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

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Message-ID: <CAMuHMdXi9s_b0xjaQ3n_-qFfdwfBtxnrhYfVuRENJM5UJ9TUwg@mail.gmail.com>
Date:   Wed, 10 Feb 2021 09:25:26 +0100
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Saravana Kannan <saravanak@...gle.com>
Cc:     Rob Herring <robh@...nel.org>, 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>,
        Android Kernel Team <kernel-team@...roid.com>,
        Joerg Roedel <joro@...tes.org>,
        Linux IOMMU <iommu@...ts.linux-foundation.org>
Subject: Re: [PATCH v4 4/8] of: property: Add fw_devlink support for optional properties

Hi Saravana,

CC iommu

On Tue, Feb 9, 2021 at 10:55 PM Saravana Kannan <saravanak@...gle.com> wrote:
> 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.

The driver cannot make that decision, cfr. below.

> > 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!

Does it really behave that way? Or does it continue without IOMMU?

> 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?

Individual I/O (IOMMU slave) drivers are completely unaware of the
presence or absence of an IOMMU; they just use the DMA API, which is the
same regardless of an IOMMU being used or not.
While for GPIO/IRQ/CLK/DMA/... have request/get_{gpio,irq,clk,dma,...}
APIs for a driver to get a reference, which can return -EPROBE_DEFER, no
such thing exists for IOMMUs.  This is handled by the IOMMU core
instead.

Using the IOMMU or not is more like a system policy decision.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-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