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

[Rob Herring <robh@kernel.org>]

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.

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).

Rob