Hi All,
I've tried to create a decent distribution list, but I'm sure I've missed
someone or some important list. Please share this with anyone you think
will be affected.
I have been playing around with some thoughts for some additions to
the devicetree FDT aka blob format.
I would like to get the affected parties thinking about how additions to
the format could improve whichever pieces of FDT related technology you
work on or care about. In my opinion, the FDT format should change
very infrequently because of the impact on so many projects that have
to work together to create a final solution, plus the many many users
of those projects.
So I would like you guys to consider what I send out in a day or so,
but I don't want to preempt your creativity by laying out the details
of my proposal right now.
I have not looked at how this would impact the devicetree compilers,
but I have hacked together a tool to convert existing blobs to the
new format. The new format is backward compatible, but transforms
the overlay related metadata into separate blocks and removes the
metadata from nodes and properties. My current proposal leaves
the fragment subtrees intact - it only transforms __symbols__,
__fixups__, and __local_fixups__.
Some Advantages and disadvantages of my proposal are:
Con:
- New blob version.
Pro:
- Backward compatible. Bootloaders and kernels that can process v17 blobs
will continue to work in the same manner with a v18 blob. They will not
be able to use the new v18 features.
Pro:
- If a bootloader passes a blob unmodified to a kernel, then the kernel will
be able to use the new v18 features.
Pro:
- If a bootloader modifies a blob before passing it to a kernel _and_
downrevs the version to v17, then the kernel will continue to work in the
same manner as it works with a v17 blob.
Con:
- If a bootloader modifies a blob before passing it to a kernel _and_ fails
to downrev the version to v17, then the kernel will most likely detect an
error and may choose to not boot.
Pro:
- A trailing magic field allows detection of a partially composed blob,
where the blob is created by a multi-pass tool.
Pro:
- Validation tools can annotate the blob to indicate validation fail or
validation warning. The bootloader and kernel can choose what to do
with the information. For example, a Linux kernel might choose to
taint itself if the blob is not validated or has a validation error.
Pro:
- A significant file size reduction for a blob that contains the symbol
information needed by overlays.
Pro:
- A significant memory size reduction for a blob that contains the symbol
information needed by overlays.
Con:
- All blobs will have a small file size increase when the symbol information
needed by overlays is not included: 125 - 140 bytes or 44 bytes less
than that if checksums not added.
Pro:
- Simplifies implementation of Linux kernel overlay application very early
in the boot, mostly before unflattening the blob. Early boot overlay
application will be complete when the unflattenning is complete.
This feature _might_ _eventually_ reduce of remove the need for the
bootloader to apply overlays.
Pro:
- Simplifies implementation of the Linux kernel overlay application to
the live tree, post-boot.
Some data would be useful, so here it is. I compiled all of the .dts files
in the Linux kernel source tree at arch/arm/boot/dts/. The following data
is the size of the blob (and for Linux the amount of memory that the
flattened tree consumes and never frees after boot). This is for v4.15-rc7.
The following sizes are in bytes.
'dtb no symbols' is the existing blob as currently built in the kernel tree.
'delta no_sym' is the extra overhead from convering 'dtb no symbols' to the
new format. Again, this could be reduced by 44 bytes if the checksums
portion of the proposal is not implemented.
'delta symbols' is the existing format build in the kernel tree, but with
the "-@" flag provided to dtc, so that the __symbols__ node is populated.
'delta new fmt' is the 'delta symbols' blobs converted to the proposed
format.
'saved' is the number of bytes saved by converting to the new format,
when symbols metadata is included in the blob.
The following table is sorted from most bytes saved, to least, with
one entry at approximately every 16 percentile:
row dtb no delta delta delta saved
symbols no_sym symbols new fmt
---- ------- ------- ------- ------- -------
99% 90531 134 42721 15766 26955 am57xx-beagle-x15-revb1.dtb
83% 44302 139 14582 5163 9419 imx6dl-tx6dl-comtft.dtb
66% 26277 132 11662 4628 7034 sun6i-a31s-sinovoip-bpi-m2.dtb
49% 21047 130 7328 2754 4574 imx53-qsb.dtb
33% 12864 137 4305 1705 2600 kirkwood-netgear_readynas_nv+_v2.dtb
16% 12009 128 2929 1520 1409 bcm911360k.dtb
0% 1220 133 68 149 -81 xenvm-4.2.dtb
Note that there are a few blobs with so little symbol metadata that the
proposed format increases the size of the blob. This can be seen in the
tail of the distribution:
2537 128 593 384 209 mt6589-aquaris5.dtb
2966 131 537 355 182 mt8127-moose.dtb
5173 132 552 372 180 spear300-evb.dtb
2473 128 485 368 117 mt6592-evb.dtb
2370 135 454 343 111 cx92755_equinox.dtb
2105 128 377 304 73 mt6580-evbp1.dtb
2357 132 346 292 54 hip01-ca9x2.dtb
1232 137 208 201 7 alphascale-asm9260-devkit.dtb
2571 126 201 222 -21 alpine-db.dtb
1220 133 68 149 -81 xenvm-4.2.dtb
I will reply to this message with the complete table of all arm blobs.
Please discuss. :-)
-Frank
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