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@xxxxxxxxxx>
> Signed-off-by: Rob Herring <robh@xxxxxxxxxx>
> Signed-off-by: Clément Léger <clement.leger@xxxxxxxxxxx>
> ---
> 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/;
> +
> +/ {
> +
> +};
