Hi Rob,

CC broonie
CC somlo

On Wed, Feb 21, 2024 at 3:06 PM Rob Herring [off-list ref] wrote:

On Fri, Feb 16, 2024 at 11:08 PM Saurabh Singh Sengar
[off-list ref] wrote:

quoted

On Fri, Feb 16, 2024 at 05:05:51PM -0800, Frank Rowand wrote:

quoted

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, the call to
unflatten(_and_copy)?_device_tree() will create an empty root node.

We make of_have_populated_dt() return true only if the DTB was loaded by
firmware so that existing callers don't change behavior after this
patch. The call in the of platform code is removed because it prevents
overlays from creating platform devices when the empty root node is
used.

[sboyd@kernel.org: Update of_have_populated_dt() to treat this empty dtb
as not populated. Drop setup_of() initcall]

Signed-off-by: Frank Rowand <redacted>
Link: https://lore.kernel.org/r/20230317053415.2254616-2-frowand.list@gmail.com (local)
Cc: Rob Herring <robh+dt@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>

Thanks for your patch, which is now commit 7b937cc243e5b1df ("of:
Create of_root if no dtb provided by firmware") in dt-rh/for-next
(next-20240312 and later).

On my OrangeCrab FPGA running Linux on LiteX/VexRiscv-V, the attached
OLED display now fails to probe:

    -ssd130x-i2c 0-003c: supply vcc not found, using dummy regulator
    -[drm] Initialized ssd130x 1.0.0 20220131 for 0-003c on minor 0
    +ssd130x-i2c 0-003c: incomplete constraints, dummy supplies not allowed
    +ssd130x-i2c 0-003c: error -ENODEV: Failed to get VCC regulator

quoted

quoted

@@ -1645,6 +1635,21 @@ static inline bool of_device_is_system_power_controller(const struct device_node
      return of_property_read_bool(np, "system-power-controller");
 }

+/**
+ * of_have_populated_dt() - Has DT been populated by bootloader
+ *
+ * Return: True if a DTB has been populated by the bootloader and it isn't the
+ * empty builtin one. False otherwise.
+ */
+static inline bool of_have_populated_dt(void)
+{
+#ifdef CONFIG_OF
+     return of_property_present(of_root, "compatible");

This adds the strict check for compatible which makes compatible
to be mandatory for root nodes. So far, DeviceTree without compatible
property in root nodes can work. Do we want to make this documented
somewhere ?

It already is in the DT spec and schemas.

How many systems in the wild violate this?

Apparently the DTS generated by LiteX does not have a root compatible
(and model) property, hence of_have_populated_dt() returns false.
While my gateware and DTS is quite old, a quick look at recent
litex_json2dts_linux.py history shows this is still true for current LiteX.

Now, how is this related to the failure I am seeing?

drivers/regulator/core.c has:

    static bool have_full_constraints(void)
    {
            return has_full_constraints || of_have_populated_dt();
    }

and

    static int __init regulator_init_complete(void)
    {
            /*
             * Since DT doesn't provide an idiomatic mechanism for
             * enabling full constraints and since it's much more natural
             * with DT to provide them just assume that a DT enabled
             * system has full constraints.
             */
            if (of_have_populated_dt())
                    has_full_constraints = true;

            ...
    }
    late_initcall_sync(regulator_init_complete);

(The latter looks a bit futile, as have_full_constraints() already
 contains a check for of_have_populated_dt()?)

When have_full_constraints() returns false, any dummy regulator will
fail to probe.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds

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