[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:

quoted

On Tue, Apr 26, 2016 at 12:55:18PM -0400, Sinan Kaya wrote:

quoted

On 4/26/2016 12:25 PM, Vinod Koul wrote:

quoted

On Tue, Apr 26, 2016 at 08:08:16AM -0400, okaya at codeaurora.org 
wrote:

quoted

On 2016-04-25 23:30, Vinod Koul wrote:

quoted

On Mon, Apr 11, 2016 at 10:21:12AM -0400, Sinan Kaya wrote:

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+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.

quoted

$ 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.