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

[Lakshmi Ramasubramanian]

On 2/12/21 6:38 AM, Rob Herring wrote:
> On Thu, Feb 11, 2021 at 7:17 PM Lakshmi Ramasubramanian
> <nramas@xxxxxxxxxxxxxxxxxxx> 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