[RFC PATCH v2] dt-bindings: dma: apple,sio: Add schema

["Martin Povišer" <povik+lin@cutebit.org>]

Describe the SIO coprocessor which serves as pretend DMA controller on
recent Apple platforms.

Signed-off-by: Martin Povišer <povik+lin@cutebit.org>
---
Since v1:
 - formatting fixes requested by Rob
 - dropped provider nodes from example
 - added 'memory-region' items range
 - tweaked 'apple,sio-firmware-params' description

Since the schema mentions a loader preparing the binding appropriately,
here's a PR with the relevant (WIP) loader code, if anyone wants to look:
https://github.com/AsahiLinux/m1n1/pull/286


 .../devicetree/bindings/dma/apple,sio.yaml    | 111 ++++++++++++++++++
 1 file changed, 111 insertions(+)
 create mode 100644 Documentation/devicetree/bindings/dma/apple,sio.yaml

diff --git a/Documentation/devicetree/bindings/dma/apple,sio.yaml b/Documentation/devicetree/bindings/dma/apple,sio.yaml
new file mode 100644
index 000000000000..0e3780ad9dd7
--- /dev/null
+++ b/Documentation/devicetree/bindings/dma/apple,sio.yaml
@@ -0,0 +1,111 @@
+# SPDX-License-Identifier: GPL-2.0-only OR BSD-2-Clause
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/dma/apple,sio.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Apple SIO Coprocessor
+
+description:
+  SIO is a coprocessor on Apple M1 and later chips (and maybe also on earlier
+  chips). Its role is to offload SPI, UART and DisplayPort audio transfers,
+  being a pretend DMA controller.
+
+maintainers:
+  - Martin Povišer <povik+lin@cutebit.org>
+
+allOf:
+  - $ref: dma-controller.yaml#
+
+properties:
+  compatible:
+    items:
+      - enum:
+          - apple,t6000-sio
+          - apple,t8103-sio
+      - const: apple,sio
+
+  reg:
+    maxItems: 1
+
+  '#dma-cells':
+    const: 1
+    description:
+      DMA clients specify a single cell that corresponds to the RTKit endpoint
+      number used for arranging the transfers in question
+
+  dma-channels:
+    maximum: 128
+
+  mboxes:
+    maxItems: 1
+
+  iommus:
+    maxItems: 1
+
+  power-domains:
+    maxItems: 1
+
+  memory-region:
+    minItems: 2
+    maxItems: 8
+    description:
+      A number of references to reserved memory regions among which are the DATA/TEXT
+      sections of coprocessor executable firmware and also auxiliary firmware data
+      describing the available DMA-enabled peripherals
+
+  apple,sio-firmware-params:
+    $ref: /schemas/types.yaml#/definitions/uint32-array
+    description: |
+      Parameters in the form of opaque key/value pairs that are to be sent to the SIO
+      coprocesssor once it boots. These parameters can point into the reserved memory
+      regions (in device address space).
+
+      Note that unlike Apple's firmware, we treat the parameters, and the data they
+      refer to, as opaque. Apple embed short data blobs into their SIO devicetree node
+      that describe the DMA-enabled peripherals (presumably with defined semantics).
+      Their driver processes those blobs and sets up data structure in mapped device
+      memory, then references this memory in the parameters sent to the SIO. At the
+      level of description we are opting for in this binding, we assume the job of
+      constructing those data structures has been done in advance, leaving behind an
+      opaque list of key/value parameter pairs to be sent by a prospective driver.
+
+      This approach is chosen for two reasons:
+
+       - It means we don't need to try to understand the semantics of Apple's blobs
+         as long as we know the transformation we need to do from Apple's devicetree
+         data to SIO data (which can be shoved away into a loader). It also means the
+         semantics of Apple's blobs (or of something to replace them) need not be part
+         of the binding and be kept up with Apple's firmware changes in the future.
+
+       - It leaves less work for the driver attaching on this binding. Instead the work
+         is done upfront in the loader which can be better suited for keeping up with
+         Apple's firmware changes.
+
+required:
+  - compatible
+  - reg
+  - '#dma-cells'
+  - dma-channels
+  - mboxes
+  - iommus
+  - power-domains
+
+additionalProperties: false
+
+examples:
+  - |
+    sio: dma-controller@36400000 {
+      compatible = "apple,t8103-sio", "apple,sio";
+      reg = <0x36400000 0x8000>;
+      dma-channels = <128>;
+      #dma-cells = <1>;
+      mboxes = <&sio_mbox>;
+      iommus = <&sio_dart 0>;
+      power-domains = <&ps_sio_cpu>;
+      memory-region = <&sio_text>, <&sio_data>,
+                      <&sio_auxdata1>, <&sio_auxdata2>; /* Filled by loader */
+      apple,sio-firmware-params = <0xb 0x10>, <0xc 0x1b80>, <0xf 0x14>,
+                                  <0x10 0x1e000>, <0x30d 0x34>, <0x30e 0x4000>,
+                                  <0x1a 0x38>, <0x1b 0x50>; /* Filled by loader */
+    };
-- 
2.33.0