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From: Geert Uytterhoeven <geert@linux-m68k.org>
To: Saravana Kannan <saravanak@google.com>
Cc: Jonathan Corbet <corbet@lwn.net>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
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Subject: Re: [PATCH v4 0/8] Make fw_devlink=on more forgiving
Date: Thu, 25 Feb 2021 10:21:56 +0100
Message-ID: <CAMuHMdUVVr8jES51_8_yPoicr-nwad_2nKLYUKweY8mbxx9GJw@mail.gmail.com> (raw)
In-Reply-To: <CAGETcx8jXkbtdgMCr6KGT4ScoaoP=AwaW6MQeEv-gsDySiY35A@mail.gmail.com>

Hi Saravana,

On Thu, Feb 18, 2021 at 12:57 AM Saravana Kannan <saravanak@google.com> wrote:
> On Tue, Feb 16, 2021 at 12:31 PM Geert Uytterhoeven
> <geert@linux-m68k.org> wrote:
> > On Tue, Feb 16, 2021 at 7:49 PM Saravana Kannan <saravanak@google.com> wrote:
> > > On Tue, Feb 16, 2021 at 12:05 AM Geert Uytterhoeven
> > > <geert@linux-m68k.org> wrote:
> > > > 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?
> > >
> > > No, I didn't do that in case the drivers that didn't need the
> > > IOMMU/DMA were sensitive to probe order.
> > >
> > > My goal was for fw_devlink=on to not affect probe order for devices
> > > that currently don't need to defer probe. But see below...
> > >
> > > >
> > > > >
> > > > > >     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.
> > >
> > > Can you explain more please? This below makes it sound like DMA just
> > > sneaks in at the last minute.
> >
> > Yes it does, as the DMAC also has a consumer link to the IOMMU.
> > If you ignore the consumer link from I2C to DMAC, the I2C device has
> > less dependencies than the DMAC, so the I2C device, and the
> > devices on the I2C bus, are probed much earlier than the DMAC.
>
> Can you give this a shot?
> https://lore.kernel.org/lkml/20210217235130.1744843-1-saravanak@google.com/T/#u
>
> It should make sure fw_devlink doesn't add a device to the deferred
> probe list too soon and change the probe ordering unnecessarily.

(FTR, to keep all info in this thread)
Yes, this makes I2C use DMA again on Salvator-XS during kernel boot-up.
I haven't run any more elaborate tests on other platforms.

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

  reply index

Thread overview: 59+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <CGME20210205222651eucas1p28ef87073dea33c1c5224c14aa203bec5@eucas1p2.samsung.com>
2021-02-05 22:26 ` Saravana Kannan
2021-02-05 22:26   ` [PATCH v4 1/8] driver core: fw_devlink: Detect supplier devices that will never be added Saravana Kannan
2021-02-05 22:26   ` [PATCH v4 2/8] of: property: Don't add links to absent suppliers Saravana Kannan
2021-02-09 21:25     ` Rob Herring
2021-02-05 22:26   ` [PATCH v4 3/8] driver core: Add fw_devlink.strict kernel param Saravana Kannan
2021-02-05 22:26   ` [PATCH v4 4/8] of: property: Add fw_devlink support for optional properties Saravana Kannan
2021-02-09 21:33     ` Rob Herring
2021-02-09 21:54       ` Saravana Kannan
2021-02-10  8:25         ` Geert Uytterhoeven
2021-02-05 22:26   ` [PATCH v4 5/8] driver core: fw_devlink: Handle suppliers that don't use driver core Saravana Kannan
2021-02-05 22:26   ` [PATCH v4 6/8] irqdomain: Mark fwnodes when their irqdomain is added/removed Saravana Kannan
2021-02-05 22:26   ` [PATCH v4 7/8] PM: domains: Mark fwnodes when their powerdomain " Saravana Kannan
2021-02-05 22:26   ` [PATCH v4 8/8] clk: Mark fwnodes when their clock provider " Saravana Kannan
2021-02-08 15:38     ` Rob Herring
2021-02-08 23:34       ` Saravana Kannan
2021-02-10 11:44     ` [PATCH] clk: Mark fwnodes when their clock provider is added Tudor Ambarus
2021-02-10 11:44       ` Tudor Ambarus
2021-02-11 13:00         ` Greg KH
2021-02-13  0:37           ` Stephen Boyd
     [not found]         ` <CGME20210325133159eucas1p297b769beb681743fb32d362a86cc6e3e@eucas1p2.samsung.com>
2021-03-25 13:31           ` Marek Szyprowski
2021-03-25 15:47             ` Geert Uytterhoeven
2021-03-25 18:25             ` Nicolas Saenz Julienne
2021-03-26 18:13               ` Stephen Boyd
2021-03-26 18:29                 ` Geert Uytterhoeven
     [not found]                   ` <161705310317.3012082.15148238105608149214@swboyd.mtv.corp.google.com>
2021-03-29 23:28                     ` Saravana Kannan
     [not found]                       ` <161706920822.3012082.10047587064612237296@swboyd.mtv.corp.google.com>
2021-03-30  6:58                         ` Geert Uytterhoeven
     [not found]                           ` <161715734080.2260335.881350237641202575@swboyd.mtv.corp.google.com>
2021-03-31  7:05                             ` Geert Uytterhoeven
     [not found]                               ` <161721871083.2260335.2392646934517115770@swboyd.mtv.corp.google.com>
2021-04-05 11:04                                 ` Nicolas Saenz Julienne
2021-04-21  3:26         ` Guenter Roeck
2021-04-21  7:00           ` Saravana Kannan
2021-02-10 19:13       ` Saravana Kannan
2021-03-30 15:42       ` Guenter Roeck
2021-03-30 16:26         ` Saravana Kannan
     [not found]     ` <161317679292.1254594.15797939257637374295@swboyd.mtv.corp.google.com>
2021-02-14 21:15       ` [PATCH v4 8/8] clk: Mark fwnodes when their clock provider is added/removed Saravana Kannan
2021-02-06  2:45   ` [PATCH v4 0/8] Make fw_devlink=on more forgiving Saravana Kannan
2021-02-06 19:41   ` Geert Uytterhoeven
2021-02-06 20:47     ` Saravana Kannan
2021-02-08  8:40   ` Marek Szyprowski
2021-02-08 23:57     ` Saravana Kannan
2021-02-10  8:19   ` Tudor.Ambarus
2021-02-10  8:54     ` Saravana Kannan
2021-02-10 10:02       ` Tudor.Ambarus
2021-02-10 19:14         ` Saravana Kannan
2021-02-11 13:00   ` Geert Uytterhoeven
2021-02-11 13:05     ` Geert Uytterhoeven
2021-02-12  2:59     ` Saravana Kannan
2021-02-12  8:14       ` Geert Uytterhoeven
2021-02-12 20:57         ` Saravana Kannan
2021-02-15 12:38       ` Geert Uytterhoeven
2021-02-15 21:26         ` Saravana Kannan
2021-02-16  8:05           ` Geert Uytterhoeven
2021-02-16 18:48             ` Saravana Kannan
2021-02-16 20:31               ` Geert Uytterhoeven
2021-02-17 23:57                 ` Saravana Kannan
2021-02-25  9:21                   ` Geert Uytterhoeven [this message]
2021-02-15 15:16       ` Geert Uytterhoeven
2021-02-15 21:57         ` Saravana Kannan
2021-02-16 12:58           ` Geert Uytterhoeven
2021-02-15 11:19     ` Geert Uytterhoeven

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