linux-kernel - Re: [PATCH stblinux.git 1/2] dt-bindings: firmware: add Broadcom's NVRAM memory mapping

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Message-ID: <93f1d985-54ae-9cbe-ef42-9e62b86d2633@gmail.com>
Date:   Mon, 8 Mar 2021 22:37:42 +0100
From:   Rafał Miłecki <zajec5@...il.com>
To:     Rob Herring <robh@...nel.org>
Cc:     Florian Fainelli <f.fainelli@...il.com>,
        Vivek Unune <npcomplete13@...il.com>,
        linux-arm-kernel@...ts.infradead.org, linux-mips@...r.kernel.org,
        devicetree@...r.kernel.org, bcm-kernel-feedback-list@...adcom.com,
        linux-kernel@...r.kernel.org,
        Rafał Miłecki <rafal@...ecki.pl>
Subject: Re: [PATCH stblinux.git 1/2] dt-bindings: firmware: add Broadcom's
 NVRAM memory mapping

On 08.03.2021 19:43, Rob Herring wrote:
> On Tue, Mar 02, 2021 at 08:44:04AM +0100, Rafał Miłecki wrote:
>> From: Rafał Miłecki <rafal@...ecki.pl>
>>
>> NVRAM structure contains device data and can be accessed using MMIO.
>>
>> Signed-off-by: Rafał Miłecki <rafal@...ecki.pl>
>> ---
>>   .../bindings/firmware/brcm,nvram.yaml         | 41 +++++++++++++++++++
>>   1 file changed, 41 insertions(+)
>>   create mode 100644 Documentation/devicetree/bindings/firmware/brcm,nvram.yaml
>>
>> diff --git a/Documentation/devicetree/bindings/firmware/brcm,nvram.yaml b/Documentation/devicetree/bindings/firmware/brcm,nvram.yaml
>> new file mode 100644
>> index 000000000000..12af8e2e7c9c
>> --- /dev/null
>> +++ b/Documentation/devicetree/bindings/firmware/brcm,nvram.yaml
>> @@ -0,0 +1,41 @@
>> +# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
>> +%YAML 1.2
>> +---
>> +$id: "http://devicetree.org/schemas/firmware/brcm,nvram.yaml#"
>> +$schema: "http://devicetree.org/meta-schemas/core.yaml#"
>> +
>> +title: Broadcom's NVRAM
>> +
>> +maintainers:
>> +  - Rafał Miłecki <rafal@...ecki.pl>
>> +
>> +description: |
>> +  NVRAM is a structure containing device specific environment variables.
>> +  It is used for storing device configuration, booting parameters and
>> +  calibration data.
> 
> The structure of the data is fully discoverable just from a genericish
> 'brcm,nvram'?

Yes, NVRAM structure is a header (with magic and length) and a list of
key-value pairs separated by \0. If you map memory at given address you
should verify magic and start reading key-value pairs.

Content example: foo=bar\0baz=qux\0quux(...)

There is no predefined order of pairs, set of keys or anything similar I
could think of. I can't think of anything more worth describing in DT.


> And it's a dedicated memory outside of regular RAM?

Yes