Re: (EXT) Re: [PATCH] of: skip disabled CPU nodes

On Wed, Aug 26, 2020 at 8:47 AM Frank Rowand [off-list ref] wrote:

Hi Rob,

On 2020-08-26 08:54, Matthias Schiffer wrote:

quoted

On Wed, 2020-08-26 at 08:01 -0500, Frank Rowand wrote:

quoted

On 2020-08-26 07:02, Matthias Schiffer wrote:

quoted

Allow disabling CPU nodes using status = "disabled".

This allows a bootloader to change the number of available CPUs
(for
example when a common DTS is used for SoC variants with different
numbers
of cores) without deleting the nodes altogether (which may require
additional fixups where the CPU nodes are referenced, e.g. a
cooling
map).

Signed-off-by: Matthias Schiffer <matthias.schiffer@ew.tq-group.com

quoted

---
 drivers/of/base.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/of/base.c b/drivers/of/base.c
index ea44fea99813..d547e9deced1 100644
--- a/drivers/of/base.c
+++ b/drivers/of/base.c

@@ -796,6 +796,8 @@ struct device_node *of_get_next_cpu_node(struct

device_node *prev)
            of_node_put(node);
    }
    for (; next; next = next->sibling) {
+           if (!__of_device_is_available(next))
+                   continue;
            if (!(of_node_name_eq(next, "cpu") ||
                  __of_node_is_type(next, "cpu")))
                    continue;

The original implementation of of_get_next_cpu_node() had
that check, but status disabled for cpu nodes has different
semantics than other nodes, and the check broke some systems.
The check was removed by c961cb3be906 "of: Fix cpu node
iterator to not ignore disabled cpu nodes".

It would be useful to document that difference in the
header comment of of_get_next_cpu_node().

-Frank

Hmm, I see. This difference in behaviour is quite unfortunate, as I'm
currently looking for a way to *really* disable a CPU core.

In arch/arm64/boot/dts/freescale/imx8mn.dtsi (and other variants of the
i.MX8M), there are 4 CPU nodes for the full-featured quad-core version.
The reduced single- and dual-core versions are currently handled in
NXP's U-Boot fork by deleting the additional nodes.

Not doing so causes the kernel to hang for a while when trying to
online the non-existent cores during boot (at least in linux-imx 5.4 -
I have not checked a more recent mainline kernel yet), but the deletion
is non-trivial to do without leaving dangling phandle references.

Any thoughts on implementing another universal property that means
something like "the hardware described by this node does not exist
or is so broken that you better not use it".

There's a couple of options:

The DT spec defines 'fail' value for status. We could use that instead
of 'disabled'.

The spec behavior with cpu 'disabled' is only on PPC AFAIK. On
arm/arm64 (probably riscv now too) we've never followed it where we
online 'disabled' CPUs. So we could just make the check conditional on
!IS_ENABLED(CONFIG_PPC). This would need some spec update.

Matthias, if Rob thinks that is a good idea, then you should start
with a new proposal that is also sent to
devicetree-spec@vger.kernel.org [off-list ref]

-Frank

quoted

Kind regards,
Matthias