On Mon, May 19, 2014 at 01:53:37PM +0100, Thierry Reding wrote: > On Mon, May 19, 2014 at 12:26:35PM +0200, Arnd Bergmann wrote: > > On Friday 16 May 2014 14:23:18 Thierry Reding wrote: > > > From: Thierry Reding <treding@xxxxxxxxxx> > > > > > > This commit introduces a generic device tree binding for IOMMU devices. > > > Only a very minimal subset is described here, but it is enough to cover > > > the requirements of both the Exynos System MMU and Tegra SMMU as > > > discussed here: > > > > > > https://lkml.org/lkml/2014/4/27/346 > > > > > > More advanced functionality such as the dma-ranges property can easily > > > be added in a backwards-compatible way. In the absence of a dma-ranges > > > property it should be safe to default to the whole address space. > > > > > > > The basic binding looks fine, but I'd like it to be more explicit > > about dma-ranges. Most importantly, what does "the whole address space" > > mean? > > The whole point was to leave out any mention of dma-ranges from the > binding until we've figured out more of the puzzle. > > So what I was trying to avoid was another lengthy discussion on the > topic of dma-ranges. Oh well... =) Apologies for the silence on this topic... I'm trying to do too many things at the sime time as usual :/ > > > A lot of IOMMUs have only 32-bit bus addresses when targetted > > by a bus master, it would also be normal for some to be smaller and > > some might even support 64-bit. > > > > For the upstream side, I'd hope we always have access to the full > > physical memory, but since this is a brand-new binding, it should > > be straightforward to just ask for upstream dma-ranges properties > > to be set all the way up to the root to confirm that. > > > > For downstream, we don't actually have a good place to put the > > dma-ranges property. > > I'm not sure I understand what you mean by upstream and downstream in > this context. > > > We can't put it into the iommu node, because that would imply translating > > to the iommu's parent bus, not the iommu's own bus space. > > My understanding was that the purpose of dma-ranges was to define a > mapping from one bus to another. So the general form of > > child-address parent-address child-size > > Would be used to translate a region of size <child-size> from the > <child-address> (the I/O address space created by the IOMMU) to the > <parent-address> (physical address space). > > > We also can't put it into the master, because dma-ranges is supposed to be > > in the parent bus. > > I don't understand. From the above I would think that the master's node > is precisely where it belongs. > > > Finally, it makes no sense to use the dma-ranges property of the master's > > parent bus, because that bus isn't actually involved in the translation. > > My understanding here is mostly based on the OpenFirmware working group > proposal for the dma-ranges property[0]. I'll give another example to > try and clarify how I had imagined this to work: > > / { > #address-cells = <2>; > #size-cells = <2>; > > iommu { > /* > * This is somewhat unusual (or maybe not) in that we > * need 2 cells to represent the size of an address > * space that is 32 bits long. > */ > #address-cells = <1>; > #size-cells = <2>; > > #iommu-cells = <1>; > }; > > master { > iommus = <&/iommu 42>; Comparing this with the other discussion thread, we have a similar concept here, in that the iommu is made a logical parent, however... Firstly, there's an implicit assumption here that the only kind of thing the master could possibly be connected to is an IOMMU, with no non-trivial interconnect in between. I'm not sure this is going to scale to complex SoCs. If a range of Stream IDs may be issued (especially from something like a PCIe RC where the stream ID may be a many-bit value), describing the IDs individually may be impractical. > /* > * Map I/O addresses 0 - 4 GiB to physical addresses > * 2 GiB - 6 GiB. > */ > dma-ranges = <0x00000000 0 0x80000000 1 0>; ... I concur with Arnd dma-ranges is now in the wrong place with respect to the (overridden) child-parent relationship, and that if this device masters to multiple destinations there's no possibility of describing different ranges mappings for each. > }; > }; > > This is somewhat incompatible with [0] in that #address-cells used to > parse the child address must be taken from the iommu node rather than > the child node. But that seems to me to be the only reasonable thing > to do, because after all the IOMMU creates a completely new address > space for the master. > > [0]: http://www.openfirmware.org/ofwg/proposals/Closed/Accepted/410-it.txt > > > My preferred option would be to always put the address range into > > the iommu descriptor, using the iommu's #address-cells. > > That could become impossible to parse. I'm not sure if such hardware > actually exists, but if for some reason we have to split the address > range into two, then there's no longer any way to determine the size > needed for the specifier. > > On the other hand what you propose makes it easy to represent multiple > master interfaces on a device. With a separate dma-ranges property how > can you define which ranges apply to which of the master interfaces? In theory, you could describe split ranges simply by multiplying out: <&/iommu 42 sub-range-1>, <&/iommu 42 sub-range-2>, ... This is only manageable if the number of subranges for each (iommu,streamID) mapping is small (and usually 1). Of course, I hope the number of subranges normally _is_ small... Cheers ---Dave > Then again if address ranges can't be broken up in the first place, then > dma-ranges could be considered to be one entry per IOMMU in the iommus > property. > > Thierry -- 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
