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SubjectRe: [PATCH v3] mux: mmio: use reg property when parent device is not a syscon
From
On 10/20/23 9:28 AM, Peter Rosin wrote:
> 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@ti.com>
>> Reviewed-by: Nishanth Menon <nm@ti.com>
>> ---
>>
>> 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@ti.com/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?
>

Ideally the only error that will be returned is ENOMEM, which happens when
this node does not have a 'reg' property, and this is also the one case we
want to do the failover. So all should be well.

> 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?
>

If we take the failover inadvertently then we will check if the parent
node is a syscon, if it is then our offset will most likely be wrong
(parent will not match child 'reg').

> 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!
>

No problem as long as we can find a way to get this in quickly (lot of
DT warning need cleaned up based on this patch).

Thanks
Andrew

> 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);

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