On 2022-10-07 16:22, Thierry Reding wrote:
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.
Yeah, that was the closest case I could think of as well, but I'd really
rather not encourage people to abuse DT that way. If a kernel driver is
loading firmware and initialising the device from scratch then it should
be able to sort everything out at runtime without DT involvement. Even
if the firmware is somehow massive enough to warrant an early dynamic
carveout rather than a regular page/CMA allocation, there's still no
good excuse for the driver not to manage its own address space constraints.
On the other hand if the device really does need its firmware at a
specific hard-coded address than that would need a fixed physical
reservation anyway, since the DT can't assume that the OS is definitely
going to use IOMMU translation.
Thanks,
Robin.
