[PATCH v1 1/2] driver core: fw_devlink: Add support for FWNODE_FLAG_BROKEN_PARENT

From: Saravana Kannan
Date: Thu Aug 26 2021 - 03:45:42 EST

If a parent device is also a supplier to a child device, fw_devlink=on
(correctly) delays the probe() of the child device until the probe() of
the parent finishes successfully.

However, some drivers of such parent devices (where parent is also a
supplier) incorrectly expect the child device to finish probing
successfully as soon as they are added using device_add() and before the
probe() of the parent device has completed successfully. While this
might have worked before, this is not guaranteed by driver core.
fw_devlink=on catches/breaks such drivers. One example of such a case is
discussed in the link mentioned below.

Add a flag to make fw_devlink=on not enforce these supplier-consumer
relationships, so these drivers can continue working. The flag is
intentionally called BROKEN_PARENT so it's clear that this flag
shouldn't be used in the normal case and that there's a problem with the

Link: https://lore.kernel.org/netdev/CAGETcx_uj0V4DChME-gy5HGKTYnxLBX=TH2rag29f_p=UcG+Tg@xxxxxxxxxxxxxx/
Signed-off-by: Saravana Kannan <saravanak@xxxxxxxxxx>
drivers/base/core.c | 22 ++++++++++++++++++++++
include/linux/fwnode.h | 3 +++
2 files changed, 25 insertions(+)

diff --git a/drivers/base/core.c b/drivers/base/core.c
index f6360490a4a3..2cc34f8ff051 100644
--- a/drivers/base/core.c
+++ b/drivers/base/core.c
@@ -1719,6 +1719,28 @@ static int fw_devlink_create_devlink(struct device *con,
struct device *sup_dev;
int ret = 0;

+ /*
+ * In some cases, a device P might also be a supplier to its child node
+ * C. However, this would defer the probe of C until the probe of P
+ * completes successfully. This is perfectly fine in the device driver
+ * model. device_add() doesn't guarantee probe completion of the device
+ * by the time it returns.
+ *
+ * However, there are a few drivers that assume C will finish probing
+ * as soon as it's added and before P finishes probing. While this is a
+ * broken assumption that needs the driver to be fixed, we don't want
+ * to block fw_devlink improvements because of these drivers.
+ *
+ * So, we provide a flag to let fw_devlink know not to delay the probe
+ * of C until the probe of P completes successfully.
+ *
+ * When such a flag is set, we can't create device links with P as the
+ * supplier of C as that would delay the probe of C.
+ */
+ if (sup_handle->flags & FWNODE_FLAG_BROKEN_PARENT &&
+ fwnode_is_ancestor_of(sup_handle, con->fwnode))
+ return -EINVAL;
sup_dev = get_dev_from_fwnode(sup_handle);
if (sup_dev) {
diff --git a/include/linux/fwnode.h b/include/linux/fwnode.h
index 59828516ebaf..9382065e6ff8 100644
--- a/include/linux/fwnode.h
+++ b/include/linux/fwnode.h
@@ -22,10 +22,13 @@ struct device;
* LINKS_ADDED: The fwnode has already be parsed to add fwnode links.
* NOT_DEVICE: The fwnode will never be populated as a struct device.
* INITIALIZED: The hardware corresponding to fwnode has been initialized.
+ * BROKEN_PARENT: The driver of this fwnode/device expects the child devices to
+ * probe as soon as they are added.

struct fwnode_handle {
struct fwnode_handle *secondary;