linux-kernel - Re: [PATCH v4 4/8] of: property: Add fw_devlink support for optional properties

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Message-ID: <CAGETcx_gHRd9UYHvSsHX_=NFF+HEJkamJp3JcpojuJob_a8_DA@mail.gmail.com>
Date:   Tue, 9 Feb 2021 13:54:52 -0800
From:   Saravana Kannan <saravanak@...gle.com>
To:     Rob Herring <robh@...nel.org>
Cc:     Jonathan Corbet <corbet@....net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Kevin Hilman <khilman@...nel.org>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Len Brown <len.brown@...el.com>, Len Brown <lenb@...nel.org>,
        Pavel Machek <pavel@....cz>,
        Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>,
        Frank Rowand <frowand.list@...il.com>,
        Marc Zyngier <maz@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        linux-clk <linux-clk@...r.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Android Kernel Team <kernel-team@...roid.com>
Subject: Re: [PATCH v4 4/8] of: property: Add fw_devlink support for optional properties

On Tue, Feb 9, 2021 at 1:33 PM Rob Herring <robh@...nel.org> wrote:
>
> On Fri, Feb 05, 2021 at 02:26:40PM -0800, Saravana Kannan wrote:
> > Not all DT bindings are mandatory bindings. Add support for optional DT
> > bindings and mark iommus, iommu-map, dmas as optional DT bindings.
>
> I don't think we can say these are optional or not. It's got to be a
> driver decision somehow.

Right, so maybe the word "optional" isn't a good name for it. I can
change that if you want.

The point being, fw_devlink can't block the probe of this driver based
on iommu property. We let the driver decide if it wants to
-EPROBE_DEFER or not or however it wants to handle this.

> For example, if IOMMU is optional, what happens with this sequence:
>
> driver probes without IOMMU
> driver calls dma_map_?()
> IOMMU driver probes
> h/w accesses DMA buffer --> BOOM!

Right. But how is this really related to fw_devlink? AFAICT, this is
an issue even today. If the driver needs the IOMMU, then it needs to
make sure the IOMMU has probed? What am I missing?

-Saravana