Re: Revisiting Devicetree Overlay Manager

[Frank Rowand <frowand.list@gmail.com>]

Hi Manivannan,

Sorry for the delay.  I'll try to get to this today or tommorrow.

-Frank


On 04/25/18 00:34, Manivannan Sadhasivam wrote:
> Hi Frank,
> 
> Did you had time to look into this? Especially I'd like to hear your
> opinion on the first pain point I have mentioned. I can understand
> that the whole point on introducing the of_overlay_fdt_apply() API
> is to remove the duplication of overlay FDT unflattening, but do you have
> any idea of how we can make this API work with overlay nodes appended
> to the base DTB?
> 
> Thanks,
> Mani
> 
> On Thu, Apr 19, 2018 at 08:08:04PM +0530, Manivannan Sadhasivam wrote:
>> Hello Everyone,
>>
>> I have just started working on the Devicetree Overlay Manager support in
>> kernel. The idea is to merge overlays at boot time specified via some
>> interface like kernel command line. The motivation for this work comes from
>> Overlay Manager [1] submitted by John last year. The mechanism which I have
>> been working on is an extension to John's work. It focusses on addressing
>> Rob's comments on the Overlay Manager which involves having multiple ways
>> to load overlays.
>>
>> Proposal:
>> =========
>>
>> 1. Pass all devicetree overlays via following methods:
>>    - Overlays appended to base DTB
>>    - Individual overlays built into kernel as firmware blobs
>>    - Any other ways?
>>
>> 2. Specify overlays to load via kernel command line as below:
>>    - overlay_mgr.overlays=<overlay1,overlay2,...>
>>
>> 3. Merge only the specified overlays during boot time. First look for the
>> overlay in the base DTB. If it is found, just apply it, else defer to firmware
>> load approach.
>>
>> The Overlay Manager code is expected to be very simple and will just do the
>> above mentioned work. Later on, it will be extended to support dynamic
>> modification of overlays from userspace with some additional security
>> features like having a property listed in the base devicetree for only
>> allowing changes to the current node and its child nodes, etc...
>>
>> Pain Points:
>> ============
>>
>> 1. Starting from 4.17 we don't have any API exposed from DT core to merge
>> individual devicetree nodes. We only have of_overlay_fdt_apply() function
>> which takes the whole FDT. This will work very well for the firmware approach
>> since we will pass the overlays blobs but not for overlays appended to base DTB,
>> where we will pass individual overlay nodes.
>>
>> 2. Using firmware load method will force us to have this Overlay Manager code
>> at late_initcall level since firmware class exists only in fs_inticall level,
>> which might be too late for some devices.
>>
>> 3. This whole approach is not expected to work very well (still not yet tested)
>> on DSI based devices since it needs to be present at very early during boot
>> process.
>>
>> The Overlay Manager propsed here will be board agnostic and it should work on
>> all platforms supporting DT. This will be a _very_ useful feature for the
>> development boards such as 96Boards, Raspberry Pi, BBB etc... and also for
>> production ready devices.
>>
>> So, I'd like to hear suggestions/feedbacks for the above mentioned proposal &
>> pain points and hope to land the most awaited feature in kernel.
>>
>> Regards,
>> Mani
>>
>> [1] https://lkml.org/lkml/2017/10/10/20
>