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From: Frank Rowand <>
To: David Gibson
	Rob Herring <>
Cc: "Devicetree Compiler"
	"Jon Loeliger" <jdl-CYoMK+44s/>,
	"Pantelis Antoniou"
	"Grant Likely"
	"Marek Vašut"
	"Tom Rini" <trini-OWPKS81ov/FWk0Htik3J/>,
	"Kyle Evans" <>,
	"Geert Uytterhoeven"
	"Alan Tull" <>,
	"Michael Ellerman" <mpe-Gsx/>
Subject: Re: [RFC] devicetree: new FDT format version
Date: Tue, 23 Jan 2018 13:17:47 -0800	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <20180123124232.GA14832@umbus>

On 01/23/18 04:42, David Gibson wrote:
> On Mon, Jan 22, 2018 at 03:01:52PM -0600, Rob Herring wrote:
>> On Mon, Jan 22, 2018 at 2:09 AM, Frank Rowand <> wrote:
>>> 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.
>> A few things discussed before:
>> - Adding type information Even just tagging phandles would be good.
> I'm a bit dubious about this.  It would have to be "hints" only -
> there's not really anyway we can put in authoritative type
> information, since dtc itself doesn't really know that.  It's also
> hard to see how that could be done in a way which wouldn't either a)
> require very awkward parallel lookup of the data and type information
> or b) not be backwards compatible (even read only).
>> - Allow applying overlays by just appending to the blob. The need for
>> this is somewhat gone now that libfdt can just fully apply overlays.
> I'm not really sure what you want here.  I mean you could easily allow
> the format to allow multiple appended overlays, and define that to
> mean the overlaid result.  At some point *something* is going to have
> to really do the application, so I'm not sure that it really buys you
> that much.  It also makes nearly every operation on the tree in libfdt
> horrible to implement, at least within the other constraints the
> interface was designed around; you'll continually have to scan the
> entire tree just in case some other overlay fragment has some bearing
> on the thing you're looking at.  It confuses the interface too: what
> does "node offset" mean if the same node could be built up from
> overlay fragments at multiple offsets.

Somewhat echoing David's comment, I'm also not sure what you mean.
And trying to not overly influence this conversation with preconceptions
from what I'm going to propose.

My first shot at the new format added a field to the FDT to indicate
that an overlay FDT was concatenated to the FDT (and the overlay FDT
in turn could set it's field to concatenate another overlay FDT).
But in the end I decided that information belonged outside the FDT,
and it was sufficient to require that all FDTs be padded out so that
if an overlay FDT was concatenated to the FDT, the overlay FDT would
be properly aligned.

For ease of typing, I'll call this "chaining" or "chained".  For
Linux, I am envisioning no kernel use of data from a chained FDT
until after the tree is unflattened.  I haven't done an exhaustive
search to determine all of the uses of data directly from the
flattened FDT, but I am optimistic that there will not be any such
access that would require data from a chained FDT (and we could
make this a rule).  I need to verify my assumptions here.  The
proof of concept code (doesn't even compile) would do phandle
resolution directly on the overlay FDT(s), and the unflatten
code would be slightly modified (I think -- I haven't prototyped
the unflatten part yet) to unflatten each FDT separately.  In
Linux, unflatten is two passes, (1) scan FDT to determine
unflattened size, (2) actually unflatten.  So this would become
(1) determine unflattened size of FDT _and_ overlay FDT(s),
(2) unflatten FDT and overlay FDT(s).

When I send out my proposal, I will note this issue, and I would
expect there will be discussion about whether information to chain
FDTs belongs in the FDT, or somewhere external.

>> - Move to an unflattened format so we don't have to unflatten the
>> tree. Or to put it another way, extend FDT enough that the tree can be
>> walked and manipulated efficiently. I think this would involve storing
>> node offsets for all the nodes. Maybe that could be combined with
>> storage for __symbols__?
> I don't think this is feasible.  If you want an unflattened tree
> you'll have to change things so much that any superficial resemblence
> to dtb will just be misleading.  You're likely to increase the size in
> nearly all cases (which seems to be a concern to people).  You're also
> almost certain to lose the benefits the serialized approach was
> written for in the first place: chiefly that you can make edits
> locally, using inserts and deletes but without having to adjust
> offsets or handles anywhere else in the structure.

With my Linux blinders on, I don't see the need for any significant
access of the data in the FDT.  We do very little access before the
tree is unflattened.

My knowledge of bootloader use of the FDT is nearly non-existent, so
I'm on thin ice here.  But I am guessing that a lot of bootloader
processing of FDT data could be removed if the kernel would unflatten
a chained FDT and overlay FDT(s).

Comments from the bootloader folks on this topic would be much

> Making a portable "libdt" that manages an in-memory unflattened tree
> with pointers, and can serialize and deserialze to dtb makes sense,
> IMO.  Trying to make the flattened and unflattened trees the *same*
> format does not.

That seems like a good approach, if the functionality is needed.

> Basically, efficient runtime manipulation is out of scope for the dtb
> format - that's not what it's for.  If you're doing non-trivial
> manipulation you really should be unflattening, then doing your work,
> and if necessary re-flattening afterwards.  Unfortunately, I think the
> fact you can manipulate while still in flattened format (and libfdt
> even makes it pretty easy to write code to do so) has meant a bunch of
> projects have postponed going to an unflattening model rather past the
> point they should have.
>> I also am not happy that labels which were purely source level
>> convenience are now part of the ABI with overlays> 
> Yeah :/.  I keep harping on it, but overlays really are a hack.  I
> think fleshing out the "connectors" approach that has been suggested
> is really a better approach than trying to make the hack less hacky.

Two topics:

(1) This maybe means that we need to have a connectors architecture
before finalizing any FDT changes, to ensure that the FDT contains
everything that connectors require.

(2) I don't see the nodes __symbols__, __fixups__, and __local_fixups__ as
ABI, even though they are in use in the wild.

I see them as an experimental implementation to provide a solution to
a problem space.  I feel that this implementation should continue to
exist, but in a deprecated fashion, for quite a while for the current

I suspect this is the will be a controversial position, but I am
strongly leaning in this direction, especially for the Linux


>>> 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.
>> What does libfdt do with a v18 blob? I'd assume it was written to
>> treat new versions the same as the last known version.
> So, the header includes a "last compatible version".  libfdt will
> attempt to work with it as long as that is <= 17, regardless of the
> version field.  Assuming version >= 17 then, yes, libfdt will treat
> the blob as if it was v17, the latest one it knows.
> If you alter the tree, libfdt will downrev the 'version' field to the
> latest one it's aware of (i.e. 17).  Current dtc will do this on any
> write operation (unless they don't call fdt_rw_check_header_(), which
> would be a bug).  I'd have to check, but I think older versions may
> only do it at the time of fdt_open_into(), which is part of the normal
> expected flow for read-write operations, but doesn't technically
> *have* to be called.
> The "in place write" functions (fdt_wip.c) _won't_ downrev the
> version, which is arguably a bug.  Most obvious extensions would not
> be broken by such in-place writes, but it's not impossible (and
> something which stored type data in parallel with the properties could
> be an example).

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  reply	other threads:[~2018-01-23 21:17 UTC|newest]

Thread overview: 33+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-01-22  8:09 Frank Rowand
     [not found] ` <>
2018-01-22  8:14   ` Frank Rowand
     [not found]     ` <>
2018-01-22 20:08       ` Frank Rowand
2018-01-22 20:08   ` Frank Rowand
     [not found]     ` <>
2018-01-25  0:27       ` Frank Rowand
     [not found]         ` <>
2018-01-27  8:48           ` David Gibson
2018-01-29  8:08             ` Frank Rowand
     [not found]               ` <>
2018-01-29 10:56                 ` David Gibson
2018-01-30  1:29                   ` Frank Rowand
2018-01-22 21:01   ` Rob Herring
     [not found]     ` <>
2018-01-23 12:42       ` David Gibson
2018-01-23 21:17         ` Frank Rowand [this message]
     [not found]           ` <>
2018-01-24 15:47             ` Rob Herring
     [not found]               ` <>
2018-01-24 21:16                 ` Frank Rowand
     [not found]                   ` <>
2018-01-24 22:27                     ` Alan Tull
2018-01-25  0:22                     ` Frank Rowand
     [not found]                       ` <>
2018-01-25 12:29                         ` David Gibson
2018-01-25 20:01                           ` Frank Rowand
     [not found]                             ` <>
2018-01-29 18:32                               ` Grant Likely
     [not found]                                 ` <>
2018-01-29 23:15                                   ` David Gibson
2018-01-26  8:56                           ` Geert Uytterhoeven
     [not found]                             ` <>
2018-01-26  8:59                               ` Geert Uytterhoeven
2018-01-26 22:08                               ` Frank Rowand
     [not found]                                 ` <>
2018-01-27  9:00                                   ` David Gibson
2018-01-27  8:56                               ` David Gibson
2018-01-25 23:11                     ` Frank Rowand
2018-01-25 12:22                 ` David Gibson
2018-01-25  9:14             ` Marek Vasut
     [not found]               ` <>
2018-01-25 12:37                 ` David Gibson
2018-01-27 20:30                   ` Marek Vasut
     [not found]                     ` <>
2018-01-29  0:53                       ` David Gibson
2018-01-23 12:05   ` David Gibson
2018-01-23 21:28     ` Frank Rowand

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