[PATCH 1/2] schemas: Add firmware node schema

[Simon Glass]

Hi Rob,

On Fri, 14 Jul 2023 at 10:58, Rob Herring <robh at kernel.org> wrote:
>
> On Tue, Jul 11, 2023 at 3:18 PM Simon Glass <sjg at chromium.org> wrote:
> >
> > Add a motivation and purpose for this new proposed node.
> >
> > Signed-off-by: Simon Glass <sjg at chromium.org>
> > ---
> >
> >  dtschema/schemas/firmware.yaml | 83 ++++++++++++++++++++++++++++++++++
> >  1 file changed, 83 insertions(+)
> >  create mode 100644 dtschema/schemas/firmware.yaml
> >
> > diff --git a/dtschema/schemas/firmware.yaml b/dtschema/schemas/firmware.yaml
> > new file mode 100644
> > index 0000000..4439a70
> > --- /dev/null
> > +++ b/dtschema/schemas/firmware.yaml
> > @@ -0,0 +1,83 @@
> > +# SPDX-License-Identifier: GPL-2.0-only OR BSD-2-clause
> > +# Copyright 2023 Google LLC
> > +#
> > +
> > +%YAML 1.2
> > +---
> > +$id: http://devicetree.org/schemas/firmware.yaml#
> > +$schema: http://devicetree.org/meta-schemas/core.yaml#
> > +
> > +title: /firmware Node
> > +
> > +maintainers:
> > +  - Simon Glass <sjg at chromium.org>
> > +
> > +description: |
> > +  The '/firmware' node does not represent a real device, but serves as a place
> > +  for recording information about the main firmware used on the device, such as
> > +  a map of its contents. This is used by the Operating System (OS), user space
> > +  programs and possibly other firmware components. Data in the '/firmware' node
> > +  does not itself represent the hardware.
> > +
> > +  Properties in this node should be common to (and used by) at least two
> > +  firmware projects, such as U-Boot and TF-A. Project-specific subnodes can be
> > +  used for properties which are specific to a single project.
> > +
> > +  Purpose of '/firmware' node
> > +  ---------------------------
> > +
> > +  Firmware has traditionally been fairly opaque to the OS, with the OS taking
> > +  no interest in its contents, version, layout or how it might be updated. This
> > +  is less than ideal, since firmware is an important part of the system and
> > +  visibility into its operation is every bit as important as visbility into the
> > +  OS and user-space programs within the system.
> > +
> > +  The traditional approach has been to let firmware deal with firmware, and the
> > +  OS deal with everything else. Updating firmware has been handled by firmware.
> > +  For example, the UEFI spec defines a way for the OS to post a 'capsule' which
> > +  is discovered next time the system boots, permitting firmware updates. But
> > +  firmware updates in firmware are highly problematic. They require a reboot
> > +  and a sometimes-lengthy wait with a strange-looking interface unfamiliar
> > +  to most users. It seems better to make the update as transparent as possible
> > +  to the user. As an example of that, ChromeOS has full knowledge of the
> > +  firmware version and layout, updates it in the background from user space and
> > +  instantly selects the new firmware when the user reboots or logs out.
>
> Perhaps if OS based firmware updates are useful, then UEFI should gain
> that capability rather than inventing some way to do it with DT. Seems
> like a worthy goal, just needs wider review IMO.

Perhaps it should, although it would involve changing the spec, etc.
In any case it would be a very strange world if we mandated UEFI
everywhere.

Yes I am looking for wider review, partly since the work to document
all the firmware-image complexity is happening mostly in U-Boot at
present.

>
> > +  A common objection to considering the system holistically is that some parts
> > +  of the system are inaccessible to the OS, such as a secure enclave. However
> > +  this does not preclude providing visibility into what is present in that
> > +  enclave. Firmware-version information is still useful. Firmware updates are
> > +  still needed and can still be initiated from user space.
> > +
> > +  Another objection is that firmware should provide an interface to the OS,
> > +  while keeping its structure private. This thinking is largely driven by
> > +  extrapolating from how firmware has been handled in the 'BIOS' days.
>
> It's also the case that the OS may not have direct access to the h/w needed.

I tried to cover that in the paragraph you quote immediately
above...what is missing?

>
> > +  It should be considered a degenerate case rather than the norm. As complexity
> > +  increases, it creates an artificial boundary between two pieces of the whole.
> > +  Mechanisms then need to be invented to cross this unnecessary chasm. An
> > +  example of this is Intel's Dynamic Platform and Thermal Framework (DPTF),
> > +  which consists of user-space, OS and firmware components all working towards
> > +  a shared goal. We need a standard description of these cross-system pieces.
> > +
> > +  In order to 'teach the OS about firmware', we need a place to put this
> > +  information. That is the purpose of this node.
> > +
> > +  In an Open Source world the entire model of firmware needs to adjust to be
> > +  more open, more visible and managed just like any other part of the system.
> > +  The major goal is to standardise how firmware is presented to the OS and user
> > +  space, so that common utilities can be used to manage the entire system,
> > +  including the firmware. For example, fwupd can look in this node for
> > +  information on how to update the firmware, similar to how VBE works. [1]
> > +  It is likely that other purposes will come to light over time.
>
> It's good we're documenting /firmware, but your use seems different to
> what's already in place. Generally, /firmware has been for providers
> which are implemented by firmware and are not on any bus. SCMI for
> example. PSCI is another example, but it predated using /firmware so
> it's typically just put at the top level (and also PSCI should have
> been made discoverable like any SMCCC interface).

The intent here is that the node is not on any bus.

I'm happy to create a new node if that is better.

>
> > +
> > +  [1] https://github.com/fwupd/fwupd/tree/main/plugins/vbe
> > +
> > +properties:
> > +  $nodename:
> > +    const: firmware
> > +
> > +  "#address-cells": true
> > +  "#size-cells": true
>
> What address space is this? It's not memory mapped because you don't
> have 'ranges'.

Probably this should be 'false' ? I suppose there may be an address
space, for example in memory-mapped SPI flash, but that is not very
common. The firmware could be stored in eMMC or UFS, which are not
memory-mapped.

Regards,
Simon