Re: [PATCH 1/2] iio: dht11: forked a driver version that polls sensor's signal from GPIO

[Harald Geyer]

Am Mittwoch, dem 01.02.2023 um 13:51 +0100 schrieb
pelzi@xxxxxxxxxxxxxxx:
> I understand that the first priority is in finding out if there is 
> actually a proper
> use case for a polling implementation at all. Only then, it might be 
> worth to extend
> the existing dht11 module by an polling alternative.
> 
> Am 31.01.23 um 11:18 schrieb harald@xxxxxxxxx:
> > On 2023-01-30 21:42, Andreas Feldner wrote:
> > > On a BananaPi M2 Zero, the existing, IRQ based dht11 driver is
> > > not 
> > > working,
> > > but missing most IRQs.
> > 
> > That's quite surprising as the driver works well on many similar
> > systems
> > based on Allwinner SoCs. I suspect the problem is with your setup.
> > Maybe
> > some other (polling?) driver is slowing everything down.
> 
> Can you give me a hint how to look for signs of such a situation?

The obvious things to try:

Enabling debug output for the dht11 driver, to look into which
interrupts are actually missing: Is it a "block" of interrupts?
Are they randomly distributed? Are they somewhat equally spaced?
This should give some hints about the nature of the problem.

Try to reproduce the problem on a minimal system:
Unload as many modules as possible.
Maybe just use a system on a ram disk.
As little user space programms running as possbile.
You might find OpenWRT helpful.

Try other kernel versions. (Unlikely, but it might be some
completely unrelated regression.)

Implement debugging output in your polling driver to investigate,
why *that* performs so bad. It probably is the same issue.

If this doesn't lead anywhere, then it is a tough problem, so
let's for now assume, you find something.


> BTW I took some pride in building the board's system image from 
> reproduceable sources: Debian kernel package 
> linux-image-5.10.0-20-armmp-lpae, and the device tree from 
> 
> arch/arm/boot/dts/sun8i-h2-plus-bananapi-m2-zero.dts
> 
> So the setup should be reproducible, unlike other approaches
> advertised 
> in the BananaPi forum...
> 
> What I did is
> 
> - check /proc/interrupts. The highest volume interrupts there are two
> instances of sunxi-mmc, one generating about 50 interrupts per
> second, 
> the other about 25. Those (and most) interrupts are GICv2, but the
> GPIO 
> releated are sunxi-pio-{level,edge}
> 
> - check dmesg: literally no messages apart from dht11_poll itself
> 
> - check top: sugov:0 is reported to eat 10% of one cpu, but I
> understand 
> that's expected and an artifact anyway. Changing the scaling governor
> to 
> "performance" eliminates this, but does not help in making the irq 
> driven dht11 work.
> 
> - check vmstat: ir is between 50 and 200 apart from short spikes,
> those 
> probably related to a certain cron job
> 
> - check sysstat cpu, mem, threads, mutex: each of the 4 cores has a
> low 
> performance (a factor of 15 lower than a Raspberrypi 3), but
> constant, 
> low stddev, etc. No surprises running e.g. 8 threads instead of 4.
> 
> So, apart from the fact that it is missing about 3/4 of the IRQs the 
> dht11 driver should get, I have no indication that something might be
> wrong with the board or its setup. Where else should I look?

There are many possible issues, that are difficult to investigate
directly. E.g. cache poisoning, some code disabling interrupts just
a bit to long etc. Thus the use of minimal systems.

> 
> > > Following the hints in Harald Geyer's comments I
> > > tried to implement a version of the driver that is polling the
> > > GPIO
> > > sensor in a busy loop, not using IRQ altogether.
> > 
> > IIRC one readout takes about 80 milliseconds. That's a very long
> > time for
> > a busy loop. I doubt this is acceptable for inclusion in the
> > kernel. Of
> > course also Jonathan's comments apply.
> 
> Seems to be a bit less, just in case that matters. Given the timing 
> chart I'd expect
> 
> on average: 200µs + 40 * 100µs = 4,2ms
> 
> worst case (device trying to send all one-bits): 200µs + 40 * 120µs =
> 5,0ms
> 

Ack.

Good luck,
Harald