Re: [PATCH 1/2] xen/device-tree: add a helper to dump an FDT in DTS-like format

["Orzel, Michal" <michal.orzel@xxxxxxx>]

On 01-Oct-26 13:16, Oleksii Kurochko wrote:
> The device tree Xen builds for a dom0less domain is only ever consumed
> by the guest, so a mistake in it surfaces as an obscure failure inside
> the domain, far away from the code which produced the tree.
> 
> Add device_tree_dump(). The output follows dtc's format so that a dump
> can be diffed against the source DTS or fed back to dtc; the value
> formatting heuristics are ported from dtc's utilfdt_print_data() for
> that reason. The walk itself goes through libfdt rather than decoding
> the tag stream by hand as dtc's fdtdump does.
> 
> Dumping a whole tree is only useful while debugging and costs both code
> size and a lot of console output, hence CONFIG_DEVICE_TREE_DEBUG. Mention
> the dump in the help text of the option.
We should also dump the DTB for dom0/hwdom in prepare_dtb_hwdom(). Please add
that to this patch.

> 
> Signed-off-by: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>
> ---
>  xen/Kconfig.debug                       |   3 +-
>  xen/common/device-tree/Makefile         |   1 +
>  xen/common/device-tree/dom0less-build.c |   4 +
>  xen/common/device-tree/fdt-dump.c       | 180 ++++++++++++++++++++++++
>  xen/include/xen/bootfdt.h               |  13 ++
>  5 files changed, 200 insertions(+), 1 deletion(-)
>  create mode 100644 xen/common/device-tree/fdt-dump.c
> 
> diff --git a/xen/Kconfig.debug b/xen/Kconfig.debug
> index d900d926c555..664f50bf8a96 100644
> --- a/xen/Kconfig.debug
> +++ b/xen/Kconfig.debug
> @@ -95,7 +95,8 @@ config DEVICE_TREE_DEBUG
>       depends on DEVICE_TREE_PARSE
>       help
>         Device tree parsing and DOM0 device tree building messages are
> -       logged in the Xen ring buffer.
> +       logged in the Xen ring buffer. The device tree generated for each
> +       dom0less domU is dumped there as well, in a DTS-like format.
I think we could have a separate option DEVICE_TREE_DUMP that depends on
DEVICE_TREE_PARSE. Sometimes all we need is just to see how the generated DTB
look like without seeing thousands of other DT debug messages. Especially that
nothing in fdt-dump.c depends on DEVICE_TREE_DEBUG.

>         If unsure, say N here.
>  
>  config SCRUB_DEBUG
> diff --git a/xen/common/device-tree/Makefile b/xen/common/device-tree/Makefile
> index 9036e455d66a..959dd2a9fd30 100644
> --- a/xen/common/device-tree/Makefile
> +++ b/xen/common/device-tree/Makefile
> @@ -6,6 +6,7 @@ obj-$(CONFIG_DOMAIN_BUILD_HELPERS) += domain-build.init.o
>  obj-$(filter $(CONFIG_DOM0LESS_BOOT),$(CONFIG_HAS_DEVICE_TREE_DISCOVERY)) += 
> dom0less-build.init.o
>  obj-$(CONFIG_DOM0LESS_BOOT) += dom0less-bindings.init.o
>  obj-$(CONFIG_OVERLAY_DTB) += dt-overlay.o
> +obj-$(CONFIG_DEVICE_TREE_DEBUG) += fdt-dump.init.o
>  obj-$(CONFIG_HAS_DEVICE_TREE_DISCOVERY) += intc.o
>  obj-$(CONFIG_DOMAIN_BUILD_HELPERS) += kernel.o
>  obj-$(CONFIG_STATIC_EVTCHN) += static-evtchn.init.o
> diff --git a/xen/common/device-tree/dom0less-build.c 
> b/xen/common/device-tree/dom0less-build.c
> index fcbeb8adbd73..182201384faf 100644
> --- a/xen/common/device-tree/dom0less-build.c
> +++ b/xen/common/device-tree/dom0less-build.c
> @@ -1,5 +1,6 @@
>  /* SPDX-License-Identifier: GPL-2.0-only */
>  
> +#include <xen/bootfdt.h>
>  #include <xen/bootinfo.h>
>  #include <xen/device_tree.h>
>  #include <xen/dom0less-build.h>
> @@ -587,6 +588,9 @@ static int __init prepare_dtb_domU(struct domain *d, 
> struct kernel_info *kinfo)
>      if ( ret < 0 )
>          goto err;
>  
> +    dt_dprintk("Device tree for %pd:\n", d);
Move this message to device_tree_dump.

> +    device_tree_dump(kinfo->fdt);
> +
>      return 0;
>  
>    err:
> diff --git a/xen/common/device-tree/fdt-dump.c 
> b/xen/common/device-tree/fdt-dump.c
> new file mode 100644
> index 000000000000..fc3d9eca5891
> --- /dev/null
> +++ b/xen/common/device-tree/fdt-dump.c
> @@ -0,0 +1,180 @@
> +/* SPDX-License-Identifier: GPL-2.0-or-later */
> +/*
> + * Dump a flattened device tree blob in a DTS-like format.
> + *
> + * is_printable_string() and print_value() are derived from
> + * util_is_printable_string() and utilfdt_print_data() in the device tree
> + * compiler (dtc).
> + *
> + * Copyright 2011 The Chromium Authors, All Rights Reserved.
> + * Copyright 2008 Jon Loeliger, Freescale Semiconductor, Inc.
> + * util_is_printable_string contributed by
> + *      Pantelis Antoniou <pantelis.antoniou AT gmail.com>
> + */
> +
> +#include <xen/bootfdt.h>
> +#include <xen/ctype.h>
> +#include <xen/init.h>
> +#include <xen/lib.h>
> +#include <xen/libfdt/libfdt.h>
> +#include <xen/types.h>
> +
> +/* Deeper nodes are still printed, but not indented any further. */
> +#define DUMP_MAX_INDENT_DEPTH 32
> +
> +/* Indentation, in characters, of a node at depth @depth. */
> +static int __init indent_of(int depth)
> +{
> +    return 2 * min(depth, DUMP_MAX_INDENT_DEPTH);
> +}
> +
> +static bool __init is_printable_string(const void *data, unsigned int len)
> +{
> +    const char *s = data;
> +    const char *se = s + len;
> +
> +    /* A zero length property, or one which isn't NUL terminated, isn't one. 
> */
> +    if ( (len == 0) || (s[len - 1] != '\0') )
> +        return false;
> +
> +    while ( s < se )
> +    {
> +        const char *ss = s;
> +
> +        while ( (s < se) && *s && isprint(*s) )
Xen's is_print() behaves differently than what dtc uses. You need to combine it
with is_ascii. Otherwise 0xff would be marked as printable.

~Michal