Re: [RFC PATCH 0/2] Add a test to catch unprobed Devicetree devices

[Rob Herring]

On Thu, Aug 10, 2023 at 04:23:49PM -0400, Nícolas F. R. A. Prado wrote:
> 
> Regressions that cause a device to no longer be probed by a driver can
> have a big impact on the platform's functionality, and despite being
> relatively common there isn't currently any generic test to detect them.
> As an example, bootrr [1] does test for device probe, but it requires
> defining the expected probed devices for each platform.
> 
> Given that the Devicetree already provides a static description of
> devices on the system, it is a good basis for building such a test on
> top.
> 
> This series introduces a test to catch regressions that prevent devices
> from probing.
> 
> Patch 1 introduces a script to parse the kernel source using Coccinelle
> and extract all compatibles that can be matched by a Devicetree node to
> a driver. Patch 2 adds a kselftest that walks over the Devicetree nodes
> on the current platform and compares the compatibles to the ones on the
> list, and on an ignore list, to point out devices that failed to be
> probed.
> 
> A compatible list is needed because not all compatibles that can show up
> in a Devicetree node can be used to match to a driver, for example the
> code for that compatible might use "OF_DECLARE" type macros and avoid
> the driver framework, or the node might be controlled by a driver that
> was bound to a different node.
> 
> An ignore list is needed for the few cases where it's common for a
> driver to match a device but not probe, like for the "simple-mfd"
> compatible, where the driver only probes if that compatible is the
> node's first compatible.
> 
> Even though there's already scripts/dtc/dt-extract-compatibles that does
> a similar job, it didn't seem to find all compatibles, returning ~3k,
> while Coccinelle found ~11k. Besides that, Coccinelle actually parses
> the C files, so it should be a more robust solution than relying on
> regexes.

I just sent a patch[1] last week fixing missing a bunch. I only looked 
at the change in count of undocumented (by schema) though.

In any case, I'm happy if we have a better solution, but really we 
should only have 1. So your script would need to replace the existing 
one.

I'd be interested in a performance comparison. IME, coccinelle is 
fairly slow. Slower is okay to a point though.

> 
> The reason for parsing the kernel source instead of relying on
> information exposed by the kernel at runtime (say, looking at modaliases
> or introducing some other mechanism), is to be able to catch issues
> where a config was renamed or a driver moved across configs, and the
> .config used by the kernel not updated accordingly. We need to parse the
> source to find all compatibles present in the kernel independent of the
> current config being run.

I've been down this route. I had another implementation using gdb to 
extract all of_device_id objects from a built kernel, but besides the 
build time, it was really slow.

Rob

[1] https://lore.kernel.org/all/20230804190130.1936566-1-robh@xxxxxxxxxx/