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Date: Wed, 27 Apr 2016 09:25:06 -0400 From: okaya@...eaurora.org To: Marc Zyngier <marc.zyngier@....com> Cc: Vinod Koul <vinod.koul@...el.com>, dmaengine@...r.kernel.org, timur@...eaurora.org, devicetree@...r.kernel.org, cov@...eaurora.org, jcm@...hat.com, shankerd@...eaurora.org, vikrams@...eaurora.org, mark.rutland@....com, eric.auger@...aro.org, agross@...eaurora.org, arnd@...db.de, linux-arm-msm@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, Dan Williams <dan.j.williams@...el.com>, Andy Shevchenko <andy.shevchenko@...il.com>, linux-kernel@...r.kernel.org Subject: Re: [PATCH V17 2/3] dmaengine: qcom_hidma: add debugfs hooks 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@...eaurora.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. Agreed. However, I need to understand how regmap really works under the covers and whether it is compatible with the hardware.
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