[PATCH] arm64: Simplify checking for populated DT

[Mark Rutland]

Hi Rob,

On Thu, Oct 28, 2021 at 01:33:55PM -0500, Rob Herring wrote:
> Use of of_scan_flat_dt() function predates libfdt and is discouraged as
> libfdt provides a nicer set of APIs. Rework dt_scan_depth1_nodes to use
> libfdt calls directly. Rather than searching for any node not /chosen or
> /hypervisor, let's just check for something always required which is the
> arch timer.

The reason for checking for /chosen and /hypervisor specifically was
that we specifically permit a "stub" DT which only contains those nodes
and nothing more. This is also mentioned in the comment block in
acpi_boot_table_init().

I'm not keen on this change because it opens the door for people to
place arbitrary things in the DT as long as they don't add a timer node,
and I'd prefer that we continue to check for /chosen and /hypervisor
specifically.

Can we re-implement the existing logic using libfdt calls? e.g. using
fdt_for_each_subnode() on the root node?

Thanks,
Mark.

> Cc: Catalin Marinas <catalin.marinas at arm.com>
> Cc: Will Deacon <will at kernel.org>
> Signed-off-by: Rob Herring <robh at kernel.org>
> ---
>  arch/arm64/kernel/acpi.c | 29 ++++++-----------------------
>  1 file changed, 6 insertions(+), 23 deletions(-)
> 
> diff --git a/arch/arm64/kernel/acpi.c b/arch/arm64/kernel/acpi.c
> index 1c9c2f7a1c04..f1bb0fb4a604 100644
> --- a/arch/arm64/kernel/acpi.c
> +++ b/arch/arm64/kernel/acpi.c
> @@ -22,6 +22,7 @@
>  #include <linux/irq_work.h>
>  #include <linux/memblock.h>
>  #include <linux/of_fdt.h>
> +#include <linux/libfdt.h>
>  #include <linux/smp.h>
>  #include <linux/serial_core.h>
>  #include <linux/pgtable.h>
> @@ -62,29 +63,12 @@ static int __init parse_acpi(char *arg)
>  }
>  early_param("acpi", parse_acpi);
>  
> -static int __init dt_scan_depth1_nodes(unsigned long node,
> -				       const char *uname, int depth,
> -				       void *data)
> +static bool __init dt_is_populated(void)
>  {
> -	/*
> -	 * Ignore anything not directly under the root node; we'll
> -	 * catch its parent instead.
> -	 */
> -	if (depth != 1)
> -		return 0;
> -
> -	if (strcmp(uname, "chosen") == 0)
> -		return 0;
> +	const void *fdt = initial_boot_params;
> +	int node = fdt_node_offset_by_compatible(fdt, -1, "arm,armv8-timer");
>  
> -	if (strcmp(uname, "hypervisor") == 0 &&
> -	    of_flat_dt_is_compatible(node, "xen,xen"))
> -		return 0;
> -
> -	/*
> -	 * This node at depth 1 is neither a chosen node nor a xen node,
> -	 * which we do not expect.
> -	 */
> -	return 1;
> +	return node > 0 ? true : false;
>  }
>  
>  /*
> @@ -205,8 +189,7 @@ void __init acpi_boot_table_init(void)
>  	 *   and ACPI has not been [force] enabled (acpi=on|force)
>  	 */
>  	if (param_acpi_off ||
> -	    (!param_acpi_on && !param_acpi_force &&
> -	     of_scan_flat_dt(dt_scan_depth1_nodes, NULL)))
> +	    (!param_acpi_on && !param_acpi_force && dt_is_populated()))
>  		goto done;
>  
>  	/*
> -- 
> 2.32.0
>