Le Wed, 22 Feb 2023 16:10:50 -0600,
Frank Rowand <frowand.list@xxxxxxxxx> a écrit :
> 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.
>
> If firmware provides a flattened device tree (FDT) then the FDT is
> unflattened via setup_arch(). Otherwise, setup_of() which is called
> immediately after setup_arch(), and will create the default root node
> if it does not exist.
>
> Signed-off-by: Frank Rowand <frowand.list@xxxxxxxxx>
> ---
>
> There are checkpatch warnings. I have reviewed them and feel
> they can be ignored.
>
> Changes since version 1:
> - refresh for 6.2-rc1
> - update Signed-off-by
> - fix typo in of_fdt.h: s/of_setup/setup_of
> - unflatten_device_tree(): validate size in header field dtb_empty_root
> that will be used to copy dtb_empty_root
> - add Kconfig option to manually select CONFIG_OF_EARLY_FLATTREE
>
> drivers/of/Kconfig | 7 ++++++-
> drivers/of/Makefile | 2 +-
> drivers/of/empty_root.dts | 6 ++++++
> drivers/of/fdt.c | 27 ++++++++++++++++++++++++++-
> include/linux/of_fdt.h | 2 ++
> init/main.c | 2 ++
> 6 files changed, 43 insertions(+), 3 deletions(-)
> create mode 100644 drivers/of/empty_root.dts
>
> diff --git a/drivers/of/Kconfig b/drivers/of/Kconfig
> index 80b5fd44ab1c..591cfe386727 100644
> --- a/drivers/of/Kconfig
> +++ b/drivers/of/Kconfig
> @@ -42,9 +42,14 @@ config OF_FLATTREE
> select CRC32
>
> config OF_EARLY_FLATTREE
> - bool
> + bool "Functions for accessing Flat Devicetree (FDT) early in boot"
> select DMA_DECLARE_COHERENT if HAS_DMA
> select OF_FLATTREE
> + help
> + Normally selected by platforms that process an FDT that has been
> + passed to the kernel by the bootloader. If the bootloader does not
> + pass an FDT to the kernel and you need an empty devicetree that
> + contains only a root node to exist, then say Y here.
>
> config OF_PROMTREE
> bool
> diff --git a/drivers/of/Makefile b/drivers/of/Makefile
> index e0360a44306e..cbae92c5ed02 100644
> --- a/drivers/of/Makefile
> +++ b/drivers/of/Makefile
> @@ -2,7 +2,7 @@
> obj-y = base.o device.o platform.o property.o
> obj-$(CONFIG_OF_KOBJ) += kobj.o
> obj-$(CONFIG_OF_DYNAMIC) += dynamic.o
> -obj-$(CONFIG_OF_FLATTREE) += fdt.o
> +obj-$(CONFIG_OF_FLATTREE) += fdt.o empty_root.dtb.o
> obj-$(CONFIG_OF_EARLY_FLATTREE) += fdt_address.o
> obj-$(CONFIG_OF_PROMTREE) += pdt.o
> obj-$(CONFIG_OF_ADDRESS) += address.o
> 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/;
> +
> +/ {
> +
> +};
> diff --git a/drivers/of/fdt.c b/drivers/of/fdt.c
> index b2272bccf85c..0d2f6d016b7e 100644
> --- a/drivers/of/fdt.c
> +++ b/drivers/of/fdt.c
> @@ -33,6 +33,13 @@
>
> #include "of_private.h"
>
> +/*
> + * __dtb_empty_root_begin[] and __dtb_empty_root_end[] magically created by
> + * cmd_dt_S_dtb in scripts/Makefile.lib
> + */
> +extern void *__dtb_empty_root_begin;
> +extern void *__dtb_empty_root_end;
> +
> /*
> * of_fdt_limit_memory - limit the number of regions in the /memory node
> * @limit: maximum entries
> @@ -1326,8 +1333,19 @@ bool __init early_init_dt_scan(void *params)
> */
> void __init unflatten_device_tree(void)
> {
> - __unflatten_device_tree(initial_boot_params, NULL, &of_root,
> + if (!initial_boot_params) {
> + initial_boot_params = (void *) __dtb_empty_root_begin;
The crash I encountered was actually not due to memblock alloc but to
this usage/declaration of the embedded dtb pointers which is wrong.
Either declare it as:
extern char __dtb_empty_root_begin[];
extern char __dtb_empty_root_end[];
and keep the existing usage in this function or keep the existing
declaration but modify this function to use &__dtb_empty_root_begin
With this modification, I finally have an empty device-tree present on
my system:
$ dtc -I fs -O dts /sys/firmware/devicetree/
/dts-v1/;
/ {
};
Thanks,
Clément
> + /* fdt_totalsize() will be used for copy size */
> + if (fdt_totalsize(initial_boot_params) >
> + __dtb_empty_root_end - __dtb_empty_root_begin) {
> + pr_err("invalid size in dtb_empty_root\n");
> + return;
> + }
> + unflatten_and_copy_device_tree();
> + } else {
> + __unflatten_device_tree(initial_boot_params, NULL, &of_root,
> early_init_dt_alloc_memory_arch, false);
> + }
>
> /* Get pointer to "/chosen" and "/aliases" nodes for use everywhere */
> of_alias_scan(early_init_dt_alloc_memory_arch);
> @@ -1367,6 +1385,13 @@ void __init unflatten_and_copy_device_tree(void)
> unflatten_device_tree();
> }
>
> +void __init setup_of(void)
> +{
> + /* if architecture did not unflatten devicetree, do it now */
> + if (!of_root)
> + unflatten_device_tree();
> +}
> +
> #ifdef CONFIG_SYSFS
> static ssize_t of_fdt_raw_read(struct file *filp, struct kobject *kobj,
> struct bin_attribute *bin_attr,
> diff --git a/include/linux/of_fdt.h b/include/linux/of_fdt.h
> index d69ad5bb1eb1..f0dc46d576da 100644
> --- a/include/linux/of_fdt.h
> +++ b/include/linux/of_fdt.h
> @@ -81,6 +81,7 @@ extern const void *of_flat_dt_match_machine(const void *default_match,
> /* Other Prototypes */
> extern void unflatten_device_tree(void);
> extern void unflatten_and_copy_device_tree(void);
> +extern void setup_of(void);
> extern void early_init_devtree(void *);
> extern void early_get_first_memblock_info(void *, phys_addr_t *);
> #else /* CONFIG_OF_EARLY_FLATTREE */
> @@ -91,6 +92,7 @@ static inline void early_init_fdt_reserve_self(void) {}
> static inline const char *of_flat_dt_get_machine_name(void) { return NULL; }
> static inline void unflatten_device_tree(void) {}
> static inline void unflatten_and_copy_device_tree(void) {}
> +static inline void setup_of(void) {}
> #endif /* CONFIG_OF_EARLY_FLATTREE */
>
> #endif /* __ASSEMBLY__ */
> diff --git a/init/main.c b/init/main.c
> index e1c3911d7c70..31e0931b5134 100644
> --- a/init/main.c
> +++ b/init/main.c
> @@ -101,6 +101,7 @@
> #include <linux/init_syscalls.h>
> #include <linux/stackdepot.h>
> #include <linux/randomize_kstack.h>
> +#include <linux/of_fdt.h>
> #include <net/net_namespace.h>
>
> #include <asm/io.h>
> @@ -961,6 +962,7 @@ asmlinkage __visible void __init __no_sanitize_address start_kernel(void)
> pr_notice("%s", linux_banner);
> early_security_init();
> setup_arch(&command_line);
> + setup_of();
> setup_boot_config();
> setup_command_line(command_line);
> setup_nr_cpu_ids();
--
Clément Léger,
Embedded Linux and Kernel engineer at Bootlin
https://bootlin.com
