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

[Rob Herring <robh@kernel.org>]

On Fri, Feb 12, 2021 at 11:19 AM Lakshmi Ramasubramanian
<nramas@linux.microsoft.com> wrote:
>
> 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.

Yes, I'd leave the 4KB in there by default and arm64 use 0.

Rob