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From:   Saravana Kannan <saravanak@google.com>
Date:   Mon, 8 Mar 2021 11:15:02 -0800
Message-ID: <CAGETcx-bmeXK4Ws2=Zeth2yiNJ0hXHZEs4GxqL7jY3+71hhF=g@mail.gmail.com>
Subject: Re: [PATCH v3] amba: Remove deferred device addition
To:     Marek Szyprowski <m.szyprowski@samsung.com>
Cc:     Russell King <linux@armlinux.org.uk>,
        Philipp Zabel <p.zabel@pengutronix.de>,
        Rob Herring <robh@kernel.org>,
        Ulf Hansson <ulf.hansson@linaro.org>,
        John Stultz <john.stultz@linaro.org>,
        Linus Walleij <linus.walleij@linaro.org>,
        Sudeep Holla <sudeep.holla@arm.com>,
        Nicolas Saenz Julienne <nsaenzjulienne@suse.de>,
        Geert Uytterhoeven <geert+renesas@glider.be>,
        Android Kernel Team <kernel-team@android.com>,
        LKML <linux-kernel@vger.kernel.org>
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On Sun, Mar 7, 2021 at 11:28 PM Marek Szyprowski
<m.szyprowski@samsung.com> wrote:
>
> Hi Saravana,
>
> On 05.03.2021 19:02, Saravana Kannan wrote:
> > On Fri, Mar 5, 2021 at 3:45 AM Marek Szyprowski
> > <m.szyprowski@samsung.com> wrote:
> >> On 04.03.2021 20:51, Saravana Kannan wrote:
> >>> The uevents generated for an amba device need PID and CID information
> >>> that's available only when the amba device is powered on, clocked and
> >>> out of reset. So, if those resources aren't available, the information
> >>> can't be read to generate the uevents. To workaround this requirement,
> >>> if the resources weren't available, the device addition was deferred and
> >>> retried periodically.
> >>>
> >>> However, this deferred addition retry isn't based on resources becoming
> >>> available. Instead, it's retried every 5 seconds and causes arbitrary
> >>> probe delays for amba devices and their consumers.
> >>>
> >>> Also, maintaining a separate deferred-probe like mechanism is
> >>> maintenance headache.
> >>>
> >>> With this commit, instead of deferring the device addition, we simply
> >>> defer the generation of uevents for the device and probing of the device
> >>> (because drivers needs PID and CID to match) until the PID and CID
> >>> information can be read. This allows us to delete all the amba specific
> >>> deferring code and also avoid the arbitrary probing delays.
> >>>
> >>> Cc: Rob Herring <robh@kernel.org>
> >>> Cc: Ulf Hansson <ulf.hansson@linaro.org>
> >>> Cc: John Stultz <john.stultz@linaro.org>
> >>> Cc: Saravana Kannan <saravanak@google.com>
> >>> Cc: Linus Walleij <linus.walleij@linaro.org>
> >>> Cc: Sudeep Holla <sudeep.holla@arm.com>
> >>> Cc: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
> >>> Cc: Geert Uytterhoeven <geert+renesas@glider.be>
> >>> Cc: Marek Szyprowski <m.szyprowski@samsung.com>
> >>> Cc: Russell King <linux@armlinux.org.uk>
> >>> Signed-off-by: Saravana Kannan <saravanak@google.com>
> >>> ---
> >>>
> >>> v1 -> v2:
> >>> - Dropped RFC tag
> >>> - Complete rewrite to not use stub devices.
> >>> v2 -> v3:
> >>> - Flipped the if() condition for hard-coded periphids.
> >>> - Added a stub driver to handle the case where all amba drivers are
> >>>     modules loaded by uevents.
> >>> - Cc Marek after I realized I forgot to add him.
> >>>
> >>> Marek,
> >>>
> >>> Would you mind testing this? It looks okay with my limited testing.
> >> It looks it works fine on my test systems. I've checked current
> >> linux-next and this patch. You can add:
> >>
> >> Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
> > Hi Marek,
> >
> > Thanks! Does your test set up have amda drivers that are loaded based
> > on uevents? That's the one I couldn't test.
>
> I've checked both, the built-in and all amba drivers compiled as
> modules, loaded by udev. Both works fine here.
>
> >> I've briefly scanned the code and I'm curious how does it work. Does it
> >> depend on the recently introduced "fw_devlink=on" feature? I don't see
> >> other mechanism, which would trigger matching amba device if pm domains,
> >> clocks or resets were not available on time to read pid/cid while adding
> >> a device...
> > No, it does not depend on fw_devlink or device links in any way.
> >
> > When a device is attempted to be probed (when it's added or during
> > deferred probe), it's matched with all the drivers on the bus.
> > When a new driver is registered to a bus, all devices in that bus are
> > matched with the driver to see if they'll work together.
> > That's how match is called. And match() can return -EPROBE_DEFER and
> > that'll cause the device to be put in the deferred probe list by
> > driver core.
> >
> > The tricky part in this patch was the uevent handling and the
> > chicken-and-egg issue I talk about in the comments.
>
> Thanks for the explanation. This EPROBE_DEFER support in match()
> callback must be something added after I crafted that periodic retry
> based workaround.
>

I think it got in just a few months before your patches, but your
patches worked :) I actually don't like match returning -EPROBE_DEFER,
but I can work around it for some of my fw_devlink optimization plans.

More context here:
https://lore.kernel.org/lkml/CAGETcx_qO4vxTSyBtBR2k7fd_3rGJF42iBbJH37HPNw=FheDKg@mail.gmail.com/

-Saravana