Re: [PATCH v4 1/2] of: create of_root if no dtb provided

[Guenter Roeck]

On 3/18/24 12:26, Rob Herring wrote:
> +Stephen
>
> On Mon, Mar 18, 2024 at 12:09 PM Guenter Roeck <linux@xxxxxxxxxxxx> wrote:
> > Hi,
> >
> > On Fri, Mar 17, 2023 at 12:34:14AM -0500, Frank Rowand wrote:
> > > 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 setup_of(), which is called
> > > immediately after setup_arch(), will create the default root node
> > > if it does not exist.
> > >
> > > Signed-off-by: Frank Rowand <frowand.list@xxxxxxxxx>
> >
> > This patch results in a crash on nios2.
>
> This patch was never applied. I assume you meant a later version of it
> that did get applied.
>
> > Building nios2:10m50-ghrd:10m50_defconfig:10m50_devboard.dts ... running ...R failed (crashed)
>
> Booting with DT?
>
> > ------------
> > qemu log:
> > earlycon: uart8250 at MMIO32 0x18001600 (options '')
> > printk: legacy bootconsole [uart8250] enabled
> > Linux version 6.8.0-11409-gf6cef5f8c37f (groeck@desktop) (nios2-linux-gcc (GCC) 11.4.0, GNU ld (GNU Binutils) 2.40) #1 Sun Mar 17 23:38:59 PDT 2024
> > Kernel panic - not syncing: early_init_dt_alloc_memory_arch: Failed to allocate 72 bytes align=0x40
> > ---[ end Kernel panic - not syncing: early_init_dt_alloc_memory_arch: Failed to allocate 72 bytes align=0x40 ]---
>
> nios2 looks utterly broken to me. This change should be a nop unless
> initial_boot_params is NULL. It looks like it is possible for r6 (dtb
> address) to be 0 depending on kconfig options, but that would have
> skipped copying and unflattening which would then panic in
> setup_cpuinfo(). If initial_boot_params is not NULL, then the same
> early_init_dt_alloc_memory_arch() calls should fail when copying the
> DT. So I don't see how nios2 booting with DT ever worked.

For nios2, in early_init_devtree():

void __init early_init_devtree(void *params)
{
        __be32 *dtb = (u32 *)__dtb_start;
	...
        if (be32_to_cpu((__be32) *dtb) == OF_DT_HEADER)
                params = (void *)__dtb_start;

That worked fine until this patch. Starting with this patch, __dtb_start
always points to a valid empty devicetree blob, which overrides the
devicetree blob passed to early_init_devtree(). This causes the problem.

Guenter