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From:   Saravana Kannan <saravanak@google.com>
Date:   Mon, 15 Feb 2021 13:26:30 -0800
Message-ID: <CAGETcx_4FGa-rzLp6bjXbm4F4R6H2W78+nM_kN=XPz5hswzANA@mail.gmail.com>
Subject: Re: [PATCH v4 0/8] Make fw_devlink=on more forgiving
To:     Geert Uytterhoeven <geert@linux-m68k.org>
Cc:     Jonathan Corbet <corbet@lwn.net>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
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On Mon, Feb 15, 2021 at 4:38 AM Geert Uytterhoeven <geert@linux-m68k.org> wrote:
>
> 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?

Because fw_devlink.strict=1 is not set and dma/iommu is considered an
"optional"/"driver decides" dependency.

>     platform e6700000.dma-controller: Linked as a consumer to
> e6150000.clock-controller

Is this the only supplier of dma-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.

This seems like a race? I'm guessing it's two different threads
probing those two devices? And it just happens to work for
"permissive" assuming the boot timing doesn't change?

> 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.

That's because fw_devlink.strict=1 isn't set. If you need DMA to be
treated as a mandatory supplier, you'll need to set the flag.

Is fw_devlink=on really breaking anything here? It just seems like
"permissive" got lucky with the timing and it could break at any point
in the future. Thought?

-Saravana