From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jon Hunter Subject: Re: [RFC PATCH 1/7] DMA: tegra-apb: Correct runtime-pm usage Date: Mon, 24 Aug 2015 14:22:49 +0100 Message-ID: <55DB1AA9.7090906@nvidia.com> References: <1439905755-25150-1-git-send-email-jonathanh@nvidia.com> <1439905755-25150-2-git-send-email-jonathanh@nvidia.com> <20150823141707.GB13546@localhost> <55DADA11.5040306@nvidia.com> <20150824092230.GI13546@localhost> Mime-Version: 1.0 Content-Type: text/plain; charset="windows-1252" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20150824092230.GI13546@localhost> Sender: linux-kernel-owner@vger.kernel.org To: Vinod Koul Cc: Laxman Dewangan , Stephen Warren , Thierry Reding , Alexandre Courbot , dmaengine@vger.kernel.org, linux-tegra@vger.kernel.org, linux-kernel@vger.kernel.org, devicetree@vger.kernel.org List-Id: linux-tegra@vger.kernel.org On 24/08/15 10:22, Vinod Koul wrote: > On Mon, Aug 24, 2015 at 09:47:13AM +0100, Jon Hunter wrote: >> >> On 23/08/15 15:17, Vinod Koul wrote: >>> On Tue, Aug 18, 2015 at 02:49:09PM +0100, Jon Hunter wrote: >>> >>>> @@ -1543,7 +1531,7 @@ static int tegra_dma_pm_suspend(struct device *dev) >>>> int ret; >>>> >>>> /* Enable clock before accessing register */ >>>> - ret = tegra_dma_runtime_resume(dev); >>>> + ret = pm_runtime_get_sync(dev); >>> >>> why is this required ? >> >> Because the clock could be disabled when this function is called. This >> function saves the DMA context so that if the context is lost during >> suspend, it can be restored. > > Have you verified this? Coz my understanding is that when PM does suspend it > will esnure you are runtime resume if runtime suspended and then will do > suspend. > So you do not need to do above I see what you are saying. I did some testing with ftrace today to trace rpm and suspend/resume calls. If the dma controller is runtime suspended and I do not call pm_runtime_get_sync() above then I do not see any runtime resume of the dma controller prior to suspend. Now I was hoping that this would cause a complete kernel crash but it did not and so the DMA clock did not appear to be needed here (at least on the one board I tested). However, I would not go as far as to remove this and prefer to keep as above. Furthermore, other drivers do similar things, including the sirf dma controller (see sirf-dma.c). Cheers Jon