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Date:   Mon, 23 Oct 2023 11:26:09 -0500
From:   Andrew Davis <afd@...com>
To:     Peter Rosin <peda@...ntia.se>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Nishanth Menon <nm@...com>,
        Vignesh Raghavendra <vigneshr@...com>
CC:     <devicetree@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] mux: mmio: use reg property when parent device is not
 a syscon

On 10/20/23 4:22 PM, Peter Rosin wrote:
> Hi!
> 
> 2023-10-20 at 18:43, Andrew Davis wrote:
>> 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@...com>
>>>> Reviewed-by: Nishanth Menon <nm@...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.
> 
> The ideal working case is usually not much of a problem. When I look at what
> device_node_to_regmap does, I find, appart from -ENOMEM, possibilities of
> -ENOENT (because no clock), and the clock may theoretically fail to prepare
> for numerous reasons hidden in clock drivers, but the clock core can
> trigger at least -EACCES and -EINPROGRESS via runtime PM.
> 
> And it definitely looks like the -EPROBE_DEFER case needs to be addressed.
> I.e., why is this call chain not a problem?
> 
> mux_mmio_probe
>   ->device_node_to_regmap
>    -> device_node_get_regmap
>     -> of_syscon_register
>      -> of_hwspin_lock_get_id
>      <- -EPROBE_DEFER
>     <- ERR_PTR(-EPROBE_DEFER)
>    <- ERR_PTR(-EPROBE_DEFER)
>   <- ERR_PTR(-EPROBE_DEFER)
> 
> As far as I can tell, if device_node_to_regmap() fails with -EPROBE_DEFER
> with your patch, then mux_mmio_probe() misbehaves. It should have aborted
> and failed with -EPROBE_DEFER, but instead throws that error away and
> goes on to try dev_get_regmap(). That, in turn, is probably futile and
> will likely error out in some way, breaking a system that might have been
> ok, if the probe had been retried some time later.
> 

This is why I liked the v1 version, dev_get_regmap() just returns a
simple NULL on error, no complex EPROBE_DEFER oddness :)

So is EPROBE_DEFER the only one we think should retry and not go
down the fallback path? I believe that is the normal assumption
for most drivers.

> As long as the above is not sufficiently explained away, or fixed, I
> consider the patch broken.
> 
>>> 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).
> 
> Hold your horses, I need the above explanation first (and perhaps an
> updated patch).
> 

I'm not normally so impatient but this went two whole kernel cycles without
any comment until rc6.. v4 on the way.

Andrew

> Cheers,
> Peter
> 
>> 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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