[alsa-devel] [RFC 00/17] clk: Add per-controller locks to fix deadlocks

[Charles Keepax]

On Tue, Aug 16, 2016 at 03:51:10PM +0200, Krzysztof Kozlowski wrote:
> On 08/16/2016 03:34 PM, Krzysztof Kozlowski wrote:
> > Hi,
> > 
> > RFC, please, do not apply, maybe except patch #1 which is harmless.
> > 
> > 
> > Introduction
> > ============
> > The patchset brings new entity: clock controller representing a hardware
> > block.  The clock controller comes with its own prepare lock which
> > is used then in many places.  The idea is to fix the deadlock mentioned
> > in commit 10ff4c5239a1 ("i2c: exynos5: Fix possible ABBA deadlock by keeping
> > I2C clock prepared") and commit 34e81ad5f0b6 ("i2c: s3c2410: fix ABBA deadlock
> > by keeping clock prepared").
> > 
> 
> Damn, I forgot to describe the overall idea. :) It is mentioned in patch
> 15 but probably not many will have enough of patience to reach it.
> 
> The locking idea
> ================
> Clock controllers representing different hardware blocks, will contain
> its own prepare lock which protects the clocks inside controller.  The
> hierarchy itself is protected by global lock.
> 
> In prepare path, the global prepare lock is removed.  This is direct
> solution for the deadlock.
> 
> Clock hierarchy imposes also hierarchy between controllers so when a
> prepare happens, also parents have to be locked.
> 
> Following locking design was chosen:
> 1. For prepare/unprepare paths: lock only clock controller and its
>    parents.
> 2. For recalc rates paths: lock global lock, the controller and its
>    children.
> 3. For reparent paths: lock entire tree up down (children and parents)
>    and the global lock as well.
> 
> 
> In each case of traversing the clock hierarchy, the locking of
> controllers is always from children to parents.
> 
> 
> Best regards,
> Krzysztof

I have been playing with these patches on my Arndale board and
they certainly do seem to resolve the interaction issues I have
been SPI and the clocking framework, which is awesome and lets me
sensibly add the clocking framework into our codec drivers. I will
keep investigating and for what its worth have a little more
detailed look through the code.

Thanks,
Charles