Re: [PATCH v3] mux: mmio: use reg property when parent device is not a syscon

[Peter Rosin]

Hi!

2023-09-11 at 17:10, 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 reg property is provided, use that to
> identify the address space for this mux. If not provided fallback to
> using the parent device as a regmap provider.
> 
> Signed-off-by: Andrew Davis <afd@xxxxxx>
> Reviewed-by: Nishanth Menon <nm@xxxxxx>
> ---
> 
> Changes from v2:
>  - Rebased on v6.6-rc1
> 
> Changes from v1:
>  - Flip logic as suggested in v1[0]
> 
> [0] https://lore.kernel.org/lkml/1c27d9d4-b1cc-c158-90f7-f7e47e02c424@xxxxxx/T/
> 
>  drivers/mux/mmio.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/mux/mmio.c b/drivers/mux/mmio.c
> index fd1d121a584ba..b6095b7853ed2 100644
> --- a/drivers/mux/mmio.c
> +++ b/drivers/mux/mmio.c
> @@ -44,10 +44,13 @@ static int mux_mmio_probe(struct platform_device *pdev)
>  	int ret;
>  	int i;
>  
> -	if (of_device_is_compatible(np, "mmio-mux"))
> +	if (of_device_is_compatible(np, "mmio-mux")) {
>  		regmap = syscon_node_to_regmap(np->parent);
> -	else
> -		regmap = dev_get_regmap(dev->parent, NULL) ?: ERR_PTR(-ENODEV);
> +	} else {
> +		regmap = device_node_to_regmap(np);

I started digging in device_node_to_regmap() to try to find an error that
could be used to trigger if the failover to dev_get_regmap() should be
tried, instead of always doing the failover on error. I got lost fairly
quickly, but it seems device_node_to_regmap() can return -EDEFER_PROBE.
While I'm not certain that it is applicable, that case should probably
not fall back to dev_get_regmap()...

Are there other error cases that should prevent the failover? I would
guess that it's perhaps just a single error that should trigger trying
the failover path? But I don't know, and which error if that's the case?

How much badness can be caused if syscon_node_to_regmap() fails for some
random obscure reason and the failover path is taken inadvertently? It
certainly smells bad for -EDEFER_PROBE, but do you have any insight in
other cases?

And after getting to approx that point a while back, I had other things
to take care of, and this fell off the table. Sorry!

Cheers,
Peter

> +		if (IS_ERR(regmap))
> +			regmap = dev_get_regmap(dev->parent, NULL) ?: ERR_PTR(-ENODEV);
> +	}
>  	if (IS_ERR(regmap)) {
>  		ret = PTR_ERR(regmap);
>  		dev_err(dev, "failed to get regmap: %d\n", ret);