linux-kernel - Re: [PATCH v5] amba: Remove deferred device addition

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Date:   Wed, 20 Jul 2022 14:12:21 +0100
From:   Sudeep Holla <sudeep.holla@....com>
To:     Saravana Kannan <saravanak@...gle.com>
Cc:     Russell King <linux@...linux.org.uk>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Sudeep Holla <sudeep.holla@....com>,
        Rob Herring <robh@...nel.org>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Nicolas Saenz Julienne <nsaenzjulienne@...e.de>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        kernel-team@...roid.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5] amba: Remove deferred device addition

On Tue, Jul 19, 2022 at 11:20:10AM -0700, 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: Saravana Kannan <saravanak@...gle.com>
> Cc: Linus Walleij <linus.walleij@...aro.org>
> Cc: Sudeep Holla <sudeep.holla@....com>

Tested-by: Sudeep Holla <sudeep.holla@....com>

on Juno with linux-next(which had the reported issue [1]) + this patch(which
fixes the issue)

--
Regards,
Sudeep

[1] https://lore.kernel.org/all/20220701150848.75eeprptmb5beip7@bogus/