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Date: Mon, 18 Mar 2024 14:31:30 -0700
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Subject: Re: [PATCH v4 1/2] of: create of_root if no dtb provided
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To: Rob Herring <robh+dt@kernel.org>, Stephen Boyd <sboyd@kernel.org>
Cc: Frank Rowand <frowand.list@gmail.com>,
 =?UTF-8?B?Q2zDqW1lbnQgTMOpZ2Vy?= <clement.leger@bootlin.com>,
 devicetree@vger.kernel.org, linux-kernel@vger.kernel.org,
 Lizhi Hou <lizhi.hou@xilinx.com>, Allan Nielsen
 <allan.nielsen@microchip.com>, Horatiu Vultur
 <horatiu.vultur@microchip.com>, Steen Hegelund
 <steen.hegelund@microchip.com>,
 Thomas Petazzoni <thomas.petazzoni@bootlin.com>
References: <20230317053415.2254616-1-frowand.list@gmail.com>
 <20230317053415.2254616-2-frowand.list@gmail.com>
 <886049ed-4f5f-4e17-86f4-1245024ade3a@roeck-us.net>
 <CAL_JsqKsF53v7d7uZ3XT4kPFy-2FBWHfvKNSFdTx2oZhmSZkDA@mail.gmail.com>
From: Guenter Roeck <linux@roeck-us.net>
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On 3/18/24 12:26, Rob Herring wrote:
> +Stephen
> 
> On Mon, Mar 18, 2024 at 12:09 PM Guenter Roeck <linux@roeck-us.net> 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@gmail.com>
>>
>> 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