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

Greg Kroah-Hartman
2019-08-27T19:43:10Z

On Wed, Aug 07, 2019 at 07:13:26PM -0700, Frank Rowand wrote:
> Hi Greg, Saravana,
>
> On 8/1/19 11:37 PM, Greg Kroah-Hartman wrote:
> > On Thu, Aug 01, 2019 at 12:59:25PM -0700, Frank Rowand wrote:
> >> On 8/1/19 12:32 PM, Greg Kroah-Hartman wrote:
> >>> On Thu, Aug 01, 2019 at 12:28:13PM -0700, Frank Rowand wrote:
> >>>> Hi Greg,
> >>>>
> >>>> On 7/31/19 11:12 PM, Greg Kroah-Hartman wrote:
> >>>>> On Wed, Jul 31, 2019 at 03:17:13PM -0700, 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.
> >>>>>
> >>>>> All now queued up in my driver-core-testing branch, and if 0-day is
> >>>>> happy with this, will move it to my "real" driver-core-next branch in a
> >>>>> day or so to get included in linux-next.
> >>>>
> >>>> I have been slow in getting my review out.
> >>>>
> >>>> This patch series is not yet ready for sending to Linus, so if putting
> >>>> this in linux-next implies that it will be in your next pull request
> >>>> to Linus, please do not put it in linux-next.
> >>>
> >>> It means that it will be in my pull request for 5.4-rc1, many many
> >>> waeeks away from now.
> >>
> >> If you are willing to revert the series before the pull request _if_ I
> >> have significant review issues in the next couple of days, then I am happy
> >> to see the patches get exposure in linux-next.
> >
> > If you have significant review issues, yes, I will be glad to revert them.
>
> Just a heads up that I have sent review issues in reply to version 7 of this
> patch series.
>
> We'll see what the responses are to my review comments, but I am expecting
> the changes are big enough to result in a new version (or a couple more
> versions) of the patch series.
>
> No rush to revert version 9 since your 5.4-rc1 pull request is still not
> near, and I am glad for whatever exposure these patches are getting in
> linux-next.

Based on the further comments on this series, and the in-person we had
at ELC, I have now reverted these, and the follow-on fixes for this
series from my tree, with the hope that an updated patch set will be
sent for review soon.

thanks,

greg k-h