Re: [PATCH v2 2/2] hwmon: add generic GPIO brownout support

Marco Felsch
2018-11-05T17:37:54Z

On 18-11-02 23:05, Trent Piepho wrote:
> On Fri, 2018-11-02 at 07:38 +0100, Marco Felsch wrote:
> > On 18-11-01 18:21, Trent Piepho wrote:
> > > On Thu, 2018-11-01 at 08:14 -0700, Guenter Roeck wrote:
> > > > On Thu, Nov 01, 2018 at 03:53:12PM +0100, Marco Felsch wrote:
> > > > >
> > > > > >
> > > > > > Isn't that configurable with devicetree flags ? I don't think a driver
> > > > > > should get involved in deciding the active edge.
> > > > >
> > > > > No, AFAIK we can only specify the active level types for gpios. This
> > > > > made sense to me, because I saw no gpio-controller which support
> > > > > 'edge-level' reporting (however it will be called) currently.
> > >
> > > Interrupts types are specific to each interrupt controller, but there
> > > is a standard set of flags that, AFAIK, every Linux controller uses.
> > > These include IRQ_TYPE_EDGE_BOTH, IRQ_TYPE_EDGE_RISING,
> > > IRQ_TYPE_LEVEL_HIGH, and so on.
> > >
> > > So you can support hardware that is inherently edge or level triggered.
> >
> > I've been spoken about gpio-controllers and AFAIK there are no edge
> > types. Interrupt-Controller are a different story, as you pointed out
> > above.
>
> You can use edge triggering with gpios. Just try writing "rising" or
> "falling" into /sys/class/gpio/gpioX/edge

Can we access the gpios trough the sysfs if they are requested by a
driver?

> It's level you can't do sysfs. The irq masking necessary isn't
> supported to get it to work in a useful way, i.e. without a live-lock
> IRQ loop.
>
> But you can in the kernel.
>
> Normal process is to call gpiod_to_irq() and then use standard IRQF
> flags to select level, edge, etc.

Currently I using the gpiod_to_irq() function to convert the sense gpio
into a irq, but I do some magic to determine the edge. I tought there
might be reasons why there are no edge defines in
include/dt-bindings/gpio/gpio.h.

> > Too fast state changes sounds like a glitch. Anyway, IMHO we should
>
> Linux has no hard real-time guarantee about interrupt latency, so
> there's no way you can be sure each interrupt is processed before the
> next.
>
> Trying to track level by interrupting on both edges doesn't work well.
> You get out of sync and stuck waiting for something that's already
> happened.

Yes, I'm with you.

> > support support both interrupts and gpios. If the users use gpios they
> > have to poll the gpio, as Guenter pointed out, else we register a
> > irq-handler.
>
> If gpio(d?)_to_irq returns a valid value, why poll? It usually works
> to call this. Plenty of call sites in the kernel that use it and don't
> fallback to polling if it doesn't work.
>
> I think it's fine to call gpiod_to_irq() and fail if that fails, and
> let a polling fallback be written if and when the need arises.

Okay, so no polling for the current solution. Let me summarize our
solution:
- no polling (currently)
- dt-node specifies a gpio instead of a interrupt
-> gpio <-> irq mapping is done by gpiod_to_irq() and fails if gpio
doesn't support irq's
- more alarms per sensor

Only one last thing I tought about:

Using a flat design like you mentioned would lead into a "virtual"
address conflict, since both sensors are on the same level. I tought
about i2c/spi/muxes/graph-devices which don't support such "addressing"
scheme.

hwmon_dev {
compatible = "gpio-alarm";
bat@0 {
reg = <0>;
label = "Battery Pack1 Voltage";
type = "voltage";
alarm-type = <GPIO_ALARM_LCRIT, GPIO_ALARM_CRIT>;
alarm-gpios = <&gpio3 15 GPIO_ACTIVE_LOW
&gpio3 16 GPIO_ACTIVE_LOW>;
};
bat@1 {
reg = <1>;
label = "Battery Pack2 Voltage";
alarm-type = <GPIO_ALARM_LCRIT, GPIO_ALARM_CRIT>;
alarm-gpios = <&gpio3 9 GPIO_ACTIVE_LOW
&gpio3 1 GPIO_ACTIVE_LOW>;
};
cputemp@0 {
reg = <0>;
label = "CPU Temperature Critical";
type = "temperature";
alarm-type = <GPIO_ALARM_GENRIC>;
alarm-gpios = <&gpio4 17 GPIO_ACTIVE_LOW>;
};
};

Where a more structured layout don't have this "issue".

hwmon_dev {
compatible = "gpio-alarm";

voltage {
bat@0 {
reg = <0>;
label = "Battery Pack1 Voltage";
alarm-type = <GPIO_ALARM_LCRIT, GPIO_ALARM_CRIT>;
alarm-gpios = <&gpio3 15 GPIO_ACTIVE_LOW
&gpio3 16 GPIO_ACTIVE_LOW>;
};
bat@1 {
reg = <1>;
label = "Battery Pack2 Voltage";
alarm-type = <GPIO_ALARM_LCRIT, GPIO_ALARM_CRIT>;
alarm-gpios = <&gpio3 9 GPIO_ACTIVE_LOW
&gpio3 1 GPIO_ACTIVE_LOW>;
};
};
temperature {
cputemp {
label = "CPU Temperature Critical";
alarm-type = <GPIO_ALARM_GENRIC>;
alarm-gpios = <&gpio4 17 GPIO_ACTIVE_LOW>;
};
};
};

We don't have to take this layout, we can also consider about devices:

hwmon_dev {
compatible = "gpio-alarm";

dev@0 {
reg = <0>;
voltage {
label = "Battery Pack1 Voltage";
alarm-type = <GPIO_ALARM_LCRIT, GPIO_ALARM_CRIT>;
alarm-gpios = <&gpio3 15 GPIO_ACTIVE_LOW
&gpio3 16 GPIO_ACTIVE_LOW>;
};
temperature {
label = "Battery Pack1 Temperature Critical";
alarm-type = <GPIO_ALARM_GENRIC>;
alarm-gpios = <&gpio4 17 GPIO_ACTIVE_LOW>;
};
};
dev@1 {
reg = <1>;
temperature {
label = "CPU Temperature Critical";
alarm-type = <GPIO_ALARM_GENRIC>;
alarm-gpios = <&gpio4 19 GPIO_ACTIVE_LOW>;
};
};
};

I don't think that is a issue at all, but I don't know the dt
maintainers opinion of this theme.

Regards,
Marco