From mboxrd@z Thu Jan 1 00:00:00 1970 From: Russell King - ARM Linux Date: Fri, 21 Jan 2011 22:02:38 +0000 Subject: Re: Locking in the clk API Message-Id: <20110121220238.GE23151@n2100.arm.linux.org.uk> List-Id: 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> In-Reply-To: MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: linux-arm-kernel@lists.infradead.org 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.