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

On Fri, Feb 12, 2021 at 11:19 AM Lakshmi Ramasubramanian
[off-list ref] wrote:

On 2/12/21 6:38 AM, Rob Herring wrote:

quoted

On Thu, Feb 11, 2021 at 7:17 PM Lakshmi Ramasubramanian
[off-list ref] wrote:

quoted

On 2/11/21 5:09 PM, Thiago Jung Bauermann wrote:

quoted

There's actually a complication that I just noticed and needs to be
addressed. More below.

<...>

quoted

quoted

+
+/*
+ * 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/ (local)

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