Re: Revisiting Devicetree Overlay Manager

[Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>]

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