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Date: Fri, 21 Jan 2011 17:35:42 -0800 From: Saravana Kannan <skannan@...eaurora.org> To: Colin Cross <ccross@...gle.com> CC: Russell King - ARM Linux <linux@....linux.org.uk>, Nicolas Pitre <nicolas.pitre@...aro.org>, Dima Zavin <dmitriyz@...gle.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>, Paul Mundt <lethal@...ux-sh.org>, linux-kernel@...r.kernel.org, Ben Dooks <ben-linux@...ff.org>, Uwe Kleine-K??nig <u.kleine-koenig@...gutronix.de>, Jeremy Kerr <jeremy.kerr@...onical.com>, linux-arm-kernel@...ts.infradead.org Subject: Re: Locking in the clk API On 01/21/2011 02:28 PM, Colin Cross wrote: > On Fri, Jan 21, 2011 at 2:02 PM, Russell King - ARM Linux > <linux@....linux.org.uk> wrote: >> On Fri, Jan 21, 2011 at 04:53:44PM -0500, Nicolas Pitre wrote: >>> So I think that the API must be augmented with more methods, such as: >>> >>> clk_slow_enable(): >>> - may sleep >>> - may be a no-op if the clk_fast_enable() is supported >>> >>> clk_fast_enable(): >>> - may not sleep, used in atomic context >>> - may be a no-op if controlling the clock takes time, in which case >>> clk_slow_enable() must have set the clock up entirely >>> >>> ... and similar for clk_slow_disable() and clk_fast_disable(). >> >> Isn't this along the same lines as my clk_prepare() vs clk_enable() >> suggestion? >> >> I suggested that clk_prepare() be callable only from non-atomic contexts, >> and do whatever's required to ensure that the clock is available. That >> may end up enabling the clock as a result. >> >> clk_enable() callable from atomic contexts, and turns the clock on if >> the hardware supports such an operation. >> >> So, if you have something like: >> >> Xtal--->PLL--->Routing/Masking--->Device >> >> clk = clk_get() returns the clock for the device. >> >> clk_prepare(clk) would walk up the clock tree, selecting the routing and >> preparing each clock. Clocks prior to _and_ including the PLL would need >> to be enabled. >> >> clk_enable(clk) would walk up the tree if the clock isn't already enabled, >> calling clk_enable() on the parent clock. As we require prepared clocks >> to already be enabled, this automatically stops at the PLL. >> >> To encourage correct usage, we just need to make sure that clk_prepare() >> has a might_sleep() thing, and clk_enable() throws a fit if it's used >> on a clk without prepare being used first. The second point is not easy >> to do in a foolproof manner though, but doing _something_ is better than >> nothing. > > I like this proposal, and I prefer the clk_prepare naming over > clk_slow_enable - too many people would call clk_slow_enable instead > of, and not as well as, clk_fast_enable. > > On Tegra, I currently use the ugly conditional mutex or spinlock > method to deal with voltage scaling based on clock frequency. Colin, MSM is in a similar situation, so thought I should bring this up to you attention -- do you have no use case for changing the rate in atomic context? If you do, the clk_prepare/unprepare() approach won't work. Do you have no such requirement? -Saravana -- Sent by an employee of the Qualcomm Innovation Center, Inc. The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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