On 01/22/18 12:08, Frank Rowand wrote:
> + Alan Tull
> + Michael Ellerman
>
> On 01/22/18 00:09, 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.
>>
>> 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:
< snip >
Here are my current thoughts on a proposed update to the devicetree
Flattened Device Tree (FDT) aka blob format.
Version 18, FDT header:
- Change version from 17 to 18.
- last_comp_version remains 16.
- Add field: u32 off_blocks
This is the offset to a new block called "blocks".
- Add field: u32 chained
If non-zero, this indicates that an overlay FDT is concatenated
to the end of this FDT.
An alternative to adding this field would be to provide chaining
information in some manner external to the FDT. One advantage of
using a data structure external to the FDT is that the information
could include extra details such as how to relocate the overlay.
For example the overlay could describe an add-on card, where the
add-on card could be located in one of several slots. For
another example, there could be multiple instances of the add-on
card and the same overlay could be relocated for each of those
slots.
The "chained" field does not preclude the use of an external
data structure to provide additional information, such as
relocations.
This field shall be set to zero by the compiler, unless the
compiler is creating a chain of FDTs. This field would
normally be set by a tools that assembles multiple FDTs
into a block of chained FDTs.
If "chained" is non-zero then the size of the FDT must provide
the required alignment for a directly appended FDT.
[[ The size/alignment intent is to simplify any tool that
assembles a block of chained FDTs. ]]
- Add field: u32 phandle_delta
If non-zero, this indicates that phandle resolution has occurred
on this FDT, and internal phandle references in properties have
been incremented by this value.
The intent of this field is for use when a running Linux kernel
provides a chained FDT to kexec, which will in turn provide the
chained FDT to a newly booting instance of the Linux kernel. If
the booting Linux kernel detects a non-zero phandle_delta then
it should decrement the phandle references by this value and then
perform phandle resolution again.
Instead of adding this field to the FDT header, I prefer to add
it to an external chaining information block. If this field is
in the FDT header, and the same FDT is applied for multiple
connectors, then a separate FDT would need to be supplied for
each instance of the overlay, because the delta would be different
for each instance. If the external chaining information block
contained several sets of relocation information for the same
FDT, then that relocation information would also contain the
phandle_delta for that instance.
Version 17 has blocks:
- mem_rsvmap
- dt_struct
- dt_strings
Version 18, add block:
- blocks
This block contains data about all blocks in the FDT, including
the blocks that exist in version 17. This means that the offsets
and sizes of the version 17 blocks will exist in the FDT header
and be duplicated in the "blocks" block. Users of version 18 and
above must use the information from the "blocks" block instead
of from the FDT header. Then after a few more version changes
(say in 10 or 15 years), the offsets and sizes in FDT header (other
than the offset of the "blocks" block) can be repurposed.
The first field of "blocks" is the number of blocks described by
"blocks".
This field is followed by a tuple of offset and size for each of
the blocks.
A c representation of "blocks" is:
struct fdt_blocks {
u32 num_blocks;
u32 blocks_off;
u32 blocks_size;
u32 csums_off;
u32 csums_size;
u32 dt_strings_off;
u32 dt_strings_size;
u32 dt_struct_off;
u32 dt_struct_size;
u32 ext_phandle_use_off;
u32 ext_phandle_use_size;
u32 int_phandle_use_off;
u32 int_phandle_use_size;
u32 mem_rsvmap_off;
u32 mem_rsvmap_size;
u32 symbols_off;
u32 symbols_size;
u32 validate_off;
u32 validate_size;
};
The num_blocks field allows adding additional blocks without
incrementing the FDT header version number. Or the specification
could require incrementing the version whenever a block is added.
If the size field of a tuple is zero, then the block does not
exist.
Version 18, add block:
- csums
Each tuple in this block contains one field, which is the
checksum of the corresponding block.
The tuples in this block are in the same order as the tuples
in the "blocks" block. This leads me to argue that the
"blocks" block tuples be in a fixed order, not allowing
tuples for non-existent blocks to be absent.
Checksums are inspired by an old suggestion from Grant Likely.
The intent was to allow a kernel to detect if a bootloader
that did not understand the new version modified the FDT in
a manner that corrupts version 18 data.
According to dgibson, "Altering a blob and not downrevving it
to the latest version you understand is definitely a bug".
That give me some assurance that the problem being protected
against should not exist. On the other hand, the checksums
do not take up a lot of space. The specification should
choose to either make the "csums" block required or make
it optional.
Version 18, add block:
- ext_phandle_use
This is the information needed to describe locations within
properties that contain the value of a phandle, where the
reference phandle property is external to this FDT.
The name could be changed to "external_phandle_use" for
more clarity.
The name change is intended to reflect "what the data is"
instead of "what the consumer is supposed to do with the
data".
The ext_phandle_use block is analagous to the data in the
__fixups__ node.
Each entry in the "ext_phandle_use" block is a tuple of:
u32 prop_value_offset
u32 symbol_offset
The prop_value_offset contains the offset within the "dt_struct"
block of the location within a property value that contains a
phandle value.
The symbol_offset contains the offset within the "dt_strings"
block that contains the name of the label corresponding to
the node that contains the referenced phandle value, where the
phandle value refers to a node in a different FDT.
The value to place at prop_value_offset will be found in the
"symbols" block of the FDT that contains the labeled node.
Version 18, add block:
- int_phandle_use
This is the information needed to describe locations within
properties that contain the value of a phandle, where the
reference phandle property is internal to this FDT.
The name could be changed to "internal_phandle_use" for
more clarity.
The int_phandle_use block is analagous to the data in the
__local_fixups__ node.
The name change is intended to reflect "what the data is"
instead of "what the consumer is supposed to do with the
data".
Each entry in the "ext_phandle_use" block is a single field of:
u32 prop_value_offset
The prop_value_offset contains the offset within the "dt_struct"
block of the location within a property value that contains a
phandle value, where the phandle value refers to a node in the
same FDT. The value of the phandle property in the referenced
node is the same as the value located at prop_value_offset.
The compiler shall create phandle property values in an increasing
contigous range, beginning with one. Exception: compiler created
values will not duplicate phandle property values that are
explicitly provided in the devicetree source file.
The value to place at prop_value_offset is an implementation
dependent value, where the value does not conflict with any
phandle property values in the active devicetree.
[[ for information only: The Linux kernel creates the replacement
value by adding a delta to all phandle properties in the FDT and
all internal phandle references. ]]
Version 18, add block:
- symbols
This is the information that describes the values of the phandle
properties in labeled nodes.
The information in the FDT "symbols" block is used to resolve
phandle references in an overlay when it is applied to the active
devicetree.
An overlay FDT may also contain a "symbols" block, which is used
to resolve references in a subsequent overlay when it is applied
to the active devicetree.
Each entry in the "ext_phandle_use" block is a tuple of:
u32 phandle_value
u32 symbol_offset
The phandle_value contains the value in this FDT of the phandle
property in the labeled node whose label name is described by
symbol_offset.
The symbol_offset contains the offset within the "dt_strings"
block that contains the name of the label corresponding to
the node that contains the phandle value.
Version 18, add block:
- validate
This is the information that describes any validation of the
FDT and/or the devicetree source that the FDT was created from.
A c representation of "validate" is:
u32 validation_done;
u32 errors_count;
u32 warnings_count;
How the client program [[ eg kernel ]] uses the data is
implementation dependent.
I created these fields as a placeholder. I would like the actual
choice of fields to flow out of the current efforts to create
devicetree validation tools.
[[ for information only: Some examples of what the Linux
kernel could use this information for:
- print a warning message if any warnings exist
- print a warning message if any errors exist
- taint the kernel if any errors exist
- refuse to boot if any errors exist
]]
One question I have is how to represent the base devicetree
(or base devicetree plus one or more applied overlays)
that this FDT was validated against when this FDT is
an overlay FDT.
Version 18, add a footer field:
- footer_magic
This field allows detection of a partially completed FDT, where
the FDT is created by a multi-pass tool. The final action of
such a tool is to set the value of this field.
The value of this field shall be u32 0xeeeefeed.
This field is located as the last u32 field in the FDT. The FDT
shall be zero padded as needed to provide proper alignment for
this field.
The use of "dt_struct" block offsets and "dt_strings" block offsets is
intended to make phandle reference resolution easy and efficient when
an overlay is applied.
The downside to using block offsets is that if a boot program deletes
a property (by replacing the property entry in the "dt_struct" block
with NOPs), then the client program must be aware of the NOPs and
not attempt to overwrite a NOP with a phandle value. I do not expect
this to be a significant complication. The alternative to this
would be for the client program to have a policy (shared agreement
with the boot program) that no phandle values are allowed to be
deleted. I think that this alternative is too restrictive, but
raise it as a possibility.
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