Re: [PATCH 3/3] mfd: syscon: Allow syscon nodes without a "syscon" compatible

[William McVicker <willmcvicker@google.com>]

Hi Rob,

Thanks for working on this!

On 12/11/2024, Rob Herring (Arm) wrote:
> of_syscon_register_regmap() was added for nodes which need a custom
> regmap setup. It's not really correct for those nodes to claim they are
> compatible with "syscon" as the default handling likely doesn't work in
> those cases. If device_node_get_regmap() happens to be called first,
> then of_syscon_register() will be called and an incorrect regmap will be
> created (barring some other error). That may lead to unknown results in
> the worst case. In the best case, of_syscon_register_regmap() will fail
> with -EEXIST. This problem remains unless these cases drop "syscon" (an
> ABI issue) or we exclude them using their specific compatible. ATM,
> there is only one user: "google,gs101-pmu"
> 
> There are also cases of adding "syscon" compatible to existing nodes
> after the fact in order to register the syscon. That presents a
> potential DT ABI problem. Instead, if there's a kernel change needing a
> syscon for a node, then it should be possible to allow the kernel to
> register a syscon without a DT change. That's only possible by using
> of_syscon_register_regmap() currently, but in the future we may want to
> support a match list for cases which don't need a custom regmap.
> 
> With this change, the lookup functions will succeed for any node
> registered by of_syscon_register_regmap() regardless of whether the node
> compatible contains "syscon".
> 
> Signed-off-by: Rob Herring (Arm) <robh@kernel.org>
> ---
>  drivers/mfd/syscon.c | 7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/mfd/syscon.c b/drivers/mfd/syscon.c
> index bfb1f69fcff1..e6df2825c14d 100644
> --- a/drivers/mfd/syscon.c
> +++ b/drivers/mfd/syscon.c
> @@ -171,8 +171,10 @@ static struct regmap *device_node_get_regmap(struct device_node *np,
>  			break;
>  		}
>  
> -	if (!syscon)
> +	if (!syscon && of_device_is_compatible(np, "syscon"))
>  		syscon = of_syscon_register(np, check_res);
> +	else
> +		syscon = ERR_PTR(-EINVAL);

This else case actually breaks Pixel 6 (Tensor) since you are now returning
-EINVAL when the syscon is created by the exynos-pmu driver and present in the
list. Instead you should only return -EINVAL if the syscon doesn't exist and
the device node is not a compatible syscon device. If you still want to check
for `of_device_is_compatible(np, "syscon")` even when the syscon is found in
the list, that should be okay , but it's probably best to check that before
inserting the regmap in the list to begin with.

This worked for me on my Pixel 6 device:

	if (!syscon) {
		if (of_device_is_compatible(np, "syscon"))
			syscon = of_syscon_register(np, check_res);
		else
			syscon = ERR_PTR(-EINVAL);
	}

Thanks,
Will