On Fri, Oct 07, 2022 at 03:21:46PM +0100, Robin Murphy wrote: > On 2022-10-07 14:54, Thierry Reding wrote: > > On Fri, Oct 07, 2022 at 02:45:31PM +0100, Robin Murphy wrote: > > > On 2022-09-23 13:35, Thierry Reding wrote: > > > > From: Thierry Reding <treding@xxxxxxxxxx> > > > > > > > > This adds the "iommu-addresses" property to reserved-memory nodes, which > > > > allow describing the interaction of memory regions with IOMMUs. Two use- > > > > cases are supported: > > > > > > > > 1. Static mappings can be described by pairing the "iommu-addresses" > > > > property with a "reg" property. This is mostly useful for adopting > > > > firmware-allocated buffers via identity mappings. One common use- > > > > case where this is required is if early firmware or bootloaders > > > > have set up a bootsplash framebuffer that a display controller is > > > > actively scanning out from during the operating system boot > > > > process. > > > > > > > > 2. If an "iommu-addresses" property exists without a "reg" property, > > > > the reserved-memory node describes an IOVA reservation. Such memory > > > > regions are excluded from the IOVA space available to operating > > > > system drivers and can be used for regions that must not be used to > > > > map arbitrary buffers. > > > > > > Bah, I've only just realised: don't we also need to change the "oneOf: > > > required: ..." schema to permit "iommu-addresses" without "reg" or "size"? > > > > Hm... good point. I think at least we'll want another: > > > > - required: > > - iommu-addresses > > > > in there. I wonder if we also need to avoid the combination of "size" > > and "iommu-addresses". When "size" is specified, is it guaranteed that > > those regions will be allocated before the direct mapping needs to be > > created? > > Well, it couldn't really be a direct mapping anyway. In general I don't > think that combination makes any sense, since the presence of > "iommu-addresses" means one of two things; either it says the IOVA range is > carved out for some special purpose or just unusable, in which case > allocating any memory to back it would surely be pointless, or it's saying > don't touch these addresses because the device is already accessing them, > thus the underlying physical memory must be allocated somewhere already. I thought perhaps there could be cases where it is known that a controller needs to access memory in a certain I/O virtual region but doesn't actually care where that lives in physical memory and also does not rely on that memory have been previously set up (pre-filled, or whatever). Say you've got a micro-controller in a system that needs its firmware in a given region, but the OS can set up that region without any other limitations. One could use "size" and "iommu-addresses" to make sure the region is allocated with a specific size and located in a specific I/O virtual region. Not sure if that's perhaps a bit exotic, though. Thierry
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