From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752473AbcD0Iry (ORCPT ); Wed, 27 Apr 2016 04:47:54 -0400 Received: from foss.arm.com ([217.140.101.70]:32844 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751241AbcD0Irv (ORCPT ); Wed, 27 Apr 2016 04:47:51 -0400 Subject: Re: [PATCH V17 2/3] dmaengine: qcom_hidma: add debugfs hooks To: Vinod Koul , Sinan Kaya References: <1460384473-5775-1-git-send-email-okaya@codeaurora.org> <1460384473-5775-3-git-send-email-okaya@codeaurora.org> <20160426033029.GB2274@localhost> <20160426162529.GK2274@localhost> <571F9D76.2060809@codeaurora.org> <20160427081501.GV2274@localhost> Cc: dmaengine@vger.kernel.org, timur@codeaurora.org, devicetree@vger.kernel.org, cov@codeaurora.org, jcm@redhat.com, shankerd@codeaurora.org, vikrams@codeaurora.org, mark.rutland@arm.com, eric.auger@linaro.org, agross@codeaurora.org, arnd@arndb.de, linux-arm-msm@vger.kernel.org, linux-arm-kernel@lists.infradead.org, Dan Williams , Andy Shevchenko , linux-kernel@vger.kernel.org From: Marc Zyngier X-Enigmail-Draft-Status: N1110 Organization: ARM Ltd Message-ID: <57207CB1.4040307@arm.com> Date: Wed, 27 Apr 2016 09:47:45 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Icedove/38.7.0 MIME-Version: 1.0 In-Reply-To: <20160427081501.GV2274@localhost> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 27/04/16 09:15, Vinod Koul wrote: > On Tue, Apr 26, 2016 at 12:55:18PM -0400, Sinan Kaya wrote: >> On 4/26/2016 12:25 PM, Vinod Koul wrote: >>> On Tue, Apr 26, 2016 at 08:08:16AM -0400, okaya@codeaurora.org wrote: >>>> On 2016-04-25 23:30, Vinod Koul wrote: >>>>> On Mon, Apr 11, 2016 at 10:21:12AM -0400, Sinan Kaya wrote: >>>>> >>>>>> +static int hidma_chan_stats(struct seq_file *s, void *unused) >>>>>> +{ >>>>>> + struct hidma_chan *mchan = s->private; >>>>>> + struct hidma_desc *mdesc; >>>>>> + struct hidma_dev *dmadev = mchan->dmadev; >>>>>> + >>>>>> + pm_runtime_get_sync(dmadev->ddev.dev); >>>>> >>>>> debug shouldn't power up device, why do you want to do that >>>> >>>> >>>> Clocks are turned off while the hw is idle. I can’t reach hw >>>> registers without restoring power. >>> >>> Hmm, have you thought about using regmap? >>> >> >> To be honest, I didn't know what regmap is but I just read some code >> and looked at how it is used. Feel free to correct me if I got it >> wrong. >> >> Regmap seems to be designed for *slow* speed peripherals to improve frequent >> accesses by the SW. It looks like it is used by MFD, SPI and I2C drivers. >> >> It seems to cache the register contents and flush/invalidate them only when >> needed. >> >> The MMIO version seems to be assuming the presence of device-tree like CLK >> API which doesn't exist on ACPI systems and is not portable. >> >> My reaction is that it is a lot of code with no added functionality to what >> HIDMA driver is trying to achieve. >> >> Given that the use case here is only for debug purposes; I think it is OK >> to keep this runtime call here. I don't want to add any overhead into the >> existing code just to support the debug use case. >> >> None of my register read/writes are slow. This file will only be used to >> troubleshoot customer issues. I'd recommend you actually run perf on a a few of your MMIO accesses. I believe the result will be eye opening. On the KVM side, we've trimmed our MMIO access as much as possible, using a memory-based cache (similar to regmap in concept). This has made some code paths about 40% faster. > $ is always faster than MMIO. This way you can give reg contents to users > without waking up hw. Indeed. MMIO access sucks rocks, even on a very fast box. Actually, the faster the box is, the slower MMIO feels (compared to memory). Thanks, M. -- Jazz is not dead. It just smells funny...