Date:   Fri, 16 Aug 2019 11:10:56 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Frank Rowand <frowand.list@...il.com>
Cc:     Saravana Kannan <saravanak@...gle.com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
        David Collins <collinsd@...eaurora.org>,
        Android Kernel Team <kernel-team@...roid.com>
Subject: Re: [PATCH v9 0/7] Solve postboot supplier cleanup and optimize
 probe ordering

On Thu, Aug 15, 2019 at 08:09:19PM -0700, Frank Rowand wrote:
> Hi Saravana,
> 
> On 8/15/19 6:50 PM, Saravana Kannan wrote:
> > On Fri, Aug 9, 2019 at 10:20 PM Frank Rowand <frowand.list@...il.com> wrote:
> >>
> >> On 8/9/19 10:00 PM, Saravana Kannan wrote:
> >>> On Fri, Aug 9, 2019 at 7:57 PM Frank Rowand <frowand.list@...il.com> 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
> >> internet.
> > 
> > I'm replying to your comments on the other 3 patches. Okay with a
> > majority of them. I'll wait for your reply to see where we settle for
> > some of the points before I send out any patches though.
> > 
> > For now I'm thinking of sending them as separate clean up patches so
> > that Greg doesn't have to deal with reverts in his "next" branch. We
> > can squash them later if we really need to rip out what's in there and
> > push it again.
> > 
> > -Saravana
> > 
> 
> Please do not do separate clean up patches.  The series that Greg has is
> not ready for acceptance and I am going to ask him to revert it as we
> work through the needed changes.
> 
> I suspect there will be at least two more versions of the series.  The
> first is to get the patches I commented in good shape.  Then I will
> look at the patches later in the series to see how they fit into the
> big picture.
> 
> In the end, there should be one coherent patch series that implements
> the feature.

Incremental patches to fix up the comments and documentation is fine, no
need to respin the whole mess.

thanks,

greg k-h

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