From mboxrd@z Thu Jan 1 00:00:00 1970 From: okaya@codeaurora.org Subject: Re: [PATCH V17 2/3] dmaengine: qcom_hidma: add debugfs hooks Date: Wed, 27 Apr 2016 09:25:06 -0400 Message-ID: <64be6f181a4c5b712fc3893e9ecd19df@codeaurora.org> 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> <57207CB1.4040307@arm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: In-Reply-To: <57207CB1.4040307@arm.com> Sender: linux-arm-msm-owner@vger.kernel.org To: Marc Zyngier Cc: Vinod Koul , 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 List-Id: devicetree@vger.kernel.org On 2016-04-27 04:47, Marc Zyngier wrote: > 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=20 >>>> wrote: >>>>> On 2016-04-25 23:30, Vinod Koul wrote: >>>>>> On Mon, Apr 11, 2016 at 10:21:12AM -0400, Sinan Kaya wrote: >>>>>>=20 >>>>>>> +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); >>>>>>=20 >>>>>> debug shouldn't power up device, why do you want to do that >>>>>=20 >>>>>=20 >>>>> Clocks are turned off while the hw is idle. I can=E2=80=99t reach= hw >>>>> registers without restoring power. >>>>=20 >>>> Hmm, have you thought about using regmap? >>>>=20 >>>=20 >>> To be honest, I didn't know what regmap is but I just read some cod= e >>> and looked at how it is used. Feel free to correct me if I got it >>> wrong. >>>=20 >>> Regmap seems to be designed for *slow* speed peripherals to improve= =20 >>> frequent >>> accesses by the SW. It looks like it is used by MFD, SPI and I2C=20 >>> drivers. >>>=20 >>> It seems to cache the register contents and flush/invalidate them=20 >>> only when >>> needed. >>>=20 >>> The MMIO version seems to be assuming the presence of device-tree=20 >>> like CLK >>> API which doesn't exist on ACPI systems and is not portable. >>>=20 >>> My reaction is that it is a lot of code with no added functionality= =20 >>> to what >>> HIDMA driver is trying to achieve. >>>=20 >>> Given that the use case here is only for debug purposes; I think it= =20 >>> is OK >>> to keep this runtime call here. I don't want to add any overhead in= to=20 >>> the >>> existing code just to support the debug use case. >>>=20 >>> None of my register read/writes are slow. This file will only be us= ed=20 >>> to >>> troubleshoot customer issues. >=20 > 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 trimme= d > our MMIO access as much as possible, using a memory-based cache=20 > (similar > to regmap in concept). This has made some code paths about 40% faster= =2E >=20 >> $ is always faster than MMIO. This way you can give reg contents to=20 >> users >> without waking up hw. >=20 > Indeed. MMIO access sucks rocks, even on a very fast box. Actually, t= he > faster the box is, the slower MMIO feels (compared to memory). >=20 > Thanks, >=20 > M. Agreed. However, I need to understand how regmap really works under the= =20 covers and whether it is compatible with the hardware.