Device Tree Microconference Notes
Welcome to Linux Plumbers Conference 2015
The structure will be short introductions to an issue or topic followed by a discussion with the audience.
Please use this etherpad to take notes. Microconf leaders will be giving a TWO MINUTE summary of their microconference during the Friday afternoon closing session.
Please remember there is no video this year, so your notes are the only record of your microconference.
Miniconf leaders: Please remember to take note of the approximate number of attendees in your session(s).
Reference Materials
Presentation slides and many references are at:
Organizers:
Frank Rowand
Schedule
1:30 Intro
1:35 Device Tree Overlays
- Pantelis Antoniou, Guenter Roeck
2:00 Overlays, some times a good idea sometimes not.
2:15 Device Tree Documentation
- Matt Porter, Frank Rowand
2:30 Chat With The dtc Maintainers, part 1
- David Gibson, Jon Loeliger
2:45 ----- Tea Break -----
3:00 Chat With The dtc Maintainers, part 2
- David Gibson, Jon Loeliger
3:10 Overlays and tools for sanity.
3:25 Device Tree Tools
3:40 Device Tree probe order and parallel device probing
3:50 Device tree round up
Notes
Total # attendees: 53
Total # participants: 20
1:30 Introduction
Frank does not suggest interfering with each other
Multiple people can write on etherpad at the same time.
1:35 Device Tree Overlays
Pantelis Antoniou
slides: http://elinux.org/images/f/fa/Plumbers_2015_dt_DT-plumbers-2015.pdf
Device trees have been static in the past, but doesn't work for many scenarios
- expansion boards, riser cards, etc.
- too many combinations, not realistic to have a device tree per combination
With dynamic use cases device trees phandles have to be resolved, similar to linking
Overlays initially created for Beagle Bone board, which has expansion boards called Capes
- can select which overlay applies to your scenario
- Proposed overlay dtb stored in eeprom of device
- Also overlays for changing what driver an I2C or PCI attached device should use, instead of changing the PCI ID
CONFIG_OF_DYNAMIC
- allowed some dynamic changes to live device tree at runtime
- Frank: some changes on powerpc in process to allow reverting PCI device tree fragments
- Changes to DT were not fully reflected in driver state
- reworking of OF_DYNAMIC made it into 3.17
- major user was dt self tests
Dynamic resolution
- Device handles used to be converted from names to numbers at compile time
- Now needed to be resolved at run-time when overlays are added
DT compiler changes
- no changes to DTB format
- Inserts a __symbols__ table with the name of the handle pointing to the node path
Changesets
- Grant Likely: current algorithm for selecting phandles sucks
- have talked about using hash tables
- if they overlap can only be reverted in order
Frank: Are things fixed to the point where the proposed .dtb format can't be changed
- generally no, things could be changed
- there are some overlays flashed in firmware already, but if necessary the old overlay content could be used to detect the hardware and look up the right overlay
We should not make decisions assuming "Linux". There are other device tree users, like BSD.
Overlays are not merged into the kernel yet, Questions about how and when overlays are applied is one of the issues.
Grant wants to have mechanism to filter out which nodes can be touched.
User space policy thoughts:
- only add things that did not exist?
- can't delete nodes?
- can only modify nodes not bound to a driver?
FPGA use case
- "bridge" driver would apply the overlay
2:00 Overlays, some times a good idea sometimes not.
xxx
2:15 Device Tree Binding Documentation
Matt Porter
slides: http://elinux.org/images/4/4a/Plumbers_2016_dt_DT_Binding_Documentation.pdf
Process is mature
- subsystem maintainers make sure new hardware adhere to the generic bindings
Problems
- bindings bit rot
- formatting is inconsistent
- terminology is inconsistent
Grant: as long as bindings are human readable, we'll have issues
Need a schema that could be applied to nodes...but this has been discussed for years and nothing has been done
Proposed solutions
- move to rigid text markup template
- revive Stephen Warren's old DT bindings doc
- separate generic bindings from other bindings
Device Tree Documentation
Frand Rowand
slides: http://elinux.org/images/1/17/Plumbers_2016_dt_device_tree_doc.pdf
What exists:
- ePAPR, 1275, etc. Lots of fragments of information
Frank is trying to organize info at elinux.org/Device_Tree
Email Frank suggestions, etc related to documentatoin and he will included it in elinux.org
Question about standardization-- talk last year about OASIS or Linux Foundation
Stuart: will send Frank info about possibility of moving ePAPR outside of power.org, which is effectively defunct.
2:30 Chat With The dtc Maintainers
David Gibson: has some concerns about overlay syntax. Conerned about living with bad decisions for a very long time.
DTC with overlay support is not in mainline DTC, is in side repos.
DTC to do list
- richer expression support
- string expressions
- way of producing node/property names from an expression
- more checks, like does "reg" have sane value, does interrupts have right number of cells?
David: wants to see patches to DTC
Frank: would like to have an option to have DTC insert a build string (e,g. SHA) into the DTB
FreeBSD guys have reimplemented DTC. (same source format, different code base).
David: who would like to see more expression support? (1 person interested)
Frank: DTC error messages are problematic-- what is the actual issue found (cause of the error message) and how to fix it.
David: DTC does track line number of all tokens, but error productions need improvements
http://elinux.org/Device_Tree_Usage
Device Tree Overlays at Juniper
Guenter Roeck
slides: http://elinux.org/images/7/71/Plumbers_2016_dt_Devicetree_Overlays_at_Juniper.pdf
don't know what board will be inserted in a chassis
use overlays to describe various boards
want to be able to hot insert cards into a system that is in the field without rebooting it
each slot has an overlay
connector driver handles insertion/removal
David: why use overlays on PCI devices
- card may have hundreds of gpio pins, and large number of i2c busses
- PCI cards don't use normal int a/b ... use GPIOs that implement interrup
Want DT overlay support on x86.
3:10 Overlays and tools for sanity.
xxx
3:25 Device Tree Tools
Frank: working on tools to identify where things went wrong. Did talk on Wednesday. Working on additional tools.
3:40 Device Tree probe order and parallel device probing
xxx
3:50 Device tree round up
xxx
4:00 LPC 2015 Readouts -- Metropolitan B
https://etherpad.openstack.org/p/LPC2015_Device_Tree
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