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

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

[Andrew Davis]

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);
+		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);
-- 
2.39.2

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@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?

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);

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

[Andrew Davis]

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);

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

[Peter Rosin]

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@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.

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.

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).

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);

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

[Andrew Davis]

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@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.
> 
> 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);