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@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 .
