On 2/23/23 05:34, Clément Léger wrote:
> 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[];
Thanks for catching this, and the fix.
I will change to uint8_t instead of char, consistent with the use
in drivers/of/unittest.c
-Frank
>
> 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();
>
>
>
