[RFC] devicetree: new FDT format version

[Frank Rowand <frowand.list-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

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