device tree variations

I'm building a kernel that can run on a handful of hardware configurations, all using OF-unaware U-Boot. I know how to make a static device tree (dts file) that works on one of these hardware variations, and how to add nodes and modify properties in the platform init code. But I don't see a nice way to deal with nodes that should be present on only some hardware configurations.

I could have the dts file contain only the common elements, and add all the variable elements in the startup code. But that means the bulk of the device tree will be expressed as relatively ugly C source instead of the much more readable and maintainable dts notation. I would much rather have the dts file contain the union of all platforms and have the platform init code delete the nodes that are not applicable, but I don't see an API to do those deletions.

I suppose I could instead compile N different dts files and have the platform init code pick the appropriate dtb blob to pass to fdt_init().

Deleting the irrelevant parts or picking a device tree to pass to fdt_init() are both reasonable solutions. libfdt which is included in the bootwrapper has functions for removing unwanted nodes: either fdt_nop_node() or fdt_del_node() will suffice. There isn't currently a dt_ops hook to call though to those functions though. You could either add one, or (knowing that your platform always has a flat dt) bypass the dt_ops hooks and call libfdt directly.

Quoted 7 lines · David Gibson

Deleting the irrelevant parts or picking a device tree to pass to fdt_init() are both reasonable solutions. libfdt which is included in the bootwrapper has functions for removing unwanted nodes: either fdt_nop_node() or fdt_del_node() will suffice. There isn't currently a dt_ops hook to call though to those functions though. You could either add one, or (knowing that your platform always has a flat dt) bypass the dt_ops hooks and call libfdt directly.

Thanks. The fdt_del_node approach works pretty nicely. I added a dt_ops hook since fdt is static in libfdt-wrapper.c.

At first I tried fdt_nop_node, fearing that find_node_by_prop_value() and fdt_del_node() would interact badly when deleting in mid-traversal,

but it turns out that fdt_nop_node() upsets find_node_by_prop_value() anyway.

Plus, the kernel prints harmless but strange messages when
there are NOPs in the tree.

So I use fdt_del_node() and rescan from
the top each time I delete a node and it works fine.

Thanks. The fdt_del_node approach works pretty nicely. I added a dt_ops hook since fdt is static in libfdt-wrapper.c.

Reposting in proper format...

Some platforms have variants that can share most of a flat device tree but need a few devices selectively pruned at boot time. This adds del_node() to ops.h to allow access to the existing fdt_del_node().

Signed-off-by Mike Ditto <[email protected]>

---

Index: arch/powerpc/boot/ops.h
===================================================================
retrieving revision 1.1.1.1
diff -u -r1.1.1.1 ops.h

powerpc/boot/ops.h +6 -0

@@ -40,6 +40,7 @@
 			const int buflen);
 	int	(*setprop)(const void *phandle, const char *name,
 			const void *buf, const int buflen);
+	int (*del_node)(const void *phandle);
 	void *(*get_parent)(const void *phandle);
 	/* The node must not already exist. */
 	void *(*create_node)(const void *parent, const char *name);

@@ -124,6 +125,11 @@
 		return dt_ops.setprop(devp, name, buf, strlen(buf) + 1);
 
 	return -1;
+}
+
+static inline int del_node(const void *devp)
+{
+	return dt_ops.del_node ? dt_ops.del_node(devp) : -1;
 }
 
 static inline void *get_parent(const char *devp)

Index: arch/powerpc/boot/libfdt-wrapper.c
===================================================================
retrieving revision 1.1.1.1
diff -u -r1.1.1.1 libfdt-wrapper.c

powerpc/boot/libfdt-wrapper.c +6 -0

@@ -105,6 +105,11 @@
 	return check_err(rc);
 }
 
+static int fdt_wrapper_del_node(const void *devp)
+{
+	return fdt_del_node(fdt, devp_offset(devp));
+}
+
 static void *fdt_wrapper_get_parent(const void *devp)
 {
 	return offset_devp(fdt_parent_offset(fdt, devp_offset(devp)));

@@ -173,6 +178,7 @@
 	dt_ops.create_node = fdt_wrapper_create_node;
 	dt_ops.find_node_by_prop_value = fdt_wrapper_find_node_by_prop_value;
 	dt_ops.find_node_by_compatible = fdt_wrapper_find_node_by_compatible;
+	dt_ops.del_node = fdt_wrapper_del_node;
 	dt_ops.get_path = fdt_wrapper_get_path;
 	dt_ops.finalize = fdt_wrapper_finalize;
 

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Quoted 6 lines · Mike Ditto

Reposting in proper format...

Some platforms have variants that can share most of a flat device tree but need a few devices selectively pruned at boot time. This adds del_node() to ops.h to allow access to the existing fdt_del_node().

Quoted 1 line

Signed-off-by Mike Ditto <[email protected]>

Looks good to me.

Acked-by Grant Likely <[email protected]>

I've fixed it up manually so no need to re-submit, but you'll know next time :-)

Reposting in proper format...

Some platforms have variants that can share most of a flat device tree but need a few devices selectively pruned at boot time. This adds del_node() to ops.h to allow access to the existing fdt_del_node().

Signed-off-by Mike Ditto <[email protected]>