Re: [alsa-devel] [PATCH v2 3/3] ASoC: TDA7802: Add turn-on diagnostic routine

[Thomas Preston <thomas.preston@codethink.co.uk>]

On 02/08/2019 12:10, Mark Brown wrote:
> On Fri, Aug 02, 2019 at 09:32:17AM +0100, Thomas Preston wrote:
>> On 02/08/2019 00:42, Mark Brown wrote:
> 
>>> Yes, that's definitely doable - we've got some other drivers with
>>> similar things like calibration triggers exposed that way.
> 
>> One problem with using a kcontrol as a trigger for the turn-on diagnostic
>> is that the diagnostic routine has a "return value".
> 
> You can use a read only control for the readback, or just have it be
> triggered by overwriting the readback value.  You can cache the result.
> 

Keeping the trigger and result together like that would be better I think,
although the routine isn't supposed to run mid way through playback. If
we're mid playback the debugfs routine has to turn off AMP_ON, take the
device back to a known state, run diagnostics, then restore. Which causes
a gap in the audible sound.

>> Hm, maybe a better idea is to have the turn on diagnostic only run on
>> device probe (as its name suggests!), and print something to dmesg:
> 
>> 	modprobe tda7802 turn_on_diagnostic=1
> 
>> 	tda7802-codec i2c-TDA7802:00: Turn on diagnostic 04 04 04 04
> 
>> Kirill Marinushkin mentioned this in the first review [0], it just didn't
>> really sink in until now!
> 
> You could do that too, yeah.  Depends on what this is diagnosing and if
> that'd be useful.
> 

The diagnostic status bits describe situations such as:
- open load (no speaker connected)
- short to GND
- short to VCC
- etc

The intention is to test if all the speakers are connected. So, one might 
have a self test which runs the diagnostic and verifies it outputs:

	00 00 00 00

For example, on my test rig there is only one speaker connected. So it
reads:

	04 04 00 04

Where the second bit is "open load". So this would fail the test.

So in the kcontrol case the test would be something like:

	amixer sset "AMP1 turn on diagnostic" on
	amixer sget "AMP1 diagnostic"

And the module parameter case:

	rmmod tda7802
	modprobe tda7802 turn_on_diagnostic=1
	dmesg | grep "Turn on diagnostic 04 04 04 04"
	rmmod tda7802
	modprobe tda7802

I think the module parameter method is more appropriate for a
"Turn-on diagnostic", even though I don't really like grepping dmesg
for the result. I'll go ahead and implement that unless anyone has a
particular preference for the kcontrol-trigger.

Thanks