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docs: Add information for RTC devices

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The `rtc` machine feature is "considered" by default for all the
machines we provide until now. This can confuse people who would want an
RTC device attached to a RaspberryPi board[1]. Clarify this via
documentation.

[1] https://github.com/agherzan/meta-raspberrypi/issues/1010

Signed-off-by: Andrei Gherzan <andrei.gherzan@huawei.com>
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Andrei Gherzan 2022-03-14 13:45:47 +01:00 committed by Andrei Gherzan
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docs/extra-build-config.md
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@ -388,3 +388,23 @@ You may need to adjust volume and toggle switches that are off by default
```
Audio capture on ReSpeaker 2 / 4 / 6 Mics Pi HAT from Seeed Studio is very noisy.
## Support for RTC devices
The RaspberryPi boards don't feature an RTC module and the machine
configurations provided in this BSP layer have this assumption (until, if at
all, some later boards will come with one).
`rtc` is handled as a `MACHINE_FEATURES` in the context of the build system
which means that if an attached device is provided for which support is needed,
the recommended way forward is to write a new machine configuration based on an
existing one. Check the documentation for
`MACHINE_FEATURES_BACKFILL_CONSIDERED` for how this is disabled for the
relevant machines.
Even when `MACHINE_FEATURES` is tweaked to include the needed `rtc` string,
make sure that your kernel configuration is supporting the attached device and
the device tree is properly tweaked. Also, mind the runtime components that
take advantage of your RTC device. You can do that by checking what is
included/configured in the build system based on the inclusion of `rtc` in
`MACHINE_FEATURES`.