Robin Murphy [off-list ref] writes:

On 16/05/18 14:37, Stefan Wahren wrote:

quoted

Recent VC4 firmware provides a hardware-independent way to retrieve the
under-voltage sensor on the following Raspberry Pi boards:
   - Raspberry Pi B+ (via GPIO on SoC)
   - Raspberry Pi A+ (via GPIO on SoC)
   - Raspberry Pi 2 B (via GPIO on SoC)
   - Raspberry Pi 3 B (via GPIO on port expander)
   - Raspberry Pi 3 B+ (via PMIC)

TODO:
   - try to make bcm2835 firmware a bus driver

Do you actually need a DT binding? From the probe routine in patch 2 it 
looks like this capability can be discovered dynamically by asking the 
firmware, therefore it might be more straightforward for the firmware 
driver itself to probe that and create a child device iff the feature is 
present, which the hwmon driver can then bind to as a pure platform driver.

Agreed, let's skip doing this in DT.  I just switched the bcm2835-camera
driver over to not need DT, and I think you could do something similar
(and use the pdev->dev.parent to get at the firmware device).
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