linux-kernel - Re: [PATCH 0/4] of: property: fw

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Date:   Thu, 16 Apr 2020 13:56:29 -0700
From:   Saravana Kannan <saravanak@...gle.com>
To:     Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        Frank Rowand <frowand.list@...il.com>,
        Rob Herring <robh+dt@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/4] of: property: fw_devlink misc fixes

On Thu, Apr 16, 2020 at 4:03 AM Nicolas Saenz Julienne
<nsaenzjulienne@...e.de> wrote:
>
> On Wed, 2020-04-15 at 11:17 -0700, Saravana Kannan wrote:
> > On Wed, Apr 15, 2020 at 8:06 AM Nicolas Saenz Julienne
> > <nsaenzjulienne@...e.de> wrote:
> > > As I'm interested in using this feature to fine-tune Raspberry Pi 4's
> >
> > You've made my day! Finally another user outside of Android. :) If
> > this does improve the boot time, I'd be super interested to see the
> > numbers.
>
> Actually making the boot time faster isn't my main objective just a nice
> possible side-effect. I'll give you some numbers nonetheless :).

Thanks!

> I have two things in mind:
>  - Exploring if fw_devlink=on can help us solve a rather convoluted device
>    initialization depency we're seeing in RPi4. It could potentially prevent us
>    from adding nasty platform specific driver code.

I hope it does! I've also noticed that fw_devlink avoids the need for
ugly hacks in drivers and side steps poorly written error handling in
drivers.

>  - See if we can use all this information to fine-tune initrd generation on
>    smaller arm devices with limited i/o speeds.

That's pretty cool. I have no idea how fw_devlink helps here, but I'm
glad it does :)

> Do you have any plans in moving the default behavior to fw_devlink=on? If so
> what is blocking us?

That's my eventual goal. The main reasons it hasn't been done yet are:
1. Cases like yours where there might be fake cycles.
2. Cases of DT with bad choice of properties. Say, something like
"nr-gpios" would cause error spew in the logs (not a functional
error).
3. Whatever other unknown reasons this might cause boot up issues.

I'm starting with trying to turn on fw_devlink=permissive so that
driver developers can stop playing chicken with initcall levels. Then
work towards setting fw_devlink=on (going to be a long road).

> Also do you think it'd be reasonable to add a DT binding to set the desired
> fw_devlink level? Something like a 'linux,fw_devlink' property under the
> /chosen node.

I don't mind that, but not sure if DT maintainers are okay with it.
But if we do have that, I'd still want the kernel command line to
override it.

Thanks,
Saravana