linux-kernel - Re: [PATCH v1 09/13] kho: Update FDT dynamically for subtree addition/removal

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Message-ID: <aRdVn-e3Wu_pFdmZ@kernel.org>
Date: Fri, 14 Nov 2025 18:15:27 +0200
From: Mike Rapoport <rppt@...nel.org>
To: Pasha Tatashin <pasha.tatashin@...een.com>
Cc: akpm@...ux-foundation.org, bhe@...hat.com, jasonmiu@...gle.com,
	arnd@...db.de, coxu@...hat.com, dave@...ilevsky.ca,
	ebiggers@...gle.com, graf@...zon.com, kees@...nel.org,
	linux-kernel@...r.kernel.org, kexec@...ts.infradead.org,
	linux-mm@...ck.org
Subject: Re: [PATCH v1 09/13] kho: Update FDT dynamically for subtree
 addition/removal

On Fri, Nov 14, 2025 at 10:53:54AM -0500, Pasha Tatashin wrote:
> Currently, sub-FDTs were tracked in a list (kho_out.sub_fdts) and the
> final FDT is constructed entirely from scratch during kho_finalize().
> 
> We can maintain the FDT dynamically:
> 1. Initialize a valid, empty FDT in kho_init().
> 2. Use fdt_add_subnode and fdt_setprop in kho_add_subtree to
>    update the FDT immediately when a subsystem registers.
> 3. Use fdt_del_node in kho_remove_subtree to remove entries.
> 
> This removes the need for the intermediate sub_fdts list and the
> reconstruction logic in kho_finalize(). kho_finalize() now
> only needs to trigger memory map serialization.
> 
> Signed-off-by: Pasha Tatashin <pasha.tatashin@...een.com>
> ---
>  kernel/liveupdate/kexec_handover.c | 144 ++++++++++++++---------------
>  1 file changed, 68 insertions(+), 76 deletions(-)
> 
> diff --git a/kernel/liveupdate/kexec_handover.c b/kernel/liveupdate/kexec_handover.c
> index 8ab77cb85ca9..822da961d4c9 100644
> --- a/kernel/liveupdate/kexec_handover.c
> +++ b/kernel/liveupdate/kexec_handover.c
> @@ -724,37 +713,67 @@ static void __init kho_reserve_scratch(void)
>   */
>  int kho_add_subtree(const char *name, void *fdt)
>  {
> -	struct kho_sub_fdt *sub_fdt;
> +	phys_addr_t phys = virt_to_phys(fdt);
> +	void *root_fdt = kho_out.fdt;
> +	int err = -ENOMEM;
> +	int off, fdt_err;
>  
> -	sub_fdt = kmalloc(sizeof(*sub_fdt), GFP_KERNEL);
> -	if (!sub_fdt)
> -		return -ENOMEM;
> +	guard(mutex)(&kho_out.lock);
> +
> +	fdt_err = fdt_open_into(root_fdt, root_fdt, PAGE_SIZE);
> +	if (fdt_err < 0)
> +		return err;
> -	INIT_LIST_HEAD(&sub_fdt->l);
> -	sub_fdt->name = name;
> -	sub_fdt->fdt = fdt;
> +	off = fdt_add_subnode(root_fdt, 0, name);

	fdt_err = fdt_add_subnode();

and then we don't need off

> +	if (off < 0) {
> +		if (off == -FDT_ERR_EXISTS)
> +			err = -EEXIST;

Is it really -ENOMEM for other FDT_ERR values?
		
> +		goto out_pack;
> +	}
> +
> +	err = fdt_setprop(root_fdt, off, PROP_SUB_FDT, &phys, sizeof(phys));
> +	if (err < 0)
> +		goto out_pack;
>  
> -	guard(mutex)(&kho_out.fdts_lock);
> -	list_add_tail(&sub_fdt->l, &kho_out.sub_fdts);
>  	WARN_ON_ONCE(kho_debugfs_fdt_add(&kho_out.dbg, name, fdt, false));
>  
> -	return 0;
> +out_pack:
> +	fdt_pack(root_fdt);
> +
> +	return err;
>  }
>  EXPORT_SYMBOL_GPL(kho_add_subtree);

-- 
Sincerely yours,
Mike.