Re: [PATCH v2 1/5] dt-bindings: reserved-memory: Document memory region specifier

[Rob Herring <robh@kernel.org>]

On Fri, Apr 23, 2021 at 06:32:30PM +0200, Thierry Reding wrote:
> From: Thierry Reding <treding@nvidia.com>
> 
> 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 <treding@nvidia.com>
> ---
>  .../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