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Date: Wed, 24 Jan 2024 09:00:02 -0600
From: Rob Herring <robh@kernel.org>
To: Amrit Anand <quic_amrianan@quicinc.com>
Cc: krzysztof.kozlowski+dt@linaro.org, conor+dt@kernel.org,
	agross@kernel.org, andersson@kernel.org, konrad.dybcio@linaro.org,
	devicetree@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-arm-msm@vger.kernel.org, kernel@quicinc.com,
	Elliot Berman <quic_eberman@quicinc.com>
Subject: Re: [PATCH 1/2] dt-bindings: hwinfo: Introduce board-id
Message-ID: <20240124150002.GB873781-robh@kernel.org>
References: <1705749649-4708-1-git-send-email-quic_amrianan@quicinc.com>
 <1705749649-4708-2-git-send-email-quic_amrianan@quicinc.com>
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On Sat, Jan 20, 2024 at 04:50:48PM +0530, Amrit Anand wrote:
> From: Elliot Berman <quic_eberman@quicinc.com>
> 
> Device manufacturers frequently ship multiple boards or SKUs under a
> single software package. These software packages will ship multiple
> devicetree blobs and require some mechanism to pick the correct DTB for
> the board the software package was deployed. Introduce a common
> definition for adding board identifiers to device trees. board-id
> provides a mechanism for bootloaders to select the appropriate DTB which
> is vendor/OEM-agnostic.
> 
> Isn't that what the compatible property is for?
> -----------------------------------------------
> The compatible property can be used for board matching, but requires
> bootloaders and/or firmware to maintain a database of possible strings
> to match against or have complex compatible string matching. Compatible
> string matching becomes complicated when there are multiple versions of
> board: the device tree selector should recognize a DTB that cares to
> distinguish between v1/v2 and a DTB that doesn't make the distinction.
> An eeprom either needs to store the compatible strings that could match
> against the board or the bootloader needs to have vendor-specific
> decoding logic for the compatible string. Neither increasing eeprom
> storage nor adding vendor-specific decoding logic is desirable.
> 
> The solution proposed here is simpler to implement and doesn't require
> updating firmware or bootloader for every new board.
> 
> How is this better than Qualcomm's qcom,msm-id/qcom,board-id?
> -------------------------------------------------------------
> The selection process for devicetrees was Qualcomm-specific and not
> useful for other devices and bootloaders that were not developed by
> Qualcomm because a complex algorithm was used to implement. Board-ids
> provide a matching solution that can be implemented by bootloaders
> without introducing vendor-specific code. Qualcomm uses three
> devicetree properties: msm-id (interchangeably: soc-id), board-id, and
> pmic-id.  This does not scale well for use casese which use identifiers,
> for example, to distinguish between a display panel. For a display
> panel, an approach could be to add a new property: display-id,
> but now	bootloaders need to be updated to also read this property. We
> want to	avoid requiring to update bootloaders with new hardware
> identifiers: a bootloader need only recognize the identifiers it can
> handle.
> 
> Signed-off-by: Elliot Berman <quic_eberman@quicinc.com>
> Signed-off-by: Amrit Anand <quic_amrianan@quicinc.com>
> ---
>  .../devicetree/bindings/hwinfo/board-id.yaml       | 53 ++++++++++++++++++++++
>  1 file changed, 53 insertions(+)
>  create mode 100644 Documentation/devicetree/bindings/hwinfo/board-id.yaml
> 
> diff --git a/Documentation/devicetree/bindings/hwinfo/board-id.yaml b/Documentation/devicetree/bindings/hwinfo/board-id.yaml
> new file mode 100644
> index 0000000..82d5ff7
> --- /dev/null
> +++ b/Documentation/devicetree/bindings/hwinfo/board-id.yaml
> @@ -0,0 +1,53 @@
> +# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
> +%YAML 1.2
> +---
> +$id: http://devicetree.org/schemas/hwinfo/board-id.yaml#
> +$schema: http://devicetree.org/meta-schemas/core.yaml#
> +
> +title: Board Identifier for Devicetree Selection
> +
> +maintainers:
> +  - Amrit Anand <quic_amrianan@quicinc.com>
> +  - Elliot Berman <quic_eberman@quicinc.com>
> +
> +description: |
> +  Device manufacturers frequently ship multiple boards under a single
> +  software package. These software packages will ship multiple devicetree
> +  blobs and require some mechanism to pick the correct DTB for the board
> +  the software package was deployed. board-id provides a mechanism for
> +  bootloaders to select the appropriate DTB which is vendor/OEM-agnostic.
> +
> +select:
> +  anyOf:
> +    - required:
> +        - 'board-id'
> +    - required:
> +        - 'board-id-types'
> +    - required:
> +        - '#board-id-cells'
> +
> +properties:
> +  $nodename:
> +    const: "/"
> +  board-id:
> +    $ref: /schemas/types.yaml#/definitions/uint32-matrix
> +    description: |
> +      A list of identifiers that can be used to match with this devicetree.
> +      The interpretatation of each cell can be matched with the
> +      board-id-type at the same index.
> +
> +  board-id-types:
> +    $ref: /schemas/types.yaml#/definitions/non-unique-string-array
> +    description:
> +      Defines the type of each cell, indicating to the DeviceTree selection
> +      mechanism how to parse the board-id.
> +
> +  '#board-id-cells':
> +    minimum: 1

This is not how #foo-cells works. It is for provider/consumer style 
bindings.

Rob