[PATCH] of/fdt: export fdt blob as /sys/firmware/fdt

[PATCH] of/fdt: export fdt blob as /sys/firmware/fdt

[Ard Biesheuvel]

Create a new /sys entry '/sys/firmware/fdt' to export the FDT blob
that was passed to the kernel by the bootloader. This allows userland
applications such as kexec to access the raw binary.

The fact that this node does not reside under /sys/firmware/device-tree
is deliberate: FDT is also used on arm64 UEFI/ACPI systems to
communicate just the UEFI and ACPI entry points, but the FDT is never
unflattened and used to configure the system.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>
---

Resending with CC LAKML and devicetree-u79uwXL29TY76Z2rM5mHXA@public.gmane.org added.
Apologies for the noise.

 drivers/of/fdt.c | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/drivers/of/fdt.c b/drivers/of/fdt.c
index d1ffca8b34ea..60f9c67dc52a 100644
--- a/drivers/of/fdt.c
+++ b/drivers/of/fdt.c
@@ -22,6 +22,7 @@
 #include <linux/libfdt.h>
 #include <linux/debugfs.h>
 #include <linux/serial_core.h>
+#include <linux/sysfs.h>
 
 #include <asm/setup.h>  /* for COMMAND_LINE_SIZE */
 #include <asm/page.h>
@@ -1103,4 +1104,23 @@ static int __init of_flat_dt_debugfs_export_fdt(void)
 module_init(of_flat_dt_debugfs_export_fdt);
 #endif
 
+#ifdef CONFIG_SYSFS
+static ssize_t of_fdt_raw_read(struct file *filp, struct kobject *kobj,
+			       struct bin_attribute *bin_attr,
+			       char *buf, loff_t off, size_t count)
+{
+	memcpy(buf, initial_boot_params + off, count);
+	return count;
+}
+
+static int __init of_fdt_raw_init(void)
+{
+	static struct bin_attribute of_fdt_raw_attr =
+		__BIN_ATTR(fdt, S_IRUSR, of_fdt_raw_read, NULL, 0);
+	of_fdt_raw_attr.size = fdt_totalsize(initial_boot_params);
+	return sysfs_create_bin_file(firmware_kobj, &of_fdt_raw_attr);
+}
+module_init(of_fdt_raw_init);
+#endif
+
 #endif /* CONFIG_OF_EARLY_FLATTREE */
-- 
1.8.3.2

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Re: [PATCH] of/fdt: export fdt blob as /sys/firmware/fdt

[Arnd Bergmann]

On Monday 10 November 2014 17:51:45 Ard Biesheuvel wrote:
> Create a new /sys entry '/sys/firmware/fdt' to export the FDT blob
> that was passed to the kernel by the bootloader. This allows userland
> applications such as kexec to access the raw binary.
> 
> The fact that this node does not reside under /sys/firmware/device-tree
> is deliberate: FDT is also used on arm64 UEFI/ACPI systems to
> communicate just the UEFI and ACPI entry points, but the FDT is never
> unflattened and used to configure the system.
> 
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>

Can you elaborate on the motivation? Initially the fdt format was
introduced to make it easy to pass the information from /proc/device-tree
to the next kernel for kexec. Are you interested in cases where this does
not work?

	Arnd
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Re: [PATCH] of/fdt: export fdt blob as /sys/firmware/fdt

[Ard Biesheuvel]

On 10 November 2014 17:57, Arnd Bergmann <arnd-r2nGTMty4D4@public.gmane.org> wrote:
> On Monday 10 November 2014 17:51:45 Ard Biesheuvel wrote:
>> Create a new /sys entry '/sys/firmware/fdt' to export the FDT blob
>> that was passed to the kernel by the bootloader. This allows userland
>> applications such as kexec to access the raw binary.
>>
>> The fact that this node does not reside under /sys/firmware/device-tree
>> is deliberate: FDT is also used on arm64 UEFI/ACPI systems to
>> communicate just the UEFI and ACPI entry points, but the FDT is never
>> unflattened and used to configure the system.
>>
>> Signed-off-by: Ard Biesheuvel <ard.biesheuvel-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>
>
> Can you elaborate on the motivation? Initially the fdt format was
> introduced to make it easy to pass the information from /proc/device-tree
> to the next kernel for kexec. Are you interested in cases where this does
> not work?
>

/sys/firmware/device-tree only gets populated if you are really
booting in DT mode, i.e., when the device tree is unflattened.
However, in ACPI mode we still need to access the data that was
communicated between UEFI and the kernel in the /chosen node if we
want to support kexec under UEFI.

-- 
Ard.
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Re: [PATCH] of/fdt: export fdt blob as /sys/firmware/fdt

[Arnd Bergmann]

On Monday 10 November 2014 18:08:03 Ard Biesheuvel wrote:
> On 10 November 2014 17:57, Arnd Bergmann <arnd-r2nGTMty4D4@public.gmane.org> wrote:
> > On Monday 10 November 2014 17:51:45 Ard Biesheuvel wrote:
> >> Create a new /sys entry '/sys/firmware/fdt' to export the FDT blob
> >> that was passed to the kernel by the bootloader. This allows userland
> >> applications such as kexec to access the raw binary.
> >>
> >> The fact that this node does not reside under /sys/firmware/device-tree
> >> is deliberate: FDT is also used on arm64 UEFI/ACPI systems to
> >> communicate just the UEFI and ACPI entry points, but the FDT is never
> >> unflattened and used to configure the system.
> >>
> >> Signed-off-by: Ard Biesheuvel <ard.biesheuvel-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>
> >
> > Can you elaborate on the motivation? Initially the fdt format was
> > introduced to make it easy to pass the information from /proc/device-tree
> > to the next kernel for kexec. Are you interested in cases where this does
> > not work?
> >
> 
> /sys/firmware/device-tree only gets populated if you are really
> booting in DT mode, i.e., when the device tree is unflattened.
> However, in ACPI mode we still need to access the data that was
> communicated between UEFI and the kernel in the /chosen node if we
> want to support kexec under UEFI.

Ah, now I get it.

	Arnd
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