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Subject: Re: [PATCH v3] amba: Remove deferred device addition
To:     Saravana Kannan <saravanak@google.com>,
        Russell King <linux@armlinux.org.uk>,
        Philipp Zabel <p.zabel@pengutronix.de>
Cc:     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>,
        kernel-team@android.com, linux-kernel@vger.kernel.org
From:   Marek Szyprowski <m.szyprowski@samsung.com>
Message-ID: <30b4141e-11bd-45a2-b890-fddf444548ea@samsung.com>
Date:   Fri, 5 Mar 2021 12:45:27 +0100
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        <20210304195101.3843496-1-saravanak@google.com>
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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@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>

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...

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