linux-kernel - Re: [PATCH V17 2/3] dmaengine: qcom

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Message-ID: <64be6f181a4c5b712fc3893e9ecd19df@codeaurora.org>
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.