From mboxrd@z Thu Jan 1 00:00:00 1970 From: Marc Zyngier Subject: Re: [PATCH V17 2/3] dmaengine: qcom_hidma: add debugfs hooks Date: Wed, 27 Apr 2016 09:47:45 +0100 Message-ID: <57207CB1.4040307@arm.com> 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> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: In-Reply-To: <20160427081501.GV2274@localhost> Sender: devicetree-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: Vinod Koul , Sinan Kaya Cc: dmaengine-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, timur-sgV2jX0FEOL9JmXXK+q4OQ@public.gmane.org, devicetree-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, cov-sgV2jX0FEOL9JmXXK+q4OQ@public.gmane.org, jcm-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org, shankerd-sgV2jX0FEOL9JmXXK+q4OQ@public.gmane.org, vikrams-sgV2jX0FEOL9JmXXK+q4OQ@public.gmane.org, mark.rutland-5wv7dgnIgG8@public.gmane.org, eric.auger-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org, agross-sgV2jX0FEOL9JmXXK+q4OQ@public.gmane.org, arnd-r2nGTMty4D4@public.gmane.org, linux-arm-msm-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-arm-kernel-IAPFreCvJWM7uuMidbF8XUB+6BGkLq7r@public.gmane.org, Dan Williams , Andy Shevchenko , linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-Id: devicetree@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-sgV2jX0FEOL9JmXXK+q4OQ@public.gmane.org wrot= e: >>>> 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 =3D s->private; >>>>>> + struct hidma_desc *mdesc; >>>>>> + struct hidma_dev *dmadev =3D 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=E2=80=99t 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=20 >> wrong.=20 >> >> 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 dri= vers. >> >> It seems to cache the register contents and flush/invalidate them on= ly when >> needed. >> >> The MMIO version seems to be assuming the presence of device-tree li= ke 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.=20 >> >> Given that the use case here is only for debug purposes; I think it = is OK=20 >> to keep this runtime call here. I don't want to add any overhead int= o the >> existing code just to support the debug use case. =20 >> >> None of my register read/writes are slow. This file will only be use= d to=20 >> 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 (simila= r 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 u= sers > 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. --=20 Jazz is not dead. It just smells funny... -- To unsubscribe from this list: send the line "unsubscribe devicetree" i= n the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org More majordomo info at http://vger.kernel.org/majordomo-info.html