From mboxrd@z Thu Jan 1 00:00:00 1970 From: Laurent Pinchart Subject: Re: [PATCH v8 1/5] PM / Runtime: Add getter for querying the IRQ safe option Date: Mon, 03 Nov 2014 18:59:37 +0200 Message-ID: <2519900.LRhHM3UCtt@avalon> References: <1413795888-18559-1-git-send-email-k.kozlowski@samsung.com> <3452823.YQ5FR2FtKz@avalon> <20141103162728.GB1870@intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: In-Reply-To: <20141103162728.GB1870@intel.com> Sender: linux-doc-owner@vger.kernel.org To: Vinod Koul Cc: Krzysztof Kozlowski , Pavel Machek , "Rafael J. Wysocki" , Len Brown , Jonathan Corbet , Russell King , Dan Williams , Ulf Hansson , Alan Stern , linux-pm@vger.kernel.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, dmaengine@vger.kernel.org, Lars-Peter Clausen , Michal Simek , Kyungmin Park , Marek Szyprowski , Bartlomiej Zolnierkiewicz List-Id: linux-pm@vger.kernel.org Hi Vinod, On Monday 03 November 2014 21:57:28 Vinod Koul wrote: > On Sat, Nov 01, 2014 at 02:29:42AM +0200, Laurent Pinchart wrote: > > On Friday 31 October 2014 15:40:16 Krzysztof Kozlowski wrote: > >> On pi=C4=85, 2014-10-31 at 15:22 +0100, Pavel Machek wrote: > >>> On Fri 2014-10-31 10:14:55, Krzysztof Kozlowski wrote: > >>>> On pon, 2014-10-20 at 11:04 +0200, Krzysztof Kozlowski wrote: > >>>>> Add a simple getter pm_runtime_is_irq_safe() for querying wheth= er > >>>>> runtime PM IRQ safe was set or not. > >>>>>=20 > >>>>> Various bus drivers implementing runtime PM may use choose to > >>>>> suspend differently based on IRQ safeness status of child drive= r > >>>>> (e.g. do not unprepare the clock if IRQ safe is not set). > >>>>>=20 > >>>>> Signed-off-by: Krzysztof Kozlowski > >>>>> Reviewed-by: Ulf Hansson > >>>>=20 > >>>> Rafael, Len, Pavel, > >>>>=20 > >>>> Is proposed API ok? Do you have any comments? > >>>>=20 > >>>> I'll upload whole patchset to Russell's patch tracking system. > >>>> However an ack from PM maintainer is probably needed. > >>>=20 > >>> I don't like the API. Having callbacks work in different context = (irq > >>> / noirq) based on what another function reports is ugly. > >>>=20 > >>> What is the penalty if we always decide callbacks are not IRQ saf= e? > >>=20 > >> Then pm_runtime_get_sync() could not be called in atomic context. = The > >> pl330 runtime PM would have to be completely reworked because one > >> pm_runtime_get_sync() is called in device_issue_pending which cann= ot > >> sleep (at least in non preemptible kernels). Probably this can be = solved > >> some way... > >=20 > > Many other drivers suffer from the same problem. While I won't reje= ct your > > proposed fix, I would prefer a more generic approach. > >=20 > > One option that has been discussed previously was to use a work que= ue to > > delay starting the DMA transfer to an interruptible context where > > pm_runtime_get_sync() could be called. However, as Russell pointed = out > > [1], > > even that won't work in all cases as the DMA slave might need the t= ransfer > > to be started before enabling part of its hardware (OMAP audio seem= to be > > such a case). > >=20 > > I've heard a rumor of a possible DMA engine rework to forbid callin= g the > > descriptor preparation API from atomic context. This could be used = as a > > base to implement runtime PM, as DMA slave drivers should not prepa= re > > descriptors if they don't need to use them. However that's a long t= erm > > plan, and we need a solution sooner than that. >=20 > Well it is not a rumour :) I didn't want to put too much pressure on you by quoting you on that :-= ) > I have been contemplating that now that async_tx will be killed so we= dont > have to worry about that usage. For the slave dma usage, we can chang= e the > prepare API to be non atomic. I think the users will be okay with app= roach. > This way drivers can use runtime pm calls in prepare. I'd certainly welcome that. There's a couple of users we will need to f= ix as=20 they call the prepare API from an atomic context. I'm thinking about AS= oC=20 drivers in particular. Do you have any time line in mind ? > > I've been toying with the idea of adding explicit open/close (or wh= atever > > we would call them) operations to the DMA engine API. Those would b= e used > > by DMA slave drivers to signal that they will start/stop using the = DMA > > engine. > >=20 > > If (1) we must start the DMA synchronously with a DMA slave call, (= 2) need > > to sleep to handle PM, and (3) don't want to keep the DMA engine po= wered > > for as long as one channel is requested, then we need to turn at le= ast > > preparation as not callable in atomic context, or introduce a new > > operation. > >=20 > > [1] http://www.spinics.net/lists/dmaengine/msg01548.html > >=20 > > > >>> --- a/Documentation/power/runtime_pm.txt > > > >>> +++ b/Documentation/power/runtime_pm.txt > > > >>>=20 > > > >>> @@ -468,6 +468,10 @@ drivers/base/power/runtime.c and > > > >>>=20 > > > >>> include/linux/pm_runtime.h: > > > >>> - set the power.irq_safe flag for the device, causing th= e > > > >>> runtime-PM callbacks to be invoked with interrupts off > > > >>>=20 > > > >>> + bool pm_runtime_is_irq_safe(struct device *dev); > > > >>> + - return true if power.irq_safe flag was set for the dev= ice, > > > >>> causing > > > >>> + the runtime-PM callbacks to be invoked with interrupts= off > > > >>> + > > > >>>=20 > > > >>> void pm_runtime_mark_last_busy(struct device *dev); > > > >>> - set the power.last_busy field to the current time --=20 Regards, Laurent Pinchart