Hi Arnd, On: 26/03/2014 11:53, Arnd wrote: > Subject: Re: [PATCH v5 6/9] ARM: shmobile: Add PCIe device tree nodes for R8A7790 > > On Wednesday 26 March 2014 11:34:41 Phil.Edworthy@xxxxxxxxxxx wrote: > > > But the ranges you specified in the property don't actually fit in those > > > constraints: you have a range with size 0x8000000 and start 0x40000000, > > > which you say can't be programmed into the hardware. > > > > Actually, the driver checks the dma-ranges against these constraints, and > > if necessary will create multiple mappings to fulfil the requested > > dma-ranges. > > Ok, I didn't notice. My initial suggestion was to not put that logic > into the driver but instead specify in the host bridge binding that each > entry in the dma-ranges property has to meet the hardware constraints. > > As long as you don't have too much complexity to detect this case, I'm > fine with it either way. Ok, good! > > > > Still, my comment about the OF PCI range code treating both 32 and > > 64-bit > > > > types the same way means that PCIe host driver has to assume it's a > > 64-bit > > > > mapping. > > > > > > I was thinking more of PCI devices than the host itself. If the host > > > driver can verify that all mappings are in the first 4GB and cover all > > of > > > RAM, we won't have to use an swiotlb for devices that don't support > > 64-bit > > > DMA, which is a very significant performance difference. > > Ok, I think I understand. However, all the other PCI host drivers just do > > 1-to-1 mapping between PCI and CPU addresses, right? Whilst it might be > > nice be able to support mapping CPU addresses > 4GiB to PCI addresses > > under 4GiB, can that be something to consider later on? > > Yes, fair enough. The current version is much simpler, so that's ok. > Just keep it in mind if you run into performance problems. Also, note > that we don't actually support swiotlb on arm32 yet, so your current > code is broken for an PCI DMA master that is not 64-bit capable. > We need swiotlb on arm32 anyway, and that will fix this problem, but > adding the hack I described would also fix it. Ok, thanks for all your comments! Phil -- To unsubscribe from this list: send the line "unsubscribe linux-pci" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
