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

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Date:	Wed, 27 Apr 2016 09:47:45 +0100
From:	Marc Zyngier <marc.zyngier@....com>
To:	Vinod Koul <vinod.koul@...el.com>,
	Sinan Kaya <okaya@...eaurora.org>
Cc:	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 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.
-- 
Jazz is not dead. It just smells funny...