Re: [PATCH v2 6/6] fdtdec: apply DT overlays from bloblist

[Tom Rini <trini@konsulko.com>]

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On Fri, Jul 04, 2025 at 06:42:05AM -0700, Raymond Mao wrote:

> During FDT setup, apply all existing DT overlays from the bloblist
> to the base FDT if bloblist is being used for handoff from previous
> boot stage.
> According to the spec update for DT overlay handoff[1], an overlay
> must have the same top-level compatible string as its target base
> DT has.
> Before applying the DTO, it checks whether sufficient space is
> reserved in the base FDT region. A margin (0x400) is used during
> estimating the space size required by the merged DT.
> A resizing happens if the reserved space is insufficient.
> After all overlays are applied, it resizes to the actual size of the
> merged DT.
> Note that the margin (0x400) is arbitrary from experience, it might
> not cover all possible scenarios as complex overlays with many
> properties might require extra spaces and lead to FDT_ERR_NOSPACE
> error.
> 
> [1] Add Transfer Entry for Devicetree Overlay
> https://github.com/FirmwareHandoff/firmware_handoff/pull/74
[snip]
> +	actual_size = fdt_totalsize(*blob);
> +	/* Add margin for extra size growth after merging */
> +	new_fdt_size = fdt_totalsize(live_fdt) + actual_size + 0x400;

This is still a fixed value and still not the right way to handle it. I
think now that we expose configuring the amount of extra space we have
for the tree it should possibly even just fail out with an error message
that points to SYS_FDT_PAD?

-- 
Tom

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