linux-kernel - Re: [PATCH] of: base: Skip CPU nodes with non-"okay"/"disabled" status

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Date:   Mon, 15 Nov 2021 12:23:29 -0500
From:   Frank Rowand <frowand.list@...il.com>
To:     Matthias Schiffer <matthias.schiffer@...tq-group.com>
Cc:     devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        Rob Herring <robh+dt@...nel.org>
Subject: Re: [PATCH] of: base: Skip CPU nodes with non-"okay"/"disabled"
 status

Hi Matthias,

On 11/15/21 3:13 AM, Matthias Schiffer wrote:
> On Sun, 2021-11-14 at 14:41 -0500, Frank Rowand wrote:
>> On 11/8/21 3:48 AM, Matthias Schiffer wrote:
>>> Allow fully disabling CPU nodes using status = "fail". Having no status
>>> property at all is still interpreted as "okay" as usual.
>>>
>>> This allows a bootloader to change the number of available CPUs (for
>>> example when a common DTS is used for SoC variants with different numbers
>>> of cores) without deleting the nodes altogether, which could require
>>> additional fixups to avoid dangling phandle references.
>>>
>>> References:
>>> - https://www.lkml.org/lkml/2020/8/26/1237
>>> - https://www.spinics.net/lists/devicetree-spec/msg01007.html
>>> - https://github.com/devicetree-org/dt-schema/pull/61
>>>
>>> Signed-off-by: Matthias Schiffer <matthias.schiffer@...tq-group.com>
>>> ---
>>>  drivers/of/base.c | 29 +++++++++++++++++++++++++++++
>>>  1 file changed, 29 insertions(+)
>>>
>>> diff --git a/drivers/of/base.c b/drivers/of/base.c
>>> index 61de453b885c..4e9973627c8d 100644
>>> --- a/drivers/of/base.c
>>> +++ b/drivers/of/base.c
>>> @@ -650,6 +650,32 @@ bool of_device_is_available(const struct device_node *device)
>>>  }
>>>  EXPORT_SYMBOL(of_device_is_available);
>>>  
>>> +/**
>>> + *  __of_device_is_disabled - check if a device has status "disabled"
>>> + *
>>> + *  @device: Node to check status for, with locks already held
>>> + *
>>> + *  Return: True if the status property is set to "disabled",
>>> + *  false otherwise
>>> + *
>>> + *  Most callers should use __of_device_is_available() instead, this function
>>> + *  only exists due to the special interpretation of the "disabled" status for
>>> + *  CPU nodes.
>>> + */
>>> +static bool __of_device_is_disabled(const struct device_node *device)
>>> +{
>>> +	const char *status;
>>> +
>>> +	if (!device)
>>> +		return false;
>>> +
>>> +	status = __of_get_property(device, "status", NULL);
>>> +	if (status == NULL)
>>> +		return false;
>>> +
>>> +	return !strcmp(status, "disabled");
>>> +}
>>> +
>>>  /**
>>>   *  of_device_is_big_endian - check if a device has BE registers
>>>   *
>>> @@ -817,6 +843,9 @@ struct device_node *of_get_next_cpu_node(struct device_node *prev)
>>>  		of_node_put(node);
>>>  	}
>>>  	for (; next; next = next->sibling) {
>>> +		if (!__of_device_is_available(next) &&
>>> +		    !__of_device_is_disabled(next))
>>
>> Shouldn't that just be a check to continue if the device is disabled?
>>
>> If adding a check for available, then all of the callers of for_each_of_cpu_node()
>> need to be checked.  There is at least one that is suspicious - arch/arm/mach-imx/platsmp.c
>> has a comment:
>>
>>  * Initialise the CPU possible map early - this describes the CPUs
>>  * which may be present or become present in the system.
> 

Thanks for the links to previous discussion you provided below.  I had
forgotten the previous discussion.

In [2] Rob ended up saying that there were two options he was fine with.
Either (or both), in of_get_next_cpu_node(),

  (1) use status of "fail" as the check or

  (2) use status of "disabled" as the check, conditional on !IS_ENABLED(CONFIG_PPC)
      "this would need some spec update"
      "Need to double check MIPS and Sparc though."

Neither of these two options are what this patch does.  It seems to me that
option 1 is probably the easiest and least intrusive method.

-Frank

> Previously, there were two option for the (effective) value of the
> status of a device_node:
> 
> - "okay", "ok" or unset
> - anything else (which includes "disabled" and "fail")
> 
> __of_device_is_available() checks which of these two is the case.
> 
> With the new code, we have 3 cases for the status of CPU nodes:
> 
> - "okay", "ok" or unset
> - "disabled"
> - anything else ("fail", ...)
> 
> My patch will only change the behaviour in one case: When a CPU node
> has a status that is not "okay", "ok", "disabled" or unset - which is
> exactly the point of my patch.
> 
> See also the change [1], which removed the !available check a while
> ago, and the discussion in [2], which led us to the conclusion that 
> of_get_next_cpu_node() must not distinguish "okay" and "disabled" CPU
> nodes and we instead need a third status to disable a CPU for real.
> 
> [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/drivers/of/base.c?id=c961cb3be9064d1097ccc019390f8b5739daafc6
> [2] https://www.lkml.org/lkml/2020/8/26/1237
> 
> 
>>
>> -Frank
>>
>>> +			continue;
>>>  		if (!(of_node_name_eq(next, "cpu") ||
>>>  		      __of_node_is_type(next, "cpu")))
>>>  			continue;
>>>
>>
>>
>