linux-kernel - Re: Locking in the clk API

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Message-ID: <20110115145358.GC15996@n2100.arm.linux.org.uk>
Date:	Sat, 15 Jan 2011 14:53:58 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Christer Weinigel <christer@...nigel.se>
Cc:	Saravana Kannan <skannan@...eaurora.org>,
	Jeremy Kerr <jeremy.kerr@...onical.com>,
	Lorenzo Pieralisi <Lorenzo.Pieralisi@....com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	linux-sh@...r.kernel.org,
	Ben Herrenschmidt <benh@...nel.crashing.org>,
	Sascha Hauer <s.hauer@...gutronix.de>,
	linux-kernel@...r.kernel.org,
	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: Locking in the clk API

On Sat, Jan 15, 2011 at 03:02:25PM +0100, Christer Weinigel wrote:
> This feels a bit like perfect being the enemy of good.
>
> On platforms that need to sleep to enable the UART clock, configuring  
> the UART as the kernel console should be equivalent to userspace opening  
> the UART device, i.e. enable the clock.  At least to me that feels like  
> an acceptable tradeoff, and if I wanted to save the last bit of power  
> I'll have to refrain from using UART as the kernel console.
>
> If both printk to the console and disabling the clock is really really  
> neccesary, add a clk_enable_busywait, but that will be a bit of a hack.

Well, we're not discussing a _new_ API here - we're discussing an API
with existing users which works completely fine on the devices its
used, with differing expectations between implementations.

> Both of these feel like they should use a call such as clk_get_atomic  
> and be able to handle EWOULDBLOCK/EAGAIN (or whatever error code is used  
> to indicate that it would have to sleep) and delegate to a worker thread  
> to enable the clock.  To catch uses of plain clk_enable from atomic  
> contects, add a WARN_ON/BUG_ON(in_atomic()).  It won't catch everything,  
> but would help a bit at least.

We've never allowed clk_get() to be called in interruptible context,
so that's not the issue.  The issue is purely about clk_enable() and
clk_disable() and whether they should be able to be called in atomic
context or not.

We've been around returning EAGAIN, WARN_ONs, BUG_ONs, having clk_enable()
vs clk_enable_atomic(), clk_enable_cansleep() vs clk_enable(), etc.

There's been a lot of talk on this issue for ages with no real progress
that I'm just going to repeat: let's unify those implementations which
use a spinlock for their clks into one consolidated solution, and
a separate consolidated solution for those which use a mutex.

This will at least allow us to have _some_ consolidation of the existing
implementations - and it doesn't add anything to the problem at hand.
It might actually help identify what can be done at code level to resolve
this issue.
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