linux-kernel - Re: [PATCH 1/6] arm64: Unconditionally call unflatten_device

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Date: Tue, 16 Jan 2024 11:51:14 +0000
From: Mark Rutland <mark.rutland@....com>
To: Stephen Boyd <sboyd@...nel.org>
Cc: Rob Herring <robh+dt@...nel.org>, linux-kernel@...r.kernel.org,
	patches@...ts.linux.dev, linux-um@...ts.infradead.org,
	linux-arm-kernel@...ts.infradead.org, kunit-dev@...glegroups.com,
	linux-kselftest@...r.kernel.org, devicetree@...r.kernel.org,
	Frank Rowand <frowand.list@...il.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>
Subject: Re: [PATCH 1/6] arm64: Unconditionally call unflatten_device_tree()

Hi Stephen,

On Fri, Jan 12, 2024 at 12:07:44PM -0800, Stephen Boyd wrote:
> Call this function unconditionally so that we can populate an empty DTB
> on platforms that don't boot with a firmware provided or builtin DTB.
> There's no harm in calling unflatten_device_tree() unconditionally.

For better or worse, that's not true: there are systems the provide both a DTB
*and* ACPI tables, and we must not consume both at the same time as those can
clash and cause all sorts of problems. In addition, we don't want people being
"clever" and describing disparate portions of their system in ACPI and DT.

It is a very deliberate choice to not unflatten the DTB when ACPI is in use,
and I don't think we want to reopen this can of worms.

Given that, I'm afraid I must NAK this patch.

Thanks
Mark.

> If there isn't a valid initial_boot_params dtb then unflatten_device_tree()
> returns early.
> 
> Cc: Rob Herring <robh+dt@...nel.org>
> Cc: Frank Rowand <frowand.list@...il.com>
> Cc: Catalin Marinas <catalin.marinas@....com>
> Cc: Will Deacon <will@...nel.org>
> Cc: <linux-arm-kernel@...ts.infradead.org>
> Signed-off-by: Stephen Boyd <sboyd@...nel.org>
> ---
>  arch/arm64/kernel/setup.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/kernel/setup.c b/arch/arm64/kernel/setup.c
> index 417a8a86b2db..ede3d59dabf0 100644
> --- a/arch/arm64/kernel/setup.c
> +++ b/arch/arm64/kernel/setup.c
> @@ -351,8 +351,7 @@ void __init __no_sanitize_address setup_arch(char **cmdline_p)
>  	/* Parse the ACPI tables for possible boot-time configuration */
>  	acpi_boot_table_init();
>  
> -	if (acpi_disabled)
> -		unflatten_device_tree();
> +	unflatten_device_tree();
>  
>  	bootmem_init();
>  
> -- 
> https://git.kernel.org/pub/scm/linux/kernel/git/clk/linux.git/
> https://git.kernel.org/pub/scm/linux/kernel/git/sboyd/spmi.git
> 
>