Messages in this thread | | | Date | Fri, 21 Jan 2011 18:22:34 -0800 | Subject | Re: Locking in the clk API | From | Colin Cross <> |
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On Fri, Jan 21, 2011 at 5:35 PM, Saravana Kannan <skannan@codeaurora.org> wrote: > On 01/21/2011 02:28 PM, Colin Cross wrote: >> >> On Fri, Jan 21, 2011 at 2:02 PM, Russell King - ARM Linux >> <linux@arm.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?
I can't think of any case that requires clk_set_rate in atomic context. We usually only scale frequencies to save power, so we can always scale up early in sleepable context for a minor power penalty. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.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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