On 5/11/2015 2:53 AM, Vesa Jääskeläinen wrote:
On 04/11/15 11:38, Lars-Peter Clausen wrote:
On 11/02/2015 07:47 PM, Vesa Jääskeläinen wrote:
Hi,
When we were using kernel 3.2 and with that board files we just got IIO
devices with static order so that we knew exactly what iio:device0 is.
Now with device tree that order is not so static anymore especially
when
using device overlays (have not yet tried that thou). If there would be
deferred probe for the device then if we have multiple iio:device's
then
those could in theory be in any order.
In example libiio uses iio device index to open the IIO device. So the
problem is to know what iio:device is what.
If we look files under /sys/bus/iio/iio:deviceX. They do not reveal
what
device this is (especially if there are multiple device of same
type). There
does not seem to be a way to give custom name for the iio device
from device
tree that could have been used as a cue.
Device tree aliases kinda sounded a good idea to try. Eg. define
adc0 alias
and then link it to actual device node in tree.
Aliases could be found under /proc/device-tree/aliases. Looking at
it shows
what device tree node it is. However there does not seem to be
actual link
from any /proc/device-tree entries to kernel drivers exposed under
sysfs.
And even the path components in device tree are not in same format
as in
sysfs. So there is no 1:1 mapping possible here.
IIO devices in /sys/bus/iio/iio:deviceX seem to be symlinks to actual
devices under /sys/devices and from there is physical path to the
device and
under that the IIO device with the same name as is under /sys/bus/iio.
So in theory we could make a mapping configuration file that specifies
logical name for iio device and then physical parent path for the
device and
then find under it iio:device* files to determine what is the iio
device
index for this physical device. Then open IIO device with that index in
example with libiio. Sounds a bit complex?
So did we miss something on how this should be defined/accessed or
is there
a slight issue here on how to identify iio devices?
Don't know how device tree overlays affect this if we first load
device tree
overlay with some configuration and then unload it and load another
one with
different configuration.
Hi,
Yes, excellent analysis. This is a real issue. As you said there is no
guarantee that the IIO device numbers are stable, they are assigned
in the
order the devices are registered. If the device are registered in a
different order for whatever reason the numbers will change. You can use
readlink on the IIO device in /sys/bus/iio/devices to get its
position in
the global device topology and be able to tell what the parent device
is,
but this path might not be completely stable either though. E.g. if your
device is on a I2C bus and the number of the I2C bus is dynamically
assigned
it might change when the probe order changes.
Alias seem to be one way to solve this issue. The big question
remains is
how to communicate the alias to userspace. For other device classes the
alias index is used as the device index e.g. a device with alias i2c0
ends
up being the i2c adapter with index 0. But with IIO where we support
a wide
range of different devices that does not really work. E.g. what to do
if you
have adc0 and dac0 as aliases for different devices. So you'd need a
different way to expose the alias.
Some bindings also use the "label" property to assign a unique name
to node.
But again the same issue how to expose that information to applications.
To continue from this "label" property idea I was wondering if we
would add it as new optional(?) file node for IIO devices.
One could then specify it like:
tscadc: tscadc@44e0d000 {
compatible = "ti,am3359-tscadc";
...
am335x_adc: adc {
compatible = "ti,am3359-adc";
...
label = "Port A";
};
};
And this would generate file /sys/bus/iio/iio:deviceX/label with
contents of "Port A".
Then during the application startup it would just need to scan all
devices under /sys/bus/iio and determine what labelled device it wants
to use.
It would be up to device's developer to determine what labels to use
in their designs. This would not break ABI and would be just an
extension for it.
One could also auto-assign label "am335x_adc" in this case too. But if
you include existing arch device tree then changing label in top level
is kinda a bit annoying as you would then need to duplicate all
properties with another label and disable arch device tree's settings.
Could cause also conflict if there are references elsewhere to
existing arch nodes.
Having following in device's device tree file would allow one to
override label or just only specify that.
#include "am33xx.dtsi"
&tscadc {
status = "okay";
adc {
ti,adc-channels = <4 5 6 7>;
label = "Port A";
};
};
I think this "label" model would be simple to understand.
Whether this needs to be implemented as per device driver feature or
could be implemented as generic IIO functionality I do not know.
Thanks,
Vesa Jääskeläinen
This would be a very useful feature as I'm encountering this problem at
the moment.
The label concept is simple and easy to use.
Regards
Phil.
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