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

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Date:	Wed, 27 Apr 2016 08:51:18 -0400
From:	okaya@...eaurora.org
To:	Vinod Koul <vinod.koul@...el.com>
Cc:	dmaengine@...r.kernel.org, timur@...eaurora.org,
	devicetree@...r.kernel.org, cov@...eaurora.org, jcm@...hat.com,
	shankerd@...eaurora.org, vikrams@...eaurora.org,
	marc.zyngier@....com, 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 04:15, 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.
> 
> $ is always faster than MMIO. This way you can give reg contents to 
> users
> without waking up hw.
> 
> Also we at Intel use regmap on ACPI systems without CLK API

I can try and see the performance impact is. What happens to registers 
that hw updates like status registers. Those will be most interesting 
during debug. How does remap get updated for those? Is there a way to 
tell it not to cache certain registers