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

Lakshmi Ramasubramanian
2021-02-12T17:19:46Z

On 2/12/21 6:38 AM, Rob Herring wrote:
> On Thu, Feb 11, 2021 at 7:17 PM Lakshmi Ramasubramanian
> <nramas@linux.microsoft.com> wrote:
>>
>> 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, I guess so. But please define the param as extra size, not total
> size. The kernel command line size addition can be in the common code.

Will do. Just to clarify -

The common code will do:

fdt_totalsize(initial_boot_params) + strlen(cmdline) + extra_fdt_size

The caller will pass "extra_fdt_size"
ARM64 => 4KB
PPC64 => fdt_totalsize(initial_boot_params) - which will be updated when
the patch Thiago had referred to is merged.

>
> The above change is also going to conflict, so I think this may have
> to wait. Or I'll take the common and arm bits and powerpc can be
> converted next cycle (or after the merge window).
>

thanks.

-lakshmi