On 5/16/23 11:19 AM, Krzysztof Kozlowski wrote:
On 16/05/2023 17:18, Andrew Davis wrote:
On 5/15/23 4:14 PM, Peter Rosin wrote:
Hi!
2023-05-15 at 21:19, Andrew Davis wrote:
The DT binding for the reg-mux compatible states it can be used when the
"parent device of mux controller is not syscon device". It also allows
for a reg property. When the parent device is indeed not a syscon device,
nor is it a regmap provider, we should fallback to using that reg
property to identify the address space to use for this mux.
We should? Says who?
Don't get me wrong, I'm not saying the change is bad or wrong, I would just
like to see an example where it matters. Or, at least some rationale for why
the code needs to change other than covering some case that looks like it
could/should be possible based on the binding. I.e., why is it not better to
"close the hole" in the binding instead?
Sure, so this all stated when I was building a checker to make sure that drivers
are not mapping overlapping register spaces. I noticed syscon nodes are a source
of that so I'm trying to look into their usage.
To start, IHMO there is only one valid use for syscon and that is when more than
one driver needs to access shared bits in a single register. DT has no way to
It has... what about all existing efuse/nvmem devices?
describe down to the bit granular level, so one must give that register to
a "syscon node", then have the device node use a phandle to the syscon node:
common_reg: syscon@10000 {
compatible = "syscon";
reg = <0x10000 0x4>;
};
consumer@1 {
syscon-efuse = <&common_reg 0x1>;
};
consumer@2 {
syscon-efuse = <&common_reg 0x2>;
};
Something like that, then regmap will take care of synchronizing access.
Syscon is not for this.
That is how it is used today, and in 5 other ways too and there is
no guidance on it. Let me know what syscon is for then.
...
Ideally DT nodes all describe their register space in a "reg"
property and all the "large collection of devices" spaces become
"simple-bus" nodes. "syscon" nodes can then be limited to only the
rare case when multiple devices share bits in a single register.
If Rob and Krzysztof agree I can send a patch with the above
guidance to the Devicetree Specification repo also.
Agree on what?
That we should provide the above guidance on when and how to use syscon
nodes. Right now it is a free for all and it is causing issues.
Andrew
Best regards,
Krzysztof
