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From:   Geert Uytterhoeven <geert@linux-m68k.org>
Date:   Mon, 15 Feb 2021 13:38:16 +0100
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Subject: Re: [PATCH v4 0/8] Make fw_devlink=on more forgiving
To:     Saravana Kannan <saravanak@google.com>
Cc:     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>,
        Rob Herring <robh+dt@kernel.org>,
        Frank Rowand <frowand.list@gmail.com>,
        Marc Zyngier <maz@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
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        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@vger.kernel.org>,
        ACPI Devel Maling List <linux-acpi@vger.kernel.org>,
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Hi Saravana,

On Fri, Feb 12, 2021 at 4:00 AM Saravana Kannan <saravanak@google.com> wrote:
> On Thu, Feb 11, 2021 at 5:00 AM Geert Uytterhoeven <geert@linux-m68k.org> wrote:
> >       - I2C on R-Car Gen3 does not seem to use DMA, according to
> >         /sys/kernel/debug/dmaengine/summary:
> >
> >             -dma4chan0    | e66d8000.i2c:tx
> >             -dma4chan1    | e66d8000.i2c:rx
> >             -dma5chan0    | e6510000.i2c:tx
>
> I think I need more context on the problem before I can try to fix it.
> I'm also very unfamiliar with that file. With fw_devlink=permissive,
> I2C was using DMA? If so, the next step is to see if the I2C relative
> probe order with DMA is getting changed and if so, why.

More detailed log:

    platform e66d8000.i2c: Linked as a consumer to e6150000.clock-controller
    platform e66d8000.i2c: Linked as a sync state only consumer to e6055400.gpio

Why is e66d8000.i2c not linked as a consumer to e6700000.dma-controller?

    platform e6700000.dma-controller: Linked as a consumer to
e6150000.clock-controller
    platform e66d8000.i2c: Added to deferred list
    platform e6700000.dma-controller: Added to deferred list

    bus: 'platform': driver_probe_device: matched device
e6700000.dma-controller with driver rcar-dmac
    bus: 'platform': really_probe: probing driver rcar-dmac with
device e6700000.dma-controller
    platform e6700000.dma-controller: Driver rcar-dmac requests probe deferral

    bus: 'platform': driver_probe_device: matched device e66d8000.i2c
with driver i2c-rcar
    bus: 'platform': really_probe: probing driver i2c-rcar with device
e66d8000.i2c

I2C becomes available...

    i2c-rcar e66d8000.i2c: request_channel failed for tx (-517)
    [...]

but DMA is not available yet, so the driver falls back to PIO.

    driver: 'i2c-rcar': driver_bound: bound to device 'e66d8000.i2c'
    bus: 'platform': really_probe: bound device e66d8000.i2c to driver i2c-rcar

    platform e6700000.dma-controller: Retrying from deferred list
    bus: 'platform': driver_probe_device: matched device
e6700000.dma-controller with driver rcar-dmac
    bus: 'platform': really_probe: probing driver rcar-dmac with
device e6700000.dma-controller
    platform e6700000.dma-controller: Driver rcar-dmac requests probe deferral
    platform e6700000.dma-controller: Added to deferred list
    platform e6700000.dma-controller: Retrying from deferred list
    bus: 'platform': driver_probe_device: matched device
e6700000.dma-controller with driver rcar-dmac
    bus: 'platform': really_probe: probing driver rcar-dmac with
device e6700000.dma-controller
    driver: 'rcar-dmac': driver_bound: bound to device 'e6700000.dma-controller'
    bus: 'platform': really_probe: bound device
e6700000.dma-controller to driver rcar-dmac

DMA becomes available.

Here userspace is entered. /sys/kernel/debug/dmaengine/summary shows
that the I2C controllers do not have DMA channels allocated, as the
kernel has performed no more I2C transfers after DMA became available.

Using i2cdetect shows that DMA is used, which is good:

    i2c-rcar e66d8000.i2c: got DMA channel for rx

With permissive devlinks, the clock controller consumers are not added
to the deferred probing list, and probe order is slightly different.
The I2C controllers are still probed before the DMA controllers.
But DMA becomes available a bit earlier, before the probing of the last
I2C slave driver.  Hence /sys/kernel/debug/dmaengine/summary shows that
some I2C transfers did use DMA.

So the real issue is that e66d8000.i2c not linked as a consumer to
e6700000.dma-controller.

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