On Fri, Apr 23, 2021 at 06:32:30PM +0200, Thierry Reding wrote:

From: Thierry Reding <redacted>

Reserved memory region phandle references can be accompanied by a
specifier that provides additional information about how that specific
reference should be treated.

One use-case is to mark a memory region as needing an identity mapping
in the system's IOMMU for the device that references the region. This is
needed for example when the bootloader has set up hardware (such as a
display controller) to actively access a memory region (e.g. a boot
splash screen framebuffer) during boot. The operating system can use the
identity mapping flag from the specifier to make sure an IOMMU identity
mapping is set up for the framebuffer before IOMMU translations are
enabled for the display controller.

Signed-off-by: Thierry Reding <redacted>
---
 .../reserved-memory/reserved-memory.txt       | 21 +++++++++++++++++++
 include/dt-bindings/reserved-memory.h         |  8 +++++++
 2 files changed, 29 insertions(+)
 create mode 100644 include/dt-bindings/reserved-memory.h

Sorry for being slow on this. I have 2 concerns.

First, this creates an ABI issue. A DT with cells in 'memory-region' 
will not be understood by an existing OS. I'm less concerned about this 
if we address that with a stable fix. (Though I'm pretty sure we've 
naively added #?-cells in the past ignoring this issue.)

Second, it could be the bootloader setting up the reserved region. If a 
node already has 'memory-region', then adding more regions is more 
complicated compared to adding new properties. And defining what each 
memory-region entry is or how many in schemas is impossible.

Both could be addressed with a new property. Perhaps something like 
'iommu-memory-region = <&phandle>;'. I think the 'iommu' prefix is 
appropriate given this is entirely because of the IOMMU being in the 
mix. I might feel differently if we had other uses for cells, but I 
don't really see it in this case. 

Rob