[PATCH 1/3] [RFC] dt-bindings: nvmem: syscon: Add syscon backed nvmem bindings
Marek Vasut
marex at denx.de
Sun Nov 27 14:05:18 PST 2022
On 10/28/22 23:28, Rob Herring wrote:
> On Fri, Oct 28, 2022 at 12:50:18AM +0200, Marek Vasut wrote:
>> Add trivial bindings for driver which permits exposing syscon backed
>> register to userspace. This is useful e.g. to expose U-Boot boot
>> counter on various platforms where the boot counter is stored in
>> random volatile register, like STM32MP15xx TAMP_BKPxR register.
>
> Generic bindings always start trivial until they get appended one
> property at a time...
>
> What happens when you have more than 1 field and/or more than 1
> register?
If it is a continuous register array, the user can use the size field to
describe such register array here.
If it is a sparse register array, multiple nvmem-syscon nodes would be
needed. I haven't seen anything which would require one node for sparse
register arrays, like boot counter distributed across multiple
non-continuous registers or such.
>> +properties:
>> + compatible:
>> + enum:
>> + - nvmem-syscon
>> +
>> + reg:
>> + maxItems: 1
>> +
>> +required:
>> + - compatible
>> + - reg
>> +
>> +additionalProperties: false
>> +
>> +examples:
>> + - |
>> + tamp at 5c00a000 {
>> + compatible = "st,stm32-tamp", "syscon", "simple-mfd";
>
> This is very common, but personally I think "syscon" and "simple-mfd"
> should be mutually exclusive. "simple-mfd" is saying the children have
> no dependency on the parent, yet the child nodes need a regmap from the
> parent. Sounds like a dependency.
So what exactly should be changed here?
>> + reg = <0x5c00a000 0x400>;
>> +
>> + nvmem-syscon {
>> + compatible = "nvmem-syscon";
>> + reg = <0x14c 0x4>;
>
> How does one identify this is the bootloader's boot count?
The user has to know where the boot counter is stored (by hardware
path). I wouldn't attempt to assign any complex logic here, since the
boot counter could be implemented in various ways. Besides, this may not
even be a boot counter, but some other variable exposed to user space.
Maybe a unique node name can be used to discern the different
nvmem-syscon nodes representing different variables if needed.
> How does the
> bootloader know it can write to this?
The bootloader implementer selected the bootcounter register based on
hardware knowledge .
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