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

[Matthias Schiffer]

On Wed, 2020-08-26 at 08:01 -0500, Frank Rowand wrote:
> On 2020-08-26 07:02, Matthias Schiffer wrote:
> > 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@xxxxxxxxxxxxxxx
> > >
> > ---
> >  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.

Kind regards,
Matthias