linux-kernel - Re: [PATCH v3] amba: Remove deferred device addition

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Date:   Fri, 5 Mar 2021 10:02:48 -0800
From:   Saravana Kannan <saravanak@...gle.com>
To:     Marek Szyprowski <m.szyprowski@...sung.com>
Cc:     Russell King <linux@...linux.org.uk>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Rob Herring <robh@...nel.org>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        John Stultz <john.stultz@...aro.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Sudeep Holla <sudeep.holla@....com>,
        Nicolas Saenz Julienne <nsaenzjulienne@...e.de>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Android Kernel Team <kernel-team@...roid.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] amba: Remove deferred device addition

On Fri, Mar 5, 2021 at 3:45 AM Marek Szyprowski
<m.szyprowski@...sung.com> wrote:
>
> Hi Saravana,
>
> 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@...nel.org>
> > Cc: Ulf Hansson <ulf.hansson@...aro.org>
> > Cc: John Stultz <john.stultz@...aro.org>
> > Cc: Saravana Kannan <saravanak@...gle.com>
> > Cc: Linus Walleij <linus.walleij@...aro.org>
> > Cc: Sudeep Holla <sudeep.holla@....com>
> > Cc: Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
> > Cc: Geert Uytterhoeven <geert+renesas@...der.be>
> > Cc: Marek Szyprowski <m.szyprowski@...sung.com>
> > Cc: Russell King <linux@...linux.org.uk>
> > Signed-off-by: Saravana Kannan <saravanak@...gle.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@...sung.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 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.

Russell,

Does this look good now? Plan to pick it up some time?

Thanks,
Saravana

>
> Best regards
> --
> Marek Szyprowski, PhD
> Samsung R&D Institute Poland
>