Hi Rob,

On Wed, Jan 17, 2024 at 6:41 PM Rob Herring [off-list ref] wrote:

On Tue, Jan 16, 2024 at 05:18:15PM -0800, Stephen Boyd wrote:

quoted

Quoting Rob Herring (2024-01-15 12:32:30)

quoted

On Fri, Jan 12, 2024 at 12:07:47PM -0800, Stephen Boyd wrote:

quoted

diff --git a/drivers/of/Kconfig b/drivers/of/Kconfig
index da9826accb1b..9628e48baa15 100644
--- a/drivers/of/Kconfig
+++ b/drivers/of/Kconfig

@@ -54,9 +54,14 @@ config OF_FLATTREE
      select CRC32

 config OF_EARLY_FLATTREE
-     bool
+     bool "Functions for accessing Flat Devicetree (FDT) early in boot"

I think we could instead just get rid of this kconfig option. Or
always enable with CONFIG_OF (except on Sparc). The only cost of
enabling it is init section functions which get freed anyways.

Getting rid of it is a more massive change. It can be the default and
kept hidden instead? If it can't be selected on Sparc then it should be
hidden there anyway.

The easier option is certainly fine for this series. I just don't want
it visible.

quoted

quoted

quoted

      select DMA_DECLARE_COHERENT if HAS_DMA && HAS_IOMEM
      select OF_FLATTREE
+     help
+       Normally selected by platforms that process an FDT that has been
+       passed to the kernel by the bootloader.  If the bootloader does not
+       pass an FDT to the kernel and you need an empty devicetree that
+       contains only a root node to exist, then say Y here.

 config OF_PROMTREE
      bool

[...]

quoted

quoted

@@ -195,6 +191,17 @@ static inline int of_node_check_flag(const struct device_node *n, unsigned long
      return test_bit(flag, &n->_flags);
 }

+/**
+ * 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)
+{
+     return of_root != NULL && !of_node_check_flag(of_root, OF_EMPTY_ROOT);

Just a side comment, but I think many/all callers of this function could
just be removed.

I don't love new flags. Another possible way to handle this would be
checking for "compatible" being present in the root node. I guess this
is fine as-is for now at least.

Ok. I can add a check for a compatible property. That's probably better
anyway. Should there be a compatible property there to signal that this
DT isn't compatible with anything? I worry about DT overlays injecting a
compatible string into the root node, but maybe that is already
prevented.

I worry about DT overlays injecting anything...

I don't think it is explicitly forbidden, but I have asked that any
general purpose interface to apply overlays be restricted to nodes
explicitly allowed (e.g. downstream of a connector node). For now, the
places (i.e. drivers) overlays are applied are limited.

We could probably restrict the root node to new nodes only and no new
or changed properties.

Changing (<wild dream>or appending to</wild dream>) the root
"compatible" and/or "model" properties is useful in case of large
extension boards, though.  This is also the case for DTBs created from
a base DTB and one or more overlays using fdtoverlay.

For the latter, see also the following threads, where you weren't
(but probably should have been) CCed:

[1] "[PATCH v9 2/2] arm64: boot: Support Flat Image Tree"
     https://lore.kernel.org/all/20231202035511.487946-3-sjg@chromium.org (local)
[2] "Proposal: FIT support for extension boards / overlays"
    https://lore.kernel.org/all/CAPnjgZ06s64C2ux1rABNAnMv3q4W++sjhNGCO_uPMH_9sTF7Mw@mail.gmail.com (local)

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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