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

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Message-ID: <7b4afaa7-13df-513a-5986-e1a9f5f5d7ed@gmail.com>
Date:   Sun, 14 Nov 2021 14:41:06 -0500
From:   Frank Rowand <frowand.list@...il.com>
To:     Matthias Schiffer <matthias.schiffer@...tq-group.com>,
        Rob Herring <robh+dt@...nel.org>
Cc:     devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] of: base: Skip CPU nodes with non-"okay"/"disabled"
 status

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.

-Frank

> +			continue;
>  		if (!(of_node_name_eq(next, "cpu") ||
>  		      __of_node_is_type(next, "cpu")))
>  			continue;
>