New dtoverlays for max6675 and max31855 thermocouples

[DougieLawson]

See forum thread: https://www.raspberrypi.org/forums/viewtopic.php?f=107&t=280929

[pelwell]

There are few small issues with this PR:

1. There is a max31885e overlay which isn't compiled or referenced.
2. All the makefile entries are in order except max6675.
3. The max38155 README entry still references MAX6675.
4. The README file doesn't like trailing whitespace, even on blank lines.
5. The dev and speed parameters aren't documented.

And there's a larger issue - although you've carefully given everything unique names, these overlays only differ functionally in their compatible strings. There are a few techniques you can use to shorten it (I won't say simplify):

i. Parameters can now include literal assignments - whatever value you give the parameter, it can also assign fixed values to properties. These can include label references.
ii. If you assign to the reg property of a node it will also change the address portion of the node name.

The combination of those two means you can create a single payload node that can be steered around by patching the target of the fragment:

...
    frag8: fragment@8 {
        target = <&spi0>;
        __overlay__ {
            thermocouple: thermocouple@0 {
                compatible = "maxim,max6675";
                reg = <0>;
                spi-max-frequency = <500000>;
            };
        };
    };
...
    __overrides {
        spi0-0 = <0>, "+0",
                 <&frag8>,"target:0=",<&spi0>,
                 <&thermocouple>,"reg:0=0";
        spi0-1 = <0>, "+1",
                 <&frag8>,"target:0=",<&spi0>,
                 <&thermocouple>,"reg:0=1";
        spi1-0 = <0>, "+2",
                 <&frag8>,"target:0=",<&spi1>,
                 <&thermocouple>,"reg:0=0";
        ...
        dev = <&thermocouple>,"compatible";
        max6675 = <&thermocouple>,"compatible=maxim,max6675";
        max31855 = <&thermocouple>,"compatible=maxim,max31855";
        max31855e = <&thermocouple>,"compatible=maxim,max31855e";
        max31855j = <&thermocouple>,"compatible=maxim,max31855j";
        ...
    };

Or is that too much magic, or too dependent on newer firmware for your needs?

[pelwell]

And I forgot to add:

6. Please don't include merge commits - we like downstream commits to be rebased, with merges only for upstream code.

[DougieLawson]

Hi Phil,

You have a distinct head start, the DTS syntax stuff is terribly arcane & isn't well documented - that's why it's a lot of hacking from the anyspi overlay. The git pull request stuff has an ever worse level of documentation.

The SPI speed is set to a fixed value in the kernel driver and can't be overridden.

I don't understand point #6, if a "merge commit" bit me on the nose I wouldn't recognise it (I'm a git novice when it comes to this stuff and it took ten hours and two goes (on two different raspberries - my zero got into swapping hell) to clone the repo).

How do I fix points 1 through 6? Or is that something you can do? Or do I scrap this PR, scrap my forked & cloned copy of the repo and try again.

Rgds, Dougie

[pelwell]

I've got a new version of this PR that I hope to share tomorrow.

[pelwell]

I've pushed an update - can you take a look? The overlay name has changed to maxtherm, and the device name is passed as a parameter to select the correct compatible string. You'll also notice that for the 31855 variants I've given the vanilla "maxim,max31855" as a fallback compatible string so it could be used before the 5.7 - let me know if that is at all useful.

I've tested it to the extent that all the parameters decode and apply correctly, and the README follows the rules, but some confirmation that it still works in one or two combinations would be great.

[DougieLawson]

Thanks Phil.

I'll drag the stuff down to my Zero with the thermocouples and give it a test drive.

[DougieLawson]

Your overlay is working. I ran rpi-update to get #1330. Then updated my config.txt with

dtdebug=1
dtparam=audio=on
gpu_mem=16
hdmi_force_hotplug=1
hdmi_group=2
hdmi_mode=81
dtoverlay=maxtherm,spi0-1,max6675
dtoverlay=maxtherm,spi0-0,max31855
dtparam=spi=on
dtparam=i2c_arm=on
disable_splash=1

Rebooted, and ran

#!/usr/bin/python3

# max31855 spi0-0
# /sys/bus/iio/devices/iio:device0/in_temp_scale
# /sys/bus/iio/devices/iio:device0/in_temp_raw
# max6675 spi0-1
# /sys/bus/iio/devices/iio:device1/in_temp_scale
# /sys/bus/iio/devices/iio:device1/in_temp_raw

from time import sleep
while True:
    with open('/sys/bus/iio/devices/iio:device0/in_temp_scale', 'r') as dev0scale:
                scale0 = float(dev0scale.read())

    with open('/sys/bus/iio/devices/iio:device0/in_temp_raw', 'r') as dev0raw:
                raw0 = float(dev0raw.read())
                dev0temp = (scale0 * raw0) / 1000.0


    with open('/sys/bus/iio/devices/iio:device1/in_temp_scale', 'r') as dev1scale:
                scale1 = float(dev1scale.read())

    with open('/sys/bus/iio/devices/iio:device1/in_temp_raw', 'r') as dev1raw:
                raw1 = float(dev1raw.read())
                dev1temp = (scale1 * raw1) / 1000.0

    print ("Max31855: ", dev0temp)
    print ("Max6675: ", dev1temp)
    sleep(2)

That gets

pi@viking:~/python/thermo$ ./readIIO.py
Max31855:  29.25
Max6675:  28.0
Max31855:  29.25
Max6675:  28.0
Max31855:  29.25
Max6675:  27.75
Max31855:  29.25
Max6675:  28.0
Max31855:  29.25
Max6675:  28.0
Max31855:  29.25
Max6675:  28.25
^CTraceback (most recent call last):
  File "./readIIO.py", line 29, in <module>
    sleep(2)
KeyboardInterrupt
pi@viking:~/python/thermo$

So a massive thank you for fixing my wonky attempt at a pull request.

[pelwell]

Merged manually.

[popcornmix]

This is in latest rpi-update kernel