Re: Dynamic overlay failure in 4.19 & 4.20

From: Frank Rowand
Date: Tue Jun 04 2019 - 14:24:45 EST

Hi Phil,

On 6/4/19 5:15 AM, Phil Elwell wrote:
> Hi,
> In the downstream Raspberry Pi kernel we are using configfs to apply overlays at
> runtime, using a patchset from Pantelis that hasn't been accepted upstream yet.
> Apart from the occasional need to adapt to upstream changes, this has been working
> well for us.
> A Raspberry Pi user recently noticed that this mechanism was failing for an overlay in
> 4.19. Although the overlay appeared to be applied successfully, pinctrl was reporting
> that one of the two fragments contained an invalid phandle, and an examination of the
> live DT agreed - the target of the reference, which was in the other fragment, was
> missing the phandle property.
> 5.0 added two patches - [1] to stop blindly copying properties from the overlay fragments
> into the live tree, and [2] to explicitly copy across the name and phandle properties.
> These two commits should be treated as a pair; the former requires the properties that
> are legitimately defined by an overlay to be added via a changeset, but this mechanism
> deliberately skips the name and phandle; the latter addresses this shortcoming. However,
> [1] was back-ported to 4.19 and 4.20 but [2] wasn't, hence the problem.

I have relied upon Greg's statement that he would handle the stable kernels, and that
the process of doing so would not impact (or would minimally impact) maintainers. If
I think something should go into stable, I will tag it as such, but otherwise I ignore
the stable branches. For overlay related code specifically, my base standard is that
overlay support is an under development, not yet ready for prime time feature and thus
I do not tag my overlay patches for stable.

Your research and analysis above sound like there are on target (thanks for providing
the clear and detailed explanation!), so if you want the stable branches to work for
overlays (out of tree, as you mentioned) I would suggest you email Greg, asking that
the second patch be added to the stable branches. Since the two patches you pointed
out are put of a larger series, you might also want to check which of the other
patches in that series were included in stable or left out from stable. My suggestion
that you request Greg add the second patch continues to rely on the concept that
stable does not add to my workload, so I have not carefully analyzed whether adding
the patch actually is the correct and full fix, but instead am relying on your good
judgment that it is.

To give you some context on the patch series, I started with 144552c78692 ("of: overlay:
add tests to validate kfrees from overlay removal") due to my concerns with how
memory is allocated and freed for overlays. The added tests exposed issues, and
the rest of the patches in the series were dealing with the issues that were

You can see the full series (with maybe an extra patch or so) with
git log --oneline 144552c78692^..eeb07c573ec3

> The effect can be seen in the "overlay" overlay in the unittest data. Although the
> overlay appears to apply correctly, the hvac-large-1 node is lacking the phandle it
> should have as a result of the hvac_2 label, and that leaves the hvac-provider property
> of ride@200 with an unresolved phandle.
> The obvious fix is to also back-port [2] to 4.19, but that leaves open the question of
> whether either the overlay application mechanism or the unit test framework should have
> detected the missing phandle.

Yes, unittest showed the missing phandle, as you described in the /proc view of
overlay.dts above. That portion of unittest depends on someone looking at the
result of what is in /proc/device-tree and is not automated and is not documented.
I intend to automate that check someday, but the task is still languishing on my
todo list.

> Phil
> [1] 8814dc46bd9e ("of: overlay: do not duplicate properties from overlay for new nodes")
> [2] f96278810150 ("of: overlay: set node fields from properties when add new overlay node")