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

Greg Kroah-Hartman
2019-08-01T19:32:53Z

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.

thanks,

greg k-h