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From:   Geert Uytterhoeven <geert@linux-m68k.org>
Date:   Tue, 16 Feb 2021 09:05:28 +0100
Message-ID: <CAMuHMdVodauqBmLMxsfi0kQtAFT8ruJ36LJL9YuQgqwQNKwHHg@mail.gmail.com>
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>,
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Hi Saravana,

On Mon, Feb 15, 2021 at 10:27 PM Saravana Kannan <saravanak@google.com> wrote:
> On Mon, Feb 15, 2021 at 4:38 AM Geert Uytterhoeven <geert@linux-m68k.org> wrote:
> > 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.

Oh, I thought dma/iommu were considered mandatory initially,
but dropped as dependencies in the late boot process?

>
> >     platform e6700000.dma-controller: Linked as a consumer to
> > e6150000.clock-controller
>
> Is this the only supplier of dma-controller?

No, e6180000.system-controller is also a supplier.

> >     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?

I don't think there is a race.  fw_devlinks calling driver_deferred_probe_add()
on all consumers has a big impact on probe order.

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