linux-kernel - Re: [PATCH] OF: Add support for built in FDT blob

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Message-Id: <20141118221628.2526EC40966@trevor.secretlab.ca>
Date:	Tue, 18 Nov 2014 22:16:28 +0000
From:	Grant Likely <grant.likely@...aro.org>
To:	Alban Bedel <albeu@...e.fr>, devicetree@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, Rob Herring <robh+dt@...nel.org>
Subject: Re: [PATCH] OF: Add support for built in FDT blob

On Fri,  7 Nov 2014 12:14:09 +0100
, Alban Bedel <albeu@...e.fr>
 wrote:
> Allow embedding an FDT blob in the kernel image, this is mostly
> useful to run new kernels on boards with old bootloaders.
> 
> Signed-off-by: Alban Bedel <albeu@...e.fr>

The kernel already supports linking in .dtb files. All you need to do is
add <filename>.dtb.o in the build objects.

> ---
>  drivers/of/Kconfig    | 16 ++++++++++++++++
>  drivers/of/Makefile   |  8 ++++++++
>  drivers/of/fdt.c      |  8 ++++++++
>  drivers/of/fdt_blob.S | 17 +++++++++++++++++
>  4 files changed, 49 insertions(+)
>  create mode 100644 drivers/of/fdt_blob.S
> 
> diff --git a/drivers/of/Kconfig b/drivers/of/Kconfig
> index 1a13f5b..97c5d18 100644
> --- a/drivers/of/Kconfig
> +++ b/drivers/of/Kconfig
> @@ -83,4 +83,20 @@ config OF_RESERVED_MEM
>  config OF_RESOLVE
>  	bool
>  
> +config OF_BUILTIN_FDT
> +	bool "Include a FDT blob in the kernel"
> +	help
> +	  Include a FDT blob in the kernel to use as default when
> +	  no blob has been passed by the bootloader. This is useful
> +	  to use devicetree on boards with old bootloaders.
> +
> +	  If unsure, say N.
> +
> +config OF_BUILTIN_FDT_BLOB
> +	string "FDT Blob name"
> +	depends on OF_BUILTIN_FDT
> +	help
> +	  Name of the FDT blob to build in the kernel, from one of the
> +	  dtbs available for the current platform.
> +

I'm not keen on making this a generic option. ARM and PowerPC already
have the mechanism for attaching a dtb, and they do not build it into
the kernel. Each architecture that wants to use this needs to enable it
separately.

g.

>  endmenu # OF
> diff --git a/drivers/of/Makefile b/drivers/of/Makefile
> index ca9209c..b96c6e9 100644
> --- a/drivers/of/Makefile
> +++ b/drivers/of/Makefile
> @@ -14,6 +14,14 @@ obj-$(CONFIG_OF_PCI_IRQ)  += of_pci_irq.o
>  obj-$(CONFIG_OF_MTD)	+= of_mtd.o
>  obj-$(CONFIG_OF_RESERVED_MEM) += of_reserved_mem.o
>  obj-$(CONFIG_OF_RESOLVE)  += resolver.o
> +obj-$(CONFIG_OF_BUILTIN_FDT) += fdt_blob.o
>  
>  CFLAGS_fdt.o = -I$(src)/../../scripts/dtc/libfdt
>  CFLAGS_fdt_address.o = -I$(src)/../../scripts/dtc/libfdt
> +
> +# The FDT blob to include
> +FDT_BLOB := arch/$(ARCH)/boot/dts/$(CONFIG_OF_BUILTIN_FDT_BLOB:"%"=%).dtb
> +# Undefine ARCH to avoid breaking the FDT path
> +AFLAGS_fdt_blob.o += -U$(ARCH) -DFDT_BLOB=$(FDT_BLOB)
> +# This dependency can't be automatically tracked
> +$(obj)/fdt_blob.o: $(obj)/../../$(FDT_BLOB)
> diff --git a/drivers/of/fdt.c b/drivers/of/fdt.c
> index d1ffca8..ffb86a7 100644
> --- a/drivers/of/fdt.c
> +++ b/drivers/of/fdt.c
> @@ -26,6 +26,9 @@
>  #include <asm/setup.h>  /* for COMMAND_LINE_SIZE */
>  #include <asm/page.h>
>  
> +extern char __fdt_blob_start[];
> +extern unsigned long __fdt_blob_size;
> +
>  /*
>   * of_fdt_limit_memory - limit the number of regions in the /memory node
>   * @limit: maximum entries
> @@ -1064,6 +1067,11 @@ void __init unflatten_and_copy_device_tree(void)
>  	int size;
>  	void *dt;
>  
> +#ifdef CONFIG_OF_BUILTIN_FDT
> +	if (!initial_boot_params && __fdt_blob_size > 0)
> +		initial_boot_params = __fdt_blob_start;
> +#endif
> +
>  	if (!initial_boot_params) {
>  		pr_warn("No valid device tree found, continuing without\n");
>  		return;
> diff --git a/drivers/of/fdt_blob.S b/drivers/of/fdt_blob.S
> new file mode 100644
> index 0000000..fcaa2aa
> --- /dev/null
> +++ b/drivers/of/fdt_blob.S
> @@ -0,0 +1,17 @@
> +#include <linux/stringify.h>
> +#include <asm-generic/vmlinux.lds.h>
> +
> +.align 4
> +.section .init.fdt_blob,"a"
> +.globl VMLINUX_SYMBOL(__fdt_blob_start)
> +VMLINUX_SYMBOL(__fdt_blob_start):
> +.incbin __stringify(FDT_BLOB)
> +__fdt_blob_end:
> +.section .init.fdt_blob.info,"a"
> +.globl VMLINUX_SYMBOL(__fdt_blob_size)
> +VMLINUX_SYMBOL(__fdt_blob_size):
> +#ifdef CONFIG_64BIT
> +        .quad __fdt_blob_end - __fdt_blob_start
> +#else
> +        .long __fdt_blob_end - __fdt_blob_start
> +#endif
> -- 
> 2.0.0
> 

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