Subject: Re: [PATCH v17 02/10] of: Add a common kexec FDT setup function

From: Lakshmi Ramasubramanian <[email-masked]>
Date: Thu, 11 Feb 2021 17:17:56 -0800

On 2/11/21 5:09 PM, Thiago Jung Bauermann wrote:
> 
> There's actually a complication that I just noticed and needs to be
> addressed. More below.
> 

<...>

>> +
>> +/*
>> + * of_kexec_alloc_and_setup_fdt - Alloc and setup a new Flattened Device Tree
>> + *
>> + * @image:		kexec image being loaded.
>> + * @initrd_load_addr:	Address where the next initrd will be loaded.
>> + * @initrd_len:		Size of the next initrd, or 0 if there will be none.
>> + * @cmdline:		Command line for the next kernel, or NULL if there will
>> + *			be none.
>> + *
>> + * Return: fdt on success, or NULL errno on error.
>> + */
>> +void *of_kexec_alloc_and_setup_fdt(const struct kimage *image,
>> +				   unsigned long initrd_load_addr,
>> +				   unsigned long initrd_len,
>> +				   const char *cmdline)
>> +{
>> +	void *fdt;
>> +	int ret, chosen_node;
>> +	const void *prop;
>> +	unsigned long fdt_size;
>> +
>> +	fdt_size = fdt_totalsize(initial_boot_params) +
>> +		   (cmdline ? strlen(cmdline) : 0) +
>> +		   FDT_EXTRA_SPACE;
> 
> Just adding 4 KB to initial_boot_params won't be enough for crash
> kernels on ppc64. The current powerpc code doubles the size of
> initial_boot_params (which is normally larger than 4 KB) and even that
> isn't enough. A patch was added to powerpc/next today which uses a more
> precise (but arch-specific) formula:
> 
> https://lore.kernel.org/linuxppc-dev/161243826811.119001.14083048209224609814.stgit@hbathini/
> 
> So I believe we need a hook here where architectures can provide their
> own specific calculation for the size of the fdt. Perhaps a weakly
> defined function providing a default implementation which an
> arch-specific file can override (a la arch_kexec_kernel_image_load())?
> 
> Then the powerpc specific hook would be the kexec_fdt_totalsize_ppc64()
> function from the patch I linked above.
> 

Do you think it'd better to add "fdt_size" parameter to 
of_kexec_alloc_and_setup_fdt() so that the caller can provide the 
desired FDT buffer size?

thanks,
  -lakshmi