linux-kernel - clk: Per controller locks (prepare & enable)

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Date:	Wed, 29 Jun 2016 09:23:13 +0200
From:	Krzysztof Kozlowski <k.kozlowski@...sung.com>
To:	Michael Turquette <mturquette@...libre.com>,
	Stephen Boyd <sboyd@...eaurora.org>, linux-clk@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
	Tomasz Figa <tomasz.figa@...il.com>,
	Sylwester Nawrocki <s.nawrocki@...sung.com>
Cc:	Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Andrzej Hajda <a.hajda@...sung.com>,
	Javier Martinez Canillas <javier@....samsung.com>
Subject: clk: Per controller locks (prepare & enable)

Hi,

Problems:
1. Deadlocks on prepare lock in following scenario:
 - prepare_enable(some external clock through i2c)
   - i2c transfer
     - prepare_enable(i2c lock in SoC)
       - deadlock
See [1]. We implemented a workaround for this in few places like
10ff4c5239a1 ("i2c: exynos5: Fix possible ABBA deadlock by keeping I2C
clock prepared") and 34e81ad5f0b6 ("i2c: s3c2410: fix ABBA deadlock by
keeping clock prepared")

The goal would be to remove also this workaround.

2. The global prepare and enable locks are over-used. I didn't test the
congestion but intuition says that they could be improved.

Solution:
Make this locks per controller. This will directly solve problem #1
because these are different controllers. Still in-controller deadlock
could occur but we couldn't find a real case with it.

Question:
What do you think about it? I know that talk is cheap and code looks
better but before starting the work I would like to hear some
comments/opinions/ideas.

Best regards,
Krzysztof

[1]
http://www.krzk.eu/builders/boot-odroid-xu3-multi_v7/builds/34/steps/Boot%20odroid/logs/serial