From mboxrd@z Thu Jan 1 00:00:00 1970 From: Russell King - ARM Linux Subject: Re: Common clock framework API vs RT patchset Date: Tue, 4 Aug 2015 16:36:41 +0100 Message-ID: <20150804153641.GR7557@n2100.arm.linux.org.uk> References: <55C0A96F.80307@ti.com> <55C0D8F3.3030105@ti.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Thomas Gleixner , Grygorii Strashko , linux-rt-users@vger.kernel.org, Felipe Balbi , Sekhar Nori , linux-clk@vger.kernel.org To: Nishanth Menon Return-path: Received: from pandora.arm.linux.org.uk ([78.32.30.218]:34620 "EHLO pandora.arm.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752692AbbHDPgs (ORCPT ); Tue, 4 Aug 2015 11:36:48 -0400 Content-Disposition: inline In-Reply-To: <55C0D8F3.3030105@ti.com> Sender: linux-rt-users-owner@vger.kernel.org List-ID: On Tue, Aug 04, 2015 at 10:23:31AM -0500, Nishanth Menon wrote: > Consider clk_enable/disable/set_parent/setfreq operations. none of these > operations are "atomic" from hardware point of view. instead, they are a > set of steps which culminates to moving from state A to state B of the > clock tree configuration. There's a world of difference between clk_enable()/clk_disable() and the rest of the clk API. clk_enable()/clk_disable() _should_ be callable from any context, since you may need to enable or disable a clock from any context. The remainder of the clk API is callable only from contexts where sleeping is permissible. The reason we have this split is because clk_enable()/clk_disable() have historically been used in interrupt handlers, and they're specifically not supposed to impose big delays. Things like waiting for a PLL to re-lock is time-consuming, so it's not something I'd expect to see behind a clk_enable() implementation (the fact you can't sleep in there is a big hint.) Such waits should be in the clk_prepare() stage instead. Now, as for clk_enable() being interrupted - if clk_enable() is interrupted and another clk_enable() comes along for the same clock, that second clk_enable() should not return until the clock has actually been enabled, and it's up to the implementation to decode how to achieve that. If that means a RT implementation using a raw spinlock, then that's one option (which basically would have the side effect of blocking until the preempted clk_enable() finishes its business.) Alternatively, if we can preempt inside clk_enable(), then the clk_enable() implementation should be written to cope with that (eg, by the second clk_enable() fiddling with the hardware, and the first thread noticing that it has nothing to do.) -- FTTC broadband for 0.8mile line: currently at 10.5Mbps down 400kbps up according to speedtest.net.