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

From: Geert Uytterhoeven <geert@linux-m68k.org>
To: Saravana Kannan <saravanak@google.com>
Cc: Rob Herring <robh@kernel.org>, Jonathan Corbet <corbet@lwn.net>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	"Rafael J. Wysocki" <rafael@kernel.org>,
	Kevin Hilman <khilman@kernel.org>,
	Ulf Hansson <ulf.hansson@linaro.org>,
	Len Brown <len.brown@intel.com>, Len Brown <lenb@kernel.org>,
	Pavel Machek <pavel@ucw.cz>,
	Michael Turquette <mturquette@baylibre.com>,
	Stephen Boyd <sboyd@kernel.org>,
	Frank Rowand <frowand.list@gmail.com>,
	Marc Zyngier <maz@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Linux Doc Mailing List <linux-doc@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Linux PM <linux-pm@vger.kernel.org>,
	linux-clk <linux-clk@vger.kernel.org>,
	"open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
	<devicetree@vger.kernel.org>,
	ACPI Devel Maling List <linux-acpi@vger.kernel.org>,
	Marek Szyprowski <m.szyprowski@samsung.com>,
	Android Kernel Team <kernel-team@android.com>,
	Joerg Roedel <joro@8bytes.org>,
	Linux IOMMU <iommu@lists.linux-foundation.org>
Subject: Re: [PATCH v4 4/8] of: property: Add fw_devlink support for optional properties
Date: Wed, 10 Feb 2021 09:25:26 +0100	[thread overview]
Message-ID: <CAMuHMdXi9s_b0xjaQ3n_-qFfdwfBtxnrhYfVuRENJM5UJ9TUwg@mail.gmail.com> (raw)
In-Reply-To: <CAGETcx_gHRd9UYHvSsHX_=NFF+HEJkamJp3JcpojuJob_a8_DA@mail.gmail.com>

Hi Saravana,

CC iommu

On Tue, Feb 9, 2021 at 10:55 PM Saravana Kannan <saravanak@google.com> wrote:
> On Tue, Feb 9, 2021 at 1:33 PM Rob Herring <robh@kernel.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@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