From mboxrd@z Thu Jan 1 00:00:00 1970 From: skannan@codeaurora.org (Saravana Kannan) Date: Fri, 21 Jan 2011 17:35:42 -0800 Subject: Locking in the clk API In-Reply-To: References: <201101111016.42819.jeremy.kerr@canonical.com> <20110111031552.GJ3760@linux-sh.org> <4D3862DB.5000708@fluff.org> <20110120185617.GI6335@n2100.arm.linux.org.uk> <4D3907BD.4040900@codeaurora.org> <20110121220238.GE23151@n2100.arm.linux.org.uk> Message-ID: <4D3A346E.9070709@codeaurora.org> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On 01/21/2011 02:28 PM, Colin Cross wrote: > On Fri, Jan 21, 2011 at 2:02 PM, Russell King - ARM Linux > 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.