Re: [PATCH v2 1/2] of: create of_root if no dtb provided

["Clément Léger" <clement.leger@bootlin.com>]

Le Wed, 22 Feb 2023 16:10:50 -0600,
Frank Rowand <frowand.list@gmail.com> 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@gmail.com>
> ---
> 
> 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