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Date:   Mon, 16 May 2022 23:22:57 -0400
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Subject: Re: [PATCH V2 Create empty OF root 1/1] of: create empty of root
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To:     Lizhi Hou <lizhi.hou@xilinx.com>, devicetree@vger.kernel.org,
        robh@kernel.org
Cc:     linux-kernel@vger.kernel.org, yilun.xu@intel.com, maxz@xilinx.com,
        sonal.santan@xilinx.com, yliu@xilinx.com, michal.simek@xilinx.com,
        stefanos@xilinx.com, trix@redhat.com, mdf@kernel.org,
        dwmw2@infradead.org, Max Zhen <max.zhen@xilinx.com>
References: <20220216050056.311496-1-lizhi.hou@xilinx.com>
 <20220216050056.311496-2-lizhi.hou@xilinx.com>
From:   Frank Rowand <frowand.list@gmail.com>
In-Reply-To: <20220216050056.311496-2-lizhi.hou@xilinx.com>
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Precedence: bulk

On 2/16/22 23:00, Lizhi Hou wrote:
> Xilinx Alveo PCIe accelerator cards use flattened device tree to describe
> describe apertures in its PCIe BARs. Each device tree node represents an
> aperture and each aperture is an hardware unit which requires a driver.
> The product detail:
>     https://www.xilinx.com/products/boards-and-kits/alveo.html
> 
> Thus a base device tree is required. Then the Alveo card driver can load
> its flattened device tree and overlay it to the base tree. However device
> tree does not always exist. To resolve this, add OF_EMPTY_ROOT config.
> When it is selected and there is not a device tree, create an empty device
> tree root node.
> 

Please run scripts/get_maintainer on your patches to see who to put in
the distribution list.

I recently stumbled across this patch series and the previous related
patch series (currently up to
"[PATCH V3 XRT Alveo Infrastructure 0/8] XRT Alveo driver infrastructure overview")
when I noticed it in an email archive.

A similar need of creating an of_root if otherwise empty is being discussed
in the thread "[PATCH 0/3] add dynamic PCI device of_node creation for overlay"
at https://lore.kernel.org/r/20220427094502.456111-1-clement.leger@bootlin.com

-Frank

> Signed-off-by: Sonal Santan <sonal.santan@xilinx.com>
> Signed-off-by: Max Zhen <max.zhen@xilinx.com>
> Signed-off-by: Lizhi Hou <lizhi.hou@xilinx.com>
> ---
>  drivers/of/Kconfig         |  5 ++++
>  drivers/of/Makefile        |  1 +
>  drivers/of/of_empty_root.c | 51 ++++++++++++++++++++++++++++++++++++++
>  3 files changed, 57 insertions(+)
>  create mode 100644 drivers/of/of_empty_root.c
> 
> diff --git a/drivers/of/Kconfig b/drivers/of/Kconfig
> index 80b5fd44ab1c..452d2316b47b 100644
> --- a/drivers/of/Kconfig
> +++ b/drivers/of/Kconfig
> @@ -94,4 +94,9 @@ config OF_DMA_DEFAULT_COHERENT
>  	# arches should select this if DMA is coherent by default for OF devices
>  	bool
>  
> +config OF_EMPTY_ROOT
> +	# driver which requires at least the empty base device tree to
> +	# overlay its own device nodes should select this.
> +	bool
> +
>  endif # OF
> diff --git a/drivers/of/Makefile b/drivers/of/Makefile
> index e0360a44306e..c65364f32935 100644
> --- a/drivers/of/Makefile
> +++ b/drivers/of/Makefile
> @@ -12,6 +12,7 @@ obj-$(CONFIG_OF_RESERVED_MEM) += of_reserved_mem.o
>  obj-$(CONFIG_OF_RESOLVE)  += resolver.o
>  obj-$(CONFIG_OF_OVERLAY) += overlay.o
>  obj-$(CONFIG_OF_NUMA) += of_numa.o
> +obj-$(CONFIG_OF_EMPTY_ROOT) += of_empty_root.o
>  
>  ifdef CONFIG_KEXEC_FILE
>  ifdef CONFIG_OF_FLATTREE
> diff --git a/drivers/of/of_empty_root.c b/drivers/of/of_empty_root.c
> new file mode 100644
> index 000000000000..5c429c7a27bd
> --- /dev/null
> +++ b/drivers/of/of_empty_root.c
> @@ -0,0 +1,51 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Copyright (C) 2022 Xilinx, Inc.
> + */
> +
> +#include <linux/of.h>
> +#include <linux/slab.h>
> +
> +#include "of_private.h"
> +
> +static int __init of_root_init(void)
> +{
> +	struct property *prop = NULL;
> +	struct device_node *node;
> +	__be32 *val = NULL;
> +
> +	if (of_root)
> +		return 0;
> +
> +	pr_info("Create empty OF root node\n");
> +	node = kzalloc(sizeof(*node), GFP_KERNEL);
> +	if (!node)
> +		return -ENOMEM;
> +	of_node_init(node);
> +	node->full_name = "/";
> +
> +	prop = kcalloc(2, sizeof(*prop), GFP_KERNEL);
> +	if (!prop)
> +		return -ENOMEM;
> +
> +	val = kzalloc(sizeof(*val), GFP_KERNEL);
> +	if (!val)
> +		return -ENOMEM;
> +	*val = cpu_to_be32(sizeof(void *) / sizeof(u32));
> +
> +	prop->name = "#address-cells";
> +	prop->value = val;
> +	prop->length = sizeof(u32);
> +	of_add_property(node, prop);
> +	prop++;
> +	prop->name = "#size-cells";
> +	prop->value = val;
> +	prop->length = sizeof(u32);
> +	of_add_property(node, prop);
> +	of_root = node;
> +	for_each_of_allnodes(node)
> +		__of_attach_node_sysfs(node);
> +
> +	return 0;
> +}
> +pure_initcall(of_root_init);