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From:   Thiago Jung Bauermann <bauerman@linux.ibm.com>
To:     Lakshmi Ramasubramanian <nramas@linux.microsoft.com>
Cc:     zohar@linux.ibm.com, robh@kernel.org, takahiro.akashi@linaro.org,
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Subject: Re: [PATCH v17 02/10] of: Add a common kexec FDT setup function
In-reply-to: <8a3aa3d2-2eba-549a-9970-a2b0fe3586c9@linux.microsoft.com>
Date:   Thu, 11 Feb 2021 22:39:11 -0300
Message-ID: <87h7mioyio.fsf@manicouagan.localdomain>
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Lakshmi Ramasubramanian <nramas@linux.microsoft.com> writes:

> 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?

Yes, that is actually simpler and better than my idea. :-)

-- 
Thiago Jung Bauermann
IBM Linux Technology Center