_mask; >> + >> + /* >> + * if dma-ranges property doesn't exist - just return else >> + * setup the dma offset >> + */ >> + ret = of_dma_get_range(dev->of_node, &dma_addr, &paddr, &size); >> + if (ret < 0) { >> + dev_dbg(dev, "no dma range information to setup\n"); >> + return; >> + } >> + >> + /* DMA ranges found. Calculate and set dma_pfn_offset */ >> + dev->dma_pfn_offset = PFN_DOWN(paddr - dma_addr); >> + dev_dbg(dev, "dma_pfn_offset(%#08lx)\n", dev->dma_pfn_offset); >> +} > > I think there should at least be a comment about why we are computing > the correct DMA mask here and then ignore that and just use DMA_BIT_MASK(32) > instead. I understand that Russell and Rob prefer it that way and I'm not > going to argue, but I find it counterintuitive and I think it deserves > an explanation in the source code for anybody who is trying to figure > out how things fit together. > In this patch, the dma_mask related code is just moved. We are not calculating dma_mask either. I was looking for the history of how DMA_BIT_MASK(32) landed up but couldn't trace it down apart from the fact that the code was carried from powerPC. May be Rob knows. How about below comment ? I didn't delibratly added point about bus intercepting drivers dma_set_*mask() call etc. /* * Set default dma-mask to 32 bit. Drivers are expected to setup * the correct supported dma_mask. */ + dev->coherent_dma_mask = DMA_BIT_MASK(32); + if (!dev->dma_mask) + dev->dma_mask = &dev->coherent_dma_mask; Regards, Santosh -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html