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Date:   Thu, 3 Dec 2020 11:05:26 -0800
From:   Saravana Kannan <saravanak@...gle.com>
To:     Tomi Valkeinen <tomi.valkeinen@...com>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Len Brown <lenb@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Frank Rowand <frowand.list@...il.com>,
        Marc Zyngier <maz@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Grygorii Strashko <grygorii.strashko@...com>,
        Android Kernel Team <kernel-team@...roid.com>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-efi <linux-efi@...r.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>
Subject: Re: [PATCH v2 00/17] Refactor fw_devlink to significantly improve
 boot time

On Tue, Nov 24, 2020 at 12:29 AM 'Tomi Valkeinen' via kernel-team
<kernel-team@...roid.com> wrote:
>
> Hi,
>
> On 21/11/2020 04:02, Saravana Kannan wrote:
> > The current implementation of fw_devlink is very inefficient because it
> > tries to get away without creating fwnode links in the name of saving
> > memory usage. Past attempts to optimize runtime at the cost of memory
> > usage were blocked with request for data showing that the optimization
> > made significant improvement for real world scenarios.
> >
> > We have those scenarios now. There have been several reports of boot
> > time increase in the order of seconds in this thread [1]. Several OEMs
> > and SoC manufacturers have also privately reported significant
> > (350-400ms) increase in boot time due to all the parsing done by
> > fw_devlink.
> >
> > So this patch series refactors fw_devlink to be more efficient. The key
> > difference now is the addition of support for fwnode links -- just a few
> > simple APIs. This also allows most of the code to be moved out of
> > firmware specific (DT mostly) code into driver core.
> >
> > This brings the following benefits:
> > - Instead of parsing the device tree multiple times (complexity was
> >   close to O(N^3) where N in the number of properties) during bootup,
> >   fw_devlink parses each fwnode node/property only once and creates
> >   fwnode links. The rest of the fw_devlink code then just looks at these
> >   fwnode links to do rest of the work.
> >
> > - Makes it much easier to debug probe issue due to fw_devlink in the
> >   future. fw_devlink=on blocks the probing of devices if they depend on
> >   a device that hasn't been added yet. With this refactor, it'll be very
> >   easy to tell what that device is because we now have a reference to
> >   the fwnode of the device.
> >
> > - Much easier to add fw_devlink support to ACPI and other firmware
> >   types. A refactor to move the common bits from DT specific code to
> >   driver core was in my TODO list as a prerequisite to adding ACPI
> >   support to fw_devlink. This series gets that done.
> >
> > Laurent and Grygorii tested the v1 series and they saw boot time
> > improvment of about 12 seconds and 3 seconds, respectively.
>
> Tested v2 on OMAP4 SDP. With my particular config, boot time to starting init went from 18.5 seconds
> to 12.5 seconds.
>
>  Tomi

Rafael,

Friendly reminder for a review.

-Saravana

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