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Message-ID: <ecf13de1-531f-b73b-b325-44ef78a3f818@gmail.com>
Date:   Thu, 23 Jun 2022 18:15:31 -0500
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Subject: Re: [PATCH v1 1/2] of: base: populate of_root node if not set
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To:     =?UTF-8?B?Q2zDqW1lbnQgTMOpZ2Vy?= <clement.leger@bootlin.com>,
        Rob Herring <robh+dt@kernel.org>
Cc:     linux-kernel@vger.kernel.org, devicetree@vger.kernel.org,
        Lizhi Hou <lizhi.hou@xilinx.com>,
        Allan Nielsen <allan.nielsen@microchip.com>,
        Horatiu Vultur <horatiu.vultur@microchip.com>,
        Steen Hegelund <steen.hegelund@microchip.com>,
        Thomas Petazzoni <thomas.petazzoni@bootlin.com>,
        Rob Herring <robh@kernel.org>
References: <20220623105044.152832-1-clement.leger@bootlin.com>
 <20220623105044.152832-2-clement.leger@bootlin.com>
From:   Frank Rowand <frowand.list@gmail.com>
In-Reply-To: <20220623105044.152832-2-clement.leger@bootlin.com>
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Hi Clement,

I said that I would send a patch to do this, but have failed to follow
through.  Sorry about that.


On 6/23/22 06:50, Clément Léger wrote:
> When enabling CONFIG_OF on a platform where of_root is not populated by
> firmware, we end up without a root node. In order to apply overlays and
> create subnodes of the root node, we need one. Create this root node
> by unflattening an empty builtin dtb with of_fdt_unflatten().
> 
> Co-developed-by: Rob Herring <robh@kernel.org>
> Signed-off-by: Rob Herring <robh@kernel.org>
> Signed-off-by: Clément Léger <clement.leger@bootlin.com>
> ---
>  drivers/of/Makefile       |  2 +-
>  drivers/of/base.c         | 18 ++++++++++++++++--
>  drivers/of/empty_root.dts |  6 ++++++
>  3 files changed, 23 insertions(+), 3 deletions(-)
>  create mode 100644 drivers/of/empty_root.dts
> 
> diff --git a/drivers/of/Makefile b/drivers/of/Makefile
> index e0360a44306e..ce56c8b95c83 100644
> --- a/drivers/of/Makefile
> +++ b/drivers/of/Makefile
> @@ -1,5 +1,5 @@
>  # SPDX-License-Identifier: GPL-2.0
> -obj-y = base.o device.o platform.o property.o
> +obj-y = base.o empty_root.dtb.o device.o platform.o property.o
>  obj-$(CONFIG_OF_KOBJ) += kobj.o
>  obj-$(CONFIG_OF_DYNAMIC) += dynamic.o
>  obj-$(CONFIG_OF_FLATTREE) += fdt.o
> diff --git a/drivers/of/base.c b/drivers/of/base.c
> index d4f98c8469ed..43e0f027a49c 100644
> --- a/drivers/of/base.c
> +++ b/drivers/of/base.c
> @@ -22,6 +22,7 @@
>  #include <linux/module.h>
>  #include <linux/of.h>
>  #include <linux/of_device.h>
> +#include <linux/of_fdt.h>
>  #include <linux/of_graph.h>
>  #include <linux/spinlock.h>
>  #include <linux/slab.h>
> @@ -163,6 +164,8 @@ void __of_phandle_cache_inv_entry(phandle handle)
>  		phandle_cache[handle_hash] = NULL;
>  }
>  
> +extern const char __dtb_empty_root_begin[];
> +
>  void __init of_core_init(void)
>  {
>  	struct device_node *np;
> @@ -176,6 +179,18 @@ void __init of_core_init(void)
>  		pr_err("failed to register existing nodes\n");
>  		return;
>  	}
> +
> +	if (!of_root) {
> +		void *dt;
> +		const unsigned long *fdt = (const unsigned long *)
> +							__dtb_empty_root_begin;
> +		dt = of_fdt_unflatten_tree(fdt, NULL, &of_root);
> +		if (!dt) {
> +			pr_err("Failed to setup empty root dt\n");
> +			return;
> +		}
> +	}
> +
>  	for_each_of_allnodes(np) {
>  		__of_attach_node_sysfs(np);
>  		if (np->phandle && !phandle_cache[of_phandle_cache_hash(np->phandle)])
> @@ -184,8 +199,7 @@ void __init of_core_init(void)
>  	mutex_unlock(&of_mutex);
>  
>  	/* Symlink in /proc as required by userspace ABI */
> -	if (of_root)
> -		proc_symlink("device-tree", NULL, "/sys/firmware/devicetree/base");
> +	proc_symlink("device-tree", NULL, "/sys/firmware/devicetree/base");
>  }

This approach is adding an additional method of unflattening the tree.
I would prefer to consolidate in a single location.

I have leveraged this patch series into a different patch series to
accomplish that.  I have boot tested with one configuration, but want
to test two more configurations before sending the new series.  It
should only take "a few minutes".

-Frank

>  
>  static struct property *__of_find_property(const struct device_node *np,
> diff --git a/drivers/of/empty_root.dts b/drivers/of/empty_root.dts
> new file mode 100644
> index 000000000000..cf9e97a60f48
> --- /dev/null
> +++ b/drivers/of/empty_root.dts
> @@ -0,0 +1,6 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/dts-v1/;
> +
> +/ {
> +
> +};