[PATCH v5 3/4] schemas: Add some common reserved-memory usages

[Simon Glass]

Hi Ard,

On Wed, Sep 6, 2023, 10:09 Ard Biesheuvel <ardb at kernel.org> wrote:

> On Wed, 6 Sept 2023 at 16:54, Simon Glass <sjg at chromium.org> wrote:
> >
> > Hi Rob, Ard,
> >
> > On Wed, 6 Sept 2023 at 08:34, Rob Herring <robh at kernel.org> wrote:
> > >
> > > On Tue, Sep 5, 2023 at 4:44 PM Ard Biesheuvel <ardb at kernel.org> wrote:
> > > >
> > > > On Thu, 31 Aug 2023 at 01:18, Simon Glass <sjg at chromium.org> wrote:
> > > > >
> > > > > The Devicetree specification skips over handling of a logical view
> of
> > > > > the memory map, pointing users to the UEFI specification.
> > > > >
> > > > > It is common to split firmware into 'Platform Init', which does the
> > > > > initial hardware setup and a "Payload" which selects the OS to be
> booted.
> > > > > Thus an handover interface is required between these two pieces.
> > > > >
> > > > > Where UEFI boot-time services are not available, but UEFI firmware
> is
> > > > > present on either side of this interface, information about memory
> usage
> > > > > and attributes must be presented to the "Payload" in some form.
> > > > >
> > > >
> > > > I don't think the UEFI references are needed or helpful here.
> > > >
> > > > > This aims to provide an small schema addition for this mapping.
> > > > >
> > > > > For now, no attempt is made to create an exhaustive binding, so
> there are
> > > > > some example types listed. More can be added later.
> > > > >
> > > > > The compatible string is not included, since the node name is
> enough to
> > > > > indicate the purpose of a node, as per the existing reserved-memory
> > > > > schema.
> > >
> > > Node names reflect the 'class', but not what's specifically in the
> > > node. So really, all reserved-memory nodes should have the same name,
> > > but that ship already sailed for existing users. 'compatible' is the
> > > right thing here. As to what the node name should be, well, we haven't
> > > defined that. I think we just used 'memory' on some platforms.
> >
> > OK
> >
> > >
> > > > > This binding does not include a binding for the memory 'attribute'
> > > > > property, defined by EFI_BOOT_SERVICES.GetMemoryMap(). It may be
> useful
> > > > > to have that as well, but perhaps not as a bit mask.
> > > > >
> > > > > Signed-off-by: Simon Glass <sjg at chromium.org>
> > > > > ---
> > > > >
> > > > > Changes in v5:
> > > > > - Drop the memory-map node (should have done that in v4)
> > > > > - Tidy up schema a bit
> > > > >
> > > > > Changes in v4:
> > > > > - Make use of the reserved-memory node instead of creating a new
> one
> > > > >
> > > > > Changes in v3:
> > > > > - Reword commit message again
> > > > > - cc a lot more people, from the FFI patch
> > > > > - Split out the attributes into the /memory nodes
> > > > >
> > > > > Changes in v2:
> > > > > - Reword commit message
> > > > >
> > > > >  .../reserved-memory/common-reserved.yaml      | 53
> +++++++++++++++++++
> > > > >  1 file changed, 53 insertions(+)
> > > > >  create mode 100644
> dtschema/schemas/reserved-memory/common-reserved.yaml
> > > > >
> > > > > diff --git a/dtschema/schemas/reserved-memory/common-reserved.yaml
> b/dtschema/schemas/reserved-memory/common-reserved.yaml
> > > > > new file mode 100644
> > > > > index 0000000..d1b466b
> > > > > --- /dev/null
> > > > > +++ b/dtschema/schemas/reserved-memory/common-reserved.yaml
> > > > > @@ -0,0 +1,53 @@
> > > > > +# SPDX-License-Identifier: GPL-2.0-only OR BSD-2-Clause
> > > > > +%YAML 1.2
> > > > > +---
> > > > > +$id:
> http://devicetree.org/schemas/reserved-memory/common-reserved.yaml#
> > > > > +$schema: http://devicetree.org/meta-schemas/core.yaml#
> > > > > +
> > > > > +title: Common memory reservations
> > > > > +
> > > > > +description: |
> > > > > +  Specifies that the reserved memory region can be used for the
> purpose
> > > > > +  indicated by its node name.
> > > > > +
> > > > > +  Clients may reuse this reserved memory if they understand what
> it is for.
> > > > > +
> > > > > +maintainers:
> > > > > +  - Simon Glass <sjg at chromium.org>
> > > > > +
> > > > > +allOf:
> > > > > +  - $ref: reserved-memory.yaml
> > > > > +
> > > > > +properties:
> > > > > +  $nodename:
> > > > > +    enum:
> > > > > +      - acpi-reclaim
> > > > > +      - acpi-nvs
> > > > > +      - boot-code
> > > > > +      - boot-data
> > > > > +      - runtime-code
> > > > > +      - runtime-data
> > > > > +
> > > >
> > > > These types are used by firmware to describe the nature of certain
> > > > memory regions to the OS. Boot code and data can be discarded, as
> well
> > > > as ACPI reclaim after its contents have been consumed. Runtime code
> > > > and data need to be mapped for runtime features to work.
> > > >
> > > > When one firmware phase communicates the purpose of a certain memory
> > > > reservation to another, it is typically not limited to whether its
> > > > needs to be preserved and when it needs to be mapped (and with which
> > > > attributes). I'd expect a memory reservation appearing under this
> node
> > > > to have a clearly defined purpose, and the subsequent phases need to
> > > > be able to discover this information.
> > > >
> > > > For example, a communication buffer for secure<->non-secure
> > > > communication or a page with spin tables used by PSCI. None of the
> > > > proposed labels are appropriate for this, and I'd much rather have a
> > > > compatible string or some other property that clarifies the nature in
> > > > a more suitable way. Note that 'no-map' already exists to indicate
> > > > that the CPU should not map this memory unless it does so for the
> > > > specific purpose that the reservation was made for.
> > >
> > > I agree. I think compatible is the better approach. Some property like
> > > 'discard' may not be sufficient information if the OS needs to consume
> > > the region first and then discard it. Better to state exactly what's
> > > there and then the OS can imply the rest.
> >
> > OK, so what sort of compatible strings?
> >
> > How about:
> > "acpi-reclaim" - holds ACPI tables; memory can be reclaimed once the
> > tables are read and no-longer needed
>
> ACPI reclaim is a policy, not a purpose. This memory could contain
> many different things.
>
> > "boot-code" - holds boot code; memory can be reclaimed once the boot
> > phase is complete
> > "runtime-code" - holds runtime code; memory can be reclaimed only if
> > this code will not be used from that point
> >
>
> These are also policies. They can be inferred from the purpose.
>
> > etc. We can then have more specific compatibles, like:
> >
> > "psci-spin-table" - holds PSCI spin tables
> >
> > so you could do:
> >
> > compatible = "runtime-code", "psci-spin-table";
> >
>
> I understand that this binding targets firmware<->firmware rather than
> firmware<->OS, which makes it much more difficult to keep it both
> generic and sufficiently descriptive.
>
> However, I still feel that all the overlap with UEFI memory types is
> not what we want here. UEFI knows how to manage its own memory map,
> what it needs to know is what memory is already in use and for which
> exact purpose. Whether or not that implies that the memory can be
> freed at some point or can be mapped or not should follow from that.
>

Can you please make a suggestion? I am unsure what you are looking for.

Regards,
Simon