[PATCH] fdtgrep: Add option to reserve free space in the output FDT

[Quentin Schulz]

Hi Simon,

On 5/4/26 2:49 PM, Simon Glass wrote:
> Hi Quentin,
> 
> On Mon, 27 Apr 2026 at 03:14, Quentin Schulz <quentin.schulz at cherry.de> wrote:
>>
>> Hi Simon, Marek,
>>
>> On 4/22/26 4:31 AM, Simon Glass wrote:
>>> When producing a DTB from fdtgrep, there is no way to reserve extra
>>> space so that more nodes/properties can be added at runtime without
>>> growing totalsize first.
>>>
>>
>> Ok but why is that an issue? What is the actual usecase? What I'm afraid
>> of is that you pad enough and you hit random bugs because you forget
>> that adding enough properties can now hit the size limit and you forgot
>> to add the logic to resize/grow the fdt when it's too small to fit new
>> properties.
> 
> I'm going to leave this question to Marek :-)
> 
>>
>>> Add a -B/--pad option that appends a caller-specified number of free
>>> bytes after the strings section and grows totalsize accordingly. The
>>> padding is applied after fdt_pack(), so it is not reclaimed when -r is
>>> used.
>>>
>>> Suggested-by: Marek Vasut <marek.vasut at mailbox.org>
>>> Signed-off-by: Simon Glass <sjg at chromium.org>
>>> ---
>>>
>>>    tools/binman/btool/fdtgrep.py |  3 ++-
>>>    tools/fdtgrep.c               | 31 ++++++++++++++++++++++++++++++-
>>>    2 files changed, 32 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/tools/binman/btool/fdtgrep.py b/tools/binman/btool/fdtgrep.py
>>> index 446b2f4144b..7b393241878 100644
>>> --- a/tools/binman/btool/fdtgrep.py
>>> +++ b/tools/binman/btool/fdtgrep.py
>>> @@ -16,9 +16,10 @@ Output formats are:
>>>        dtb - device tree blob (sets -Hmt automatically)
>>>        bin - device tree fragment (may not be a valid .dtb)
>>>
>>> -Options: -[haAc:b:C:defg:G:HIlLmn:N:o:O:p:P:rRsStTvhV]
>>> +Options: -[haAB:c:b:C:defg:G:HIlLmn:N:o:O:p:P:rRsStTvhV]
>>>      -a, --show-address                 Display address
>>>      -A, --colour                       Show all nodes/tags, colour those that match
>>> +  -B, --pad <arg>                    Append free space to the output FDT, for later growth
>>>      -b, --include-node-with-prop <arg> Include contains containing property
>>>      -c, --include-compat <arg>         Compatible nodes to include in grep
>>>      -C, --exclude-compat <arg>         Compatible nodes to exclude in grep
>>
>> I'm tempted to remove this and just tell people to look at the
>> documentation in tools/fdtgrep.c. Especially since we don't actually
>> allow the user to pass any of those parameters themselves via the
>> bintool, so it isn't actually useful.
> 
> I would like fdtgrep to give useful help, though. It is a tool which
> can stand alone.
> 

Yes, but this here is docstring for the fdtgrep *Bintool* module which 
actually doesn't support most of the arguments specified here. So I 
would say it's more confusing to have it there than not have anything. 
For sure we want to keep documentation of fdtgrep, the tool as in 
tools/fdtgrep.c, complete and up-to-date!

Cheers,
Quentin