On 19 September 2016 at 13:23, Jan Kandziora <j...@gmx.de> wrote:
> Am 19.09.2016 um 10:42 schrieb Colin Law:
>> I have been trying
>> to get to grips with device tree and (on pi with raspbian jessie) in
>> /boot/overlays/README it says
>> "Device Tree makes it possible
>> to support many hardware configurations with a single kernel and without the
>> need to explicitly load or blacklist kernel modules."
>> which I think means that it should not be necessary to use modprobe
>> w1-gpio or to put the module in /etc/modules.
> This is a misunderstanding.
> If you don't modprobe the module, someone else has to do. It depends on
> your udev setup whether udev does it for you. The kernel doesn't load
> modules automatically just because a device is listed in the loaded
> device tree.

Are you sure about that?
says, in section 3.1,
"With a Device Tree, the kernel will automatically search for and load
modules that support the indicated, enabled devices. As a result, by
creating an appropriate DT overlay for a device, you save users of the
device from having to edit /etc/modules; all of the configuration goes
in config.txt, and in the case of a HAT, even that step is
unnecessary. Note, however, that layered modules such as i2c-dev still
need to be loaded explicitly."

> Before device trees, hardware layout has been hardcoded in the driver
> modules or have to be specified by module options. That's what is no
> longer necessary.
>> ...
>> Does this mean I can (should?) use
>> dtoverlay=w1-gpio,gpiopin=4,pullup=y
> Haven't checked yet. Technically it is possible to use the internal push
> transistor of the GPIO port to act as a strong pullup.
> HOWEVER, you should use a series resistor outside the GPIO port then.
> Because, overcurrent from shorting the Onewire too long may kill the
> GPIO4 push transistor inside the BCM2835 SoC.

I didn't realise there was a push transistor in the GPIO port. I
haven't been able to find any information on this. Do you have any
references/specs I can look up?  If the push does work then presumably
one can get away with a higher value of pullup on the data line.

>> and remove the modprobe command?
> Check with lsmod whether the module gets loaded automatically by udev.

I can confirm that using raspbian Jessie w1-gpio is loaded without
using modprobe or putting it in /etc/modules



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