This is a note to let you know that I've just added the patch titled

driver core: fw_devlink: Improve handling of cyclic dependencies

to my driver-core git tree which can be found at git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/driver-core.git in the driver-core-linus branch.

The patch will show up in the next release of the linux-next tree (usually sometime within the next 24 hours during the week.)

The patch will hopefully also be merged in Linus's tree for the next -rc kernel release.

If you have any questions about this process, please let me know.

From 2de9d8e0d2fe3a1eb632def2245529067cb35db5 Mon Sep 17 00:00:00 2001

From: Saravana Kannan saravanak@google.com Date: Wed, 15 Sep 2021 10:09:37 -0700 Subject: driver core: fw_devlink: Improve handling of cyclic dependencies

When we have a dependency of the form:

Device-A -> Device-C Device-B

Device-C -> Device-B

Where, * Indentation denotes "child of" parent in previous line. * X -> Y denotes X is consumer of Y based on firmware (Eg: DT).

We have cyclic dependency: device-A -> device-C -> device-B -> device-A

fw_devlink current treats device-C -> device-B dependency as an invalid dependency and doesn't enforce it but leaves the rest of the dependencies as is.

While the current behavior is necessary, it is not sufficient if the false dependency in this example is actually device-A -> device-C. When this is the case, device-C will correctly probe defer waiting for device-B to be added, but device-A will be incorrectly probe deferred by fw_devlink waiting on device-C to probe successfully. Due to this, none of the devices in the cycle will end up probing.

To fix this, we need to go relax all the dependencies in the cycle like we already do in the other instances where fw_devlink detects cycles. A real world example of this was reported[1] and analyzed[2].

[1] - https://lore.kernel.org/lkml/0a2c4106-7f48-2bb5-048e-8c001a7c3fda@samsung.co... [2] - https://lore.kernel.org/lkml/CAGETcx8peaew90SWiux=TyvuGgvTQOmO4BFALz7aj0Za5Q...

Fixes: f9aa460672c9 ("driver core: Refactor fw_devlink feature") Cc: stable stable@vger.kernel.org Reported-by: Marek Szyprowski m.szyprowski@samsung.com Tested-by: Marek Szyprowski m.szyprowski@samsung.com Signed-off-by: Saravana Kannan saravanak@google.com Link: https://lore.kernel.org/r/20210915170940.617415-2-saravanak@google.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- drivers/base/core.c | 17 ++++++++++++----- 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/drivers/base/core.c b/drivers/base/core.c index e65dd803a453..316df6027093 100644 --- a/drivers/base/core.c +++ b/drivers/base/core.c @@ -1772,14 +1772,21 @@ static int fw_devlink_create_devlink(struct device *con, * be broken by applying logic. Check for these types of cycles and * break them so that devices in the cycle probe properly. * - * If the supplier's parent is dependent on the consumer, then - * the consumer-supplier dependency is a false dependency. So, - * treat it as an invalid link. + * If the supplier's parent is dependent on the consumer, then the + * consumer and supplier have a cyclic dependency. Since fw_devlink + * can't tell which of the inferred dependencies are incorrect, don't + * enforce probe ordering between any of the devices in this cyclic + * dependency. Do this by relaxing all the fw_devlink device links in + * this cycle and by treating the fwnode link between the consumer and + * the supplier as an invalid dependency. */ sup_dev = fwnode_get_next_parent_dev(sup_handle); if (sup_dev && device_is_dependent(con, sup_dev)) { - dev_dbg(con, "Not linking to %pfwP - False link\n", - sup_handle); + dev_info(con, "Fixing up cyclic dependency with %pfwP (%s)\n", + sup_handle, dev_name(sup_dev)); + device_links_write_lock(); + fw_devlink_relax_cycle(con, sup_dev); + device_links_write_unlock(); ret = -EINVAL; } else { /*