Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755238AbbG3Ju2 (ORCPT ); Thu, 30 Jul 2015 05:50:28 -0400 Received: from mail-wi0-f173.google.com ([209.85.212.173]:33372 "EHLO mail-wi0-f173.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754636AbbG3Ju0 (ORCPT ); Thu, 30 Jul 2015 05:50:26 -0400 Date: Thu, 30 Jul 2015 10:50:14 +0100 From: Lee Jones To: Michael Turquette Cc: Maxime Ripard , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, kernel@stlinux.com, sboyd@codeaurora.org, devicetree@vger.kernel.org, geert@linux-m68k.org, s.hauer@pengutronix.de Subject: Re: [PATCH v7 3/5] clk: Supply the critical clock {init, enable, disable} framework Message-ID: <20150730095014.GD14642@x1> References: <1437570255-21049-1-git-send-email-lee.jones@linaro.org> <1437570255-21049-4-git-send-email-lee.jones@linaro.org> <20150727072549.GP2564@lukather> <20150727085338.GW3436@x1> <20150728114022.GW2564@lukather> <20150728130055.GV14943@x1> <20150730011932.642.85168@quantum> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20150730011932.642.85168@quantum> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 9100 Lines: 213 On Wed, 29 Jul 2015, Michael Turquette wrote: > Quoting Lee Jones (2015-07-28 06:00:55) > > On Tue, 28 Jul 2015, Maxime Ripard wrote: > > > > > On Mon, Jul 27, 2015 at 09:53:38AM +0100, Lee Jones wrote: > > > > On Mon, 27 Jul 2015, Maxime Ripard wrote: > > > > > > > > > On Wed, Jul 22, 2015 at 02:04:13PM +0100, Lee Jones wrote: > > > > > > These new API calls will firstly provide a mechanisms to tag a clock as > > > > > > critical and secondly allow any knowledgeable driver to (un)gate clocks, > > > > > > even if they are marked as critical. > > > > > > > > > > > > Suggested-by: Maxime Ripard > > > > > > Signed-off-by: Lee Jones > > > > > > --- > > > > > > drivers/clk/clk.c | 45 ++++++++++++++++++++++++++++++++++++++++++++ > > > > > > include/linux/clk-provider.h | 2 ++ > > > > > > include/linux/clk.h | 30 +++++++++++++++++++++++++++++ > > > > > > 3 files changed, 77 insertions(+) > > > > > > > > > > > > diff --git a/drivers/clk/clk.c b/drivers/clk/clk.c > > > > > > index 61c3fc5..486b1da 100644 > > > > > > --- a/drivers/clk/clk.c > > > > > > +++ b/drivers/clk/clk.c > > > > > > @@ -46,6 +46,21 @@ static struct clk_core *clk_core_lookup(const char *name); > > > > > > > > > > > > /*** private data structures ***/ > > > > > > > > > > > > +/** > > > > > > + * struct critical - Provides 'play' over critical clocks. A clock can be > > > > > > + * marked as critical, meaning that it should not be > > > > > > + * disabled. However, if a driver which is aware of the > > > > > > + * critical behaviour wants to control it, it can do so > > > > > > + * using clk_enable_critical() and clk_disable_critical(). > > > > > > + * > > > > > > + * @enabled Is clock critical? Once set, doesn't change > > > > > > + * @leave_on Self explanatory. Can be disabled by knowledgeable drivers > > > > > > + */ > > > > > > +struct critical { > > > > > > + bool enabled; > > > > > > + bool leave_on; > > > > > > +}; > > > > > > + > > > > > > struct clk_core { > > > > > > const char *name; > > > > > > const struct clk_ops *ops; > > > > > > @@ -75,6 +90,7 @@ struct clk_core { > > > > > > struct dentry *dentry; > > > > > > #endif > > > > > > struct kref ref; > > > > > > + struct critical critical; > > > > > > }; > > > > > > > > > > > > struct clk { > > > > > > @@ -995,6 +1011,10 @@ static void clk_core_disable(struct clk_core *clk) > > > > > > if (WARN_ON(clk->enable_count == 0)) > > > > > > return; > > > > > > > > > > > > + /* Refuse to turn off a critical clock */ > > > > > > + if (clk->enable_count == 1 && clk->critical.leave_on) > > > > > > + return; > > > > > > + > > > > > > > > > > I think it should be handled by a separate counting. Otherwise, if you > > > > > have two users that marked the clock as critical, and then one of them > > > > > disable it... > > > > > > > > > > > if (--clk->enable_count > 0) > > > > > > return; > > > > > > > > > > > > @@ -1037,6 +1057,13 @@ void clk_disable(struct clk *clk) > > > > > > } > > > > > > EXPORT_SYMBOL_GPL(clk_disable); > > > > > > > > > > > > +void clk_disable_critical(struct clk *clk) > > > > > > +{ > > > > > > + clk->core->critical.leave_on = false; > > > > > > > > > > .. you just lost the fact that it was critical in the first place. > > > > > > > > I thought about both of these points, which is why I came up with this > > > > strategy. > > > > > > > > Any device which uses the *_critical() API should a) have knowledge of > > > > what happens when a particular critical clock is gated and b) have > > > > thought about the consequences. > > > > > > Indeed. > > > > > > > I don't think we can use reference counting, because we'd need as > > > > many critical clock owners as there are critical clocks. > > > > > > Which we can have if we replace the call to clk_prepare_enable you add > > > in your fourth patch in __set_critical_clocks. > > > > What should it be replaced with? > > > > > > Cast your mind back to the reasons for this critical clock API. One > > > > of the most important intentions of this API is the requirement > > > > mitigation for each of the critical clocks to have an owner > > > > (driver). > > > > > > > > With regards to your second point, that's what 'critical.enabled' > > > > is for. Take a look at clk_enable_critical(). > > > > > > I don't think this addresses the issue, if you just throw more > > > customers at it, the issue remain with your implementation. > > > > > > If you have three customers that used the critical API, and if on of > > > these calls clk_disable_critical, you're losing leave_on. > > > > That's the idea. See my point above, the one you replied "Indeed" > > to. So when a driver uses clk_disable_critical() it's saying, "I know > > why this clock is a critical clock, and I know that nothing terrible > > will happen if I disable it, as I have that covered". So then if it's > > not the last user to call clk_disable(), the last one out the door > > will be allowed to finally gate the clock, regardless whether it's > > critical aware or not. > > > > Then, when we come to enable the clock again, the critical aware user > > then re-marks the clock as leave_on, so not critical un-aware user can > > take the final reference and disable the clock. > > > > > Which means that if there's one of the two users left that calls > > > clk_disable on it, the clock will actually be disabled, which is > > > clearly not what we want to do, as we have still a user that want the > > > clock to be enabled. > > > > That's not what happens (at least it shouldn't if I've coded it up > > right). The API _still_ requires all of the users to give-up their > > reference. > > > > > It would be much more robust to have another count for the critical > > > stuff, initialised to one by the __set_critical_clocks function. > > > > If I understand you correctly, we already have a count. We use the > > original reference count. No need for one of our own. > > > > Using your RAM Clock (Clock 4) as an example > > -------------------------------------------- > > > > Early start-up: > > Clock 4 is marked as critical and a reference is taken (ref == 1) > > > > Driver probe: > > SPI enables Clock 4 (ref == 2) > > I2C enables Clock 4 (ref == 3) > > > > Suspend (without RAM driver's permission): > > SPI disables Clock 4 (ref == 2) > > I2C disables Clock 4 (ref == 1) > > /* > > * Clock won't be gated because: > > * .leave_on is True - can't dec final reference > > I am clearly missing the point. The clock won't be gated because the > enable_count is still 1! What does .leave_on do here? The point of _this_ (the extended) part of the API is so that the clock _can_ be turned off. Without the possibility to disable .leave_on and the logic with accompanies it (i.e. clk_disable_critical()) the clock will _never_ be gated. > > */ > > > > Suspend (with RAM driver's permission): > > /* Order is unimportant */ > > SPI disables Clock 4 (ref == 2) > > RAM disables Clock 4 (ref == 1) /* Won't turn off here (ref > 0) > > I2C disables Clock 4 (ref == 0) /* (.leave_on == False) last ref can be taken */ > > /* > > * Clock will be gated because: > > * .leave_on is False, so (ref == 0) > > Again, .leave_on does nothing new here. We gate the clock because the > reference count is 0. It's the fact that .leave_on has been disabled in clk_disable_critical() that allows the final reference to be taken. > > */ > > > > Resume: > > /* Order is unimportant */ > > SPI enables Clock 4 (ref == 1) > > RAM enables Clock 4 and re-enables .leave_on (ref == 2) > > I2C enables Clock 4 (ref == 3) > > Same again. As soon as RAM calls clk_enable_critical the ref count goes > up. .leave_on does nothing as far as I can tell. The all works because > of the reference counting, which already exists before this patch > series. So fundamentally you're right in what you say. All you really need to disable a critical clock is write a knowledgeable driver, which is intentionally unbalanced i.e. just calls clk_disable(). All this extended API really does is makes the process more official and ensures that an unintentionally unbalanced driver doesn't bugger up the running platform. We could also add a new WARN() to say that said driver is unbalanced, as it just tried to turn off a critical clock. What do you think is best? -- Lee Jones Linaro STMicroelectronics Landing Team Lead Linaro.org │ Open source software for ARM SoCs Follow Linaro: Facebook | Twitter | Blog -- 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/