Re: [PATCH v9 0/7] Solve postboot supplier cleanup and optimize probe ordering

From: Frank Rowand
Date: Sat Aug 10 2019 - 01:20:59 EST

On 8/9/19 10:00 PM, Saravana Kannan wrote:
> On Fri, Aug 9, 2019 at 7:57 PM Frank Rowand <frowand.list@xxxxxxxxx> wrote:
>> Hi Saravana,
>> On 7/31/19 3:17 PM, Saravana Kannan wrote:
>>> Add device-links to track functional dependencies between devices
>>> after they are created (but before they are probed) by looking at
>>> their common DT bindings like clocks, interconnects, etc.
>>> Having functional dependencies automatically added before the devices
>>> are probed, provides the following benefits:
>>> - Optimizes device probe order and avoids the useless work of
>>> attempting probes of devices that will not probe successfully
>>> (because their suppliers aren't present or haven't probed yet).
>>> For example, in a commonly available mobile SoC, registering just
>>> one consumer device's driver at an initcall level earlier than the
>>> supplier device's driver causes 11 failed probe attempts before the
>>> consumer device probes successfully. This was with a kernel with all
>>> the drivers statically compiled in. This problem gets a lot worse if
>>> all the drivers are loaded as modules without direct symbol
>>> dependencies.
>>> - Supplier devices like clock providers, interconnect providers, etc
>>> need to keep the resources they provide active and at a particular
>>> state(s) during boot up even if their current set of consumers don't
>>> request the resource to be active. This is because the rest of the
>>> consumers might not have probed yet and turning off the resource
>>> before all the consumers have probed could lead to a hang or
>>> undesired user experience.
>>> Some frameworks (Eg: regulator) handle this today by turning off
>>> "unused" resources at late_initcall_sync and hoping all the devices
>>> have probed by then. This is not a valid assumption for systems with
>>> loadable modules. Other frameworks (Eg: clock) just don't handle
>>> this due to the lack of a clear signal for when they can turn off
>>> resources. This leads to downstream hacks to handle cases like this
>>> that can easily be solved in the upstream kernel.
>>> By linking devices before they are probed, we give suppliers a clear
>>> count of the number of dependent consumers. Once all of the
>>> consumers are active, the suppliers can turn off the unused
>>> resources without making assumptions about the number of consumers.
>>> By default we just add device-links to track "driver presence" (probe
>>> succeeded) of the supplier device. If any other functionality provided
>>> by device-links are needed, it is left to the consumer/supplier
>>> devices to change the link when they probe.
>>> v1 -> v2:
>>> - Drop patch to speed up of_find_device_by_node()
>>> - Drop depends-on property and use existing bindings
>>> v2 -> v3:
>>> - Refactor the code to have driver core initiate the linking of devs
>>> - Have driver core link consumers to supplier before it's probed
>>> - Add support for drivers to edit the device links before probing
>>> v3 -> v4:
>>> - Tested edit_links() on system with cyclic dependency. Works.
>>> - Added some checks to make sure device link isn't attempted from
>>> parent device node to child device node.
>>> - Added way to pause/resume sync_state callbacks across
>>> of_platform_populate().
>>> - Recursively parse DT node to create device links from parent to
>>> suppliers of parent and all child nodes.
>>> v4 -> v5:
>>> - Fixed copy-pasta bugs with linked list handling
>>> - Walk up the phandle reference till I find an actual device (needed
>>> for regulators to work)
>>> - Added support for linking devices from regulator DT bindings
>>> - Tested the whole series again to make sure cyclic dependencies are
>>> broken with edit_links() and regulator links are created properly.
>>> v5 -> v6:
>>> - Split, squashed and reordered some of the patches.
>>> - Refactored the device linking code to follow the same code pattern for
>>> any property.
>>> v6 -> v7:
>>> - No functional changes.
>>> - Renamed i to index
>>> - Added comment to clarify not having to check property name for every
>>> index
>>> - Added "matched" variable to clarify code. No functional change.
>>> - Added comments to include/linux/device.h for add_links()
>>> v7 -> v8:
>>> - Rebased on top of linux-next to handle device link changes in [1]
>>> v8 -> v9:
>>> - Fixed kbuild test bot reported errors (docs and const)
>> Some maintainers have strong opinions about whether change logs should be:
>> (1) only in patch 0
>> (2) only in the specific patches that are changed
>> (3) both in patch 0 and in the specific patches that are changed.
>> I can adapt to any of the three styles. But for style "(1)" please
>> list which specific patch has changed for each item in the change list.
> Thanks for the context Frank. I'm okay with (1) or (2) but I'll stick
> with (1) for this series. Didn't realize there were options (2) and
> (3). Since you started reviewing from v7, I'll do that in the future
> updates? Also, I haven't forgotten your emails. Just tied up with
> something else for a few days. I'll get to your emails next week.

Yes, starting with future updates is fine, no need to redo the v9
change logs.

No problem on the timing. I figured you were busy or away from the


> Thanks,
> Saravana