On Thu, May 24, 2018 at 2:01 PM, Greg Kroah-Hartman
[off-list ref] wrote:

On Thu, May 24, 2018 at 12:50:18PM -0500, Rob Herring wrote:

quoted

Deferring probe can wait forever on dependencies that may never appear
for a variety of reasons. This can be difficult to debug especially if
the console has dependencies or userspace fails to boot to a shell. Add
a timeout to retry probing without possibly optional dependencies and to
dump out the deferred probe pending list after retrying.

This mechanism is intended for debug purposes. It won't work for the
console which needs to be enabled before userspace starts. However, if
the console's dependencies are resolved, then the kernel log will be
printed (as opposed to no output).

Signed-off-by: Rob Herring <robh@kernel.org>
---
 .../admin-guide/kernel-parameters.txt         |  7 +++++
 drivers/base/dd.c                             | 28 ++++++++++++++++++-
 2 files changed, 34 insertions(+), 1 deletion(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 11fc28ecdb6d..dd3f40b34a24 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt

@@ -809,6 +809,13 @@
                      Defaults to the default architecture's huge page size
                      if not specified.

+     deferred_probe_timeout=
+                     [KNL] Set a timeout in seconds for deferred probe to
+                     give up waiting on dependencies to probe. Only specific
+                     dependencies (subsystems or drivers) that have opted in
+                     will be ignored. This option also dumps out devices
+                     still on the deferred probe list after retrying.

Doesn't sound like a debugging-only option.  I can see devices enabling
this when they figure out that's the only way their platform can boot :)

Here's some rope...

No doubt it can be abused. So are you saying don't call it a debug
option or hide it behind a config option? And for the latter, what's
one that distros don't just turn on?

Rob