Re: [RFC PATCH 0/3] m68k goes DT

[Greg Ungerer <gerg@kernel.org>]

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Hi Daniel,

On 5/1/25 17:14, Daniel Palmer wrote:
> Short version:
> I want to start converting m68k (including nommu) to use DT for booting
> so I can add few cool boards I have (e.g. dual 040/060 VME board..).
> I need ideas, help etc. Maybe if someone converted an m68k machine they
> are using to DT alongside me this would have some hope of actually happening?

I have been thinking about this for a while for the ColdFire targets.
In a few cases the drivers its uses are already devicetree enabled,
since they are used on some of the NXP/Freescale ARM SoC devices.
So this is interesting work to me.


> All of my WIP in-progress very messy code etc is here:
> https://github.com/fifteenhex/m68kjunk
> 
> Longer version:
> 
> As of today I have:
> - Modern (2024.07) u-boot fork that works on 000/030/040 real hardware/QEMU
>    that supports booting a Linux ELF image. For nommu a FDT blob address
>    is passed via a regiser, for mmu the normal bootinfo is created and
>    the FDT is passed via a bootinfo tag. nommu never used bootinfo and doesn't
>    need to.
> 
> - A Linux branch for nommu that removes most of the current DragonBall
>    code, makes it into a devicetree based machine and adds a bunch of
>    drivers etc. This works in a fork of QEMU I have and on the real hardware.
>    The DragonBall even has a working 1bpp framebuffer. The nommu branch also
>    works on 010 and better.
> 
> - A Linux branch for mmu that uses the crappy patches in this series and some
>    other bits to start moving the MVME147 board over to using DT.
> 
> - A buildroot fork that can build a 000 nommu userland etc. A patch to add
>    support for building for 030 is already in buildroot, I plan to send one
>    for 060 later.
> 
> - A bunch of DragonBall machines, an MVME147 030 machine, an Eltec E27 dual
>    socket VME board with one 040 at the moment but I have 060s to go in it once
>    I work out the need jumper changes, some other 060 VME boards, Amigas...
> 
> What I'm thinking:
> 
> - We initially add passing of an FDT via bootinfo for mmu
> - Add support for a generic machine that can boot almost anything so I can
>    bring up my new (to Linux) machines.
> - I will migrate MVME147 to device tree.
> - Some like minded person migrates a machine they have to device tree. :)
> - Maybe embed a FDT for machines that'll never get a bootloader that
>    supports this and use the machine type to select the embedded FDT
>    and move all of that stuff over without needing to mess with bootloaders.

To that end that is what I have been playing with. Doing it in a similar
way to what MIPS does. Example patch attached of what I did for non-MMU
ColdFire. So no boot loader support required.

Regards
Greg



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diff --git a/arch/m68k/Kconfig b/arch/m68k/Kconfig
index 7c4f7bcc89d7..a4949b69e1dc 100644
--- a/arch/m68k/Kconfig
+++ b/arch/m68k/Kconfig
@@ -39,6 +39,8 @@ config M68K
 	select OLD_SIGSUSPEND3
 	select UACCESS_MEMCPY if !MMU
 	select ZONE_DMA
+	select OF if COLDFIRE
+	select OF_EARLY_FLATTREE if COLDFIRE
 
 config CPU_BIG_ENDIAN
 	def_bool y
diff --git a/arch/m68k/Kconfig.devices b/arch/m68k/Kconfig.devices
index e6e3efac1840..ea0bce4db639 100644
--- a/arch/m68k/Kconfig.devices
+++ b/arch/m68k/Kconfig.devices
@@ -144,3 +144,14 @@ config SERIAL_CONSOLE
 endmenu
 
 endif
+
+config EMBEDDED_DTB
+	bool "Embedded devicetree ELF section"
+	help
+	  If there is no capability for the boot loader to specify a
+	  devicetree (DTB) then this option allows for it to be embedded
+	  within the linux binary itself. Typically you can do this with
+	  something like this:
+
+		objcopy --update-section .embedded_dtb=<filename>.dtb vmlinux
+
diff --git a/arch/m68k/coldfire/head.S b/arch/m68k/coldfire/head.S
index c6d7fd28c602..88fd8b5b3bda 100644
--- a/arch/m68k/coldfire/head.S
+++ b/arch/m68k/coldfire/head.S
@@ -133,6 +133,15 @@ _init_sp:
 .long	0
 #endif
 
+#ifdef CONFIG_EMBEDDED_DTB
+/*
+ * If embedding a DTB in the kernel ELF binary then create a place holder
+ * for that section. It will be inserted after the final kernel build/link.
+ */
+.section ".embedded_dtb","aw"
+.long 0
+#endif
+
 /*****************************************************************************/
 
 __HEAD
diff --git a/arch/m68k/kernel/setup_no.c b/arch/m68k/kernel/setup_no.c
index c926da9d5ec2..d1d79c41c6f0 100644
--- a/arch/m68k/kernel/setup_no.c
+++ b/arch/m68k/kernel/setup_no.c
@@ -33,6 +33,8 @@
 #include <linux/initrd.h>
 #include <linux/root_dev.h>
 #include <linux/rtc.h>
+#include <linux/of.h>
+#include <linux/of_fdt.h>
 
 #include <asm/setup.h>
 #include <asm/bootinfo.h>
@@ -70,6 +72,31 @@ void (*mach_halt)(void);
 #define	CPU_NAME	"UNKNOWN"
 #endif
 
+#ifdef CONFIG_EMBEDDED_DTB
+static void __init m68k_setup_fdt(void)
+{
+	extern void *embedded_dtb;
+	phys_addr_t fdt = (phys_addr_t) &embedded_dtb;
+
+	pr_info("m68k generic DT machine support, FDT blob at 0x%08x\n", fdt);
+	if (!early_init_dt_verify(__va(fdt), fdt)) {
+		pr_err("FDT blob is bad?!\n");
+		return;
+	}
+	early_init_dt_scan_nodes();
+}
+
+static void __init m68k_dtb_model(void)
+{
+	const char *model;
+	model = of_flat_dt_get_machine_name();
+	if (model)
+		pr_info("DTB reports model = %s\n", model);
+	else
+		pr_warn("DTB has no model type?\n");
+}
+#endif /* CONFIG_EMBEDDED_DTB */
+
 /*
  * Different cores have different instruction execution timings.
  * The old/traditional 68000 cores are basically all the same, at 16.
@@ -166,6 +193,11 @@ void __init setup_arch(char **cmdline_p)
 	 * Get kmalloc into gear.
 	 */
 	paging_init();
+
+#ifdef CONFIG_EMBEDDED_DTB
+	m68k_setup_fdt();
+	m68k_dtb_model();
+#endif
 }
 
 /*
diff --git a/arch/m68k/kernel/vmlinux-nommu.lds b/arch/m68k/kernel/vmlinux-nommu.lds
index 2624fc18c131..1be3bfe31ba4 100644
--- a/arch/m68k/kernel/vmlinux-nommu.lds
+++ b/arch/m68k/kernel/vmlinux-nommu.lds
@@ -70,6 +70,15 @@ SECTIONS {
 	INIT_TEXT_SECTION(PAGE_SIZE)
 	INIT_DATA_SECTION(16)
 	PERCPU_SECTION(16)
+
+#ifdef CONFIG_EMBEDDED_DTB
+	STRUCT_ALIGN();
+	.embedded_dtb : {
+		embedded_dtb = .;
+		*(.embedded_dtb)
+		KEEP(*(.embedded_dtb))
+	}
+#endif
 	.m68k_fixup : {
 		__start_fixup = .;
 		*(.m68k_fixup)