Re: [PATCH V17 2/3] dmaengine: qcom_hidma: add debugfs hooks

[okaya]

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@xxxxxxxxxxxxxx 
> > > > 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 canât 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.