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From:   Geert Uytterhoeven <geert@linux-m68k.org>
Date:   Tue, 27 Apr 2021 19:10:41 +0200
Message-ID: <CAMuHMdWHRb0+BS=s0OU_JBMtEuc4=tFkrZhb=3Wjm2AUxL-ryQ@mail.gmail.com>
Subject: Re: [PATCH v1 3/3] Revert "Revert "driver core: Set fw_devlink=on by default""
To:     Florian Fainelli <f.fainelli@gmail.com>
Cc:     Sudeep Holla <sudeep.holla@arm.com>,
        Cristian Marussi <cristian.marussi@arm.com>,
        Jim Quinlan <james.quinlan@broadcom.com>,
        Saravana Kannan <saravanak@google.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        Al Cooper <alcooperx@gmail.com>,
        Michael Walle <michael@walle.cc>,
        Jon Hunter <jonathanh@nvidia.com>,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        Guenter Roeck <linux@roeck-us.net>,
        Android Kernel Team <kernel-team@android.com>,
        LKML <linux-kernel@vger.kernel.org>
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Hi Florian,

On Tue, Apr 27, 2021 at 6:50 PM Florian Fainelli <f.fainelli@gmail.com> wrote:
> On 4/27/2021 9:39 AM, Sudeep Holla wrote:
> > On Tue, Apr 27, 2021 at 09:24:55AM -0700, Florian Fainelli wrote:
> >
> > [...]
> >
> >> This is a self inflicted problem that we have in that the bootloader
> >> provides a Device Tree to the kernel which is massaged in different ways
> >> and intends to stay backwards compatible as much as possible. And indeed
> >> after removing the 'mboxes' property gets us going with fw_devlink=on.
> >>
> >
> > I assume the bootloader checks the presence of SMC support and modifies
> > the DT node accordingly. Can't it remove the mbox properties as it make
> > no sense with SMC compatible ? However ...
>
> The bootloader has always assumed the SMC support was there from the day
> we introduced it because it was. What changed is the way we advertised
> to Linux that support. We used to have a custom mailbox driver that
> would be pretty much what the ARM SMC transport eventually came to be.
>
> Since we still support earlier kernels that were deployed with the old
> mailbox we cannot arbitrarily break that setup, especially as our
> customers tend to be slow in picking up new kernel versions, fortunately
> before they get to 5.13 we can mandate a new bootloader that may not be
> compatible with their 4.1 kernel anymore, or at least not without some
> backporting of the ARM SMC transport, that's all fair IMHO.
>
> >>> 2. IIUC, the fw_devlink might use information from DT to establish the
> >>>    dependency and having mailbox information in this context may be
> >>>    considered wrong as there is no dependency if it is using SMC.
> >>
> >> Right, unfortunately, short of having some special casing for SCMI and
> >> checking that if we have both an "arm,smc-id" and "mboxes" phandle we
> >> should prefer the former, there is not probably much that can be done
> >> here. Do we want to do that?
> >
> > I *think* we could do that in the SCMI drivers, but:
> > 1. I am not sure if that helps fw_devlinks if they are deriving the info
> >    purely based on DT
> > 2. I am also afraid that someone might come up with exactly opposite
> >    requirement that let us prefer mailbox over SMC as they would use
> >    SMC only if h/w lacks proper mailbox support. I fear that we will get
> >    into rabbit hole trying to do something like that.
>
> That is true, and to get to the SCMI driver, even the base protocol you
> must have been probed, so we have a nice chicken and egg problem. I
> highly appreciate your time understanding the context and trying to find
> a solution it is pretty clear that we must fix our FDT now.

Alternatively, you can have a quirk in the kernel that removes the
phandle from the FDT during early boot.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds